THE PHOTOCHEMICAL ACCELERATION OFTHE URANIUM (IV) - URANIUM (VI) ELECTRON EXCHANGE REACTION Thesis for the Degree of Ph. D. MICHIGAN STATE UNIVERSITY JAMES DAVID HOESCHELE 1969 This is to certify that the thesis entitled THE PHOTOCHEMICAL ACCELERATION OF THE URANIUM(|V) - URANIUM(V|) ELECTRON EXCHANGE REACTION presented by James David Hoeschele has been accepted towards fulfillment of the requirements for M degree inghflmisltL Major professor ' a Date July 25, 1969 0-169 ABSTRACT THE PHOTOCHEMICAL ACCELERATION OF THE URANIUM(IV) - URANIUM(VI) ELECTRON EXCHANGE REACTION By James David Hoeschele The kinetics of the uranium(IV)-uranium(VI) electron exchange reaction were investigated in aqueous perchloric acid solutions under conditions of constant incident light intensity. A low-pressure mercury— vapor lamp, Model L0 73SA-7 (Hanovia), was used as the light source which emitted principally 2537 A radiation. The exchange was inhibited by uranium(VI) and hydrogen ion, but accelerated by uranium(IV) and by increasing the temperature. Non-linear order graphs were obtained for uranium(IV), uranium(VI), and hydrogen ion, having the approximate orders of 0.41, -1.3, and -0.65, respectively. Overall quantum yields for exchange ranged from 0.01 to 0.1, based on the absorption of light by uranium(VI), and were determined by means of potassium ferrioxalate actinometry. Plausible exchange mechanisms are discussed in terms of a uranium(V) intermediate as produced by one or more of the following steps: (1) U(IV) + O -—-+ U(V) + H0 2 2 (2) U(IV) + U(VI) --+ 2U(V) (3) UO§+-H20 -§3+ U0: + OH + H+ A.mechanism based on step (3) as the principal exchange path is in rea- sonable qualitative agreement with the experimental results. THE PHOTOCHEMICAL ACCELERATION OF THE URANIUM(IV) - URANIUM(VI) ELECTRON EXCHANGE REACTION By James David Hoeschele A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Chemistry 1969 DEDICATION To Todd, Lisa, David, Mark, and Heidi . may they profit from my mistakes ii ACKNOWLEDGEMENTS The author gratefully acknowledges the advice and encouragement of Professor Carl H. Brubaker, the patience and understanding of his wife, Joyce, and the financial support of the Atomic Energy Commission. Special thanks goes to Don E. Ferguson, Raymond G. Wymer, and Rex E. Leuze, of the Chemical Technology Division, Oak Ridge National Laboratory, for their interest and encouragement. iii TABLE OF CONTENTS PAGE I. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . 1 II. HISTORICAL . . . . . . . . . . . . . . . . . . . . . . . . . . 11 A. Aqueous Chemistry of Uranium . . . . . . . . . . . . . . 11 Hydrolysis . . . . . . . . . . . . . . . . . . . . . . 13 Spectra. . . . . . . . . . . . . . . . . . . . . . . . 14 Photochemistry of U(VI). . . . . . . . . . . . . . . . 18 Electron Transfer Reactions. . . . . . . . . . . . . . 20 B. Uranium(IV)-Uranium(VI) Exchange Studies . . . . . . . . 23 Thermal Exchange Studies . . . . . . . . . . . . . . . 24 Photochemical Exchange Studies . . . . . . . . . . . . 27 III. THEORETICAL . . . . . . . . . . . . . . . . . . . . . . . . . 29 IV. EXPERIMENTAL . . . . . . . . . . . . . . . . . . . . . . . . . 35 A. Preparation and Standardization of Reagents. . . . . . . 35 Sodium Hydroxide . . . . . . . . . . . . . . . . . . . 35 Perchloric Acid. . . . . . . . . . . . . . . . . . . . 36 Sodium Perchlorate . . . . . . . . . . . . . . . . . . 36 Cerium(IV) in Sulfuric Acid. . . . . . . . . . . . . . 37 Nitrogen Purification. . . . . . . . . . . . . . . . . 38 238Uranium(VI) Perchlorate . . . . . . . . . . . . . . 40 233U-enriched 238Uranium(VI) Perchlorate . . . . . . . 42 Uranium(IV) Perchlorate. . . . . . . . . . . . . . . . 42 Analysis of Uranium Stock Solutions. . . . . . . . . . 46 iv B. Exchange Experiments . . . . . . . . . . Preparation of Exchange Solutions. . . Photolysis of Exchange Solutions . . . Processing of the Exchange Solutions Separation . . . . . . . . . . . . Preparation of Counting Samples. . . . Counting Techniques. . . . . . . . . . C. Chemical Actinometry . . . . . . . . . . Preparation of Calibration Graph . . . Photolysis of Actinometer Solutions. Quantum Yield Determinations . . . . v. RESULTS 0 O O O O O O O O O O O O O O O O O O O A. Light Intensities and Overall Quantum Yields . . . B. Calculation of Exchange Results. . . . . Uranium(VI) Concentration Dependence . Uranium(IV) Concentration Dependence . Hydrogen Ion Concentration Dependence. Radiolysis and Temperature Dependences Tartaric Acid Concentration Dependence VI 0 DISCUSSION. 0 O O O O O O O O O O O O O O O O O A..Discuseion of Errors and Reproducibility of Results. Photochemical Rate Data. . . . . . . . . Thermal Exchange Results . . . . . . . . B. Interpretation of Photochemical Data. . . VII. SUMMARY . . . . . . . . . . . . . . . . . . LIST OF REFERENCES I O O O O O I O I O O O O O O O I 0 PAGE 47 48 51 58 6O 62 63 64 65 67 68 76 76 8O 81 83 88 96 98 101 101 101 102 104 113 114 PAGE APPENDICES O O O O O O O O O O O O O O O O O O O O O I O O O O O O O 122 A. Relative Calibration of a Low—Pressure Mercury-Vapor Lamp (Model LO 735A-7, Hanovia) . . . . . . . . . . . . . . . . . 122 B. Light Absorption Data for Uranium(IV)-Uranium(VI) Solutions. . 130 C. Computer Program and Sample of Input—Output Data . . . . . . . 133 D. Original Kinetic Data. . . . . . . . . . . . . . . . . . . . . 137 vi LIST OF TABLES TABLE 1. Selected decay properties for 233U and 238U . . . . . . . . 2. Emission intensities of a mercury-vapor lamp, Model LO 735A-7 (HanOVia) O O O O O O O O C O O O I O O O O O O I O O O O O 3. Total incident intensities for lamps I, II, and III . . . . 4. Dependence of exchange rate and overall quantum yields on uranium(VI) concentration . . . . . . . . . . . . . . . . . 5. Dependence of exchange rate and overall quantum yields on uranium(IV) concentration . . . . . . . . . . . . . . . . . 6. Dependence of exchange rate and overall quantum yield on hydrogen ion concentration. . . . . . . . . . . . . . . . . 7. Dependence of exchange rate on the 233U concentration . . . . 8. Dependence of exchange rate on temperature. . . . . . . . . . 9. Dependence of exchange rate on tartaric acid concentration. . 10. Exchange rates for comparable experiments . . . . . . . . . . Al. The spectral radiance and Cary response for the standard tungsten lamp (U-202) and the spectral response factors for a Cary 14 spectrophotometer . . . . . . . . . . . . . A2. Cary Model 14 resolution parameters and relative intensity data for a mercury-vapor lamp, Model L0 735A—7 (Hanovia). . Bl. Molar absorptivities of uranium(IV) and uranium(VI) for the wavelengths of the mercury-vapor emission spectrum. . . . . BZ. Fractions of light absorbed by exchange solutions and uranium ions 0 O O O O O C O O O O O O O O O O O O I O O I O O O O 0 B3. Summary of data used in correlating exchange rates with calculated absorbed intensities . . . . . . . . . . . . . . D1. Dependence of exchange rate on concentration of uranium(IV) . D2. Dependence of exchange rate on uranium(VI) concentration. . . vii PAGE 42 54 77 84 87 93 96 98 100 101 125 129 130 131 132 137 139 D3. D4. D5. D6. Dependence Dependence Dependence Dependence of of of of exchange rate exchange rate exchange rate exchange rate on hydrogen ion concentration . 0n temperature. I O O O O O O O 233 on the U concentration . . . on tartaric acid concentration. viii PAGE 142 145 146 148 FIGURE 1. Formal reduction potentials of uranium in l M_HC1O4 at 25°. . . 2. Absorption spectra of uranium(IV) and uranium(VI) and fluorescence (emission) spectrum of the uranyl ion in perchlorate media . . . . . . . . . . . . . . . . . . . . . . 3. Nitrogen purification train . . . . . . . . . . . . . . . . . . 4. Preparation and storage of uranium(IV) perchlorate. . . . . . . 5. Apparatus for the de-aeration and maintenance of a nitrogen atmosphere over exchange solutions. . . . . . . . . . . . . 6. Photolysis apparatus. . . . . . . . . . . . . . . . . . . . . . 7. Lamp and associated circuitry . . . . . . . . . . . . . . . . . 8. Photolysis apparatus and associated equipment . . . . . . . . . 9. Flow diagram of exchange solution processing. . . . . . . . . . 10. Absorbance !§_length of photolysis. . . . . . . . . . . . . . 11. Absorbance !§_uranium(IV) concentration for uranium(IV) variation 0 O O O O O O O I O O O O D O O O O O O O O O 0 O O 12. Absorbance !§_uranium(VI) concentration for uranium(VI) variation 0 O O I O I O C O O O I O O O O O O O O O O O O O O 13. Absorbance X§_potassium ferrioxalate (K3[Fe(C204)3] concentration . . . . . . . . . . . . . . . . . . . . . . . . l4. Quanta absorbed (2537 A radiation)/ml—sec y§_absorbance of potassium ferrioxalate solutions at 2537 A . . . . . . . . 15. Typical graphs of 1n(1 - F) ythime for the hydrogen ion dependence. . . . . . . . . . . . . . . . . . . . . . . . . . 16. Logarithm of exchange rate z§_1ogarithm of uranium(IV) and uranium(VI) concentrations. . . . . . . . . . . . . . . . . . 17. Logarithm of overall quantum yield yg_logarithm of uranium(VI) LIST OF FIGURES dependence 0 O O C O O O I O O I O I O I O I O O O O 0 O 0 O 0 PAGE ll 16 39 44 49 52 53 57 59 69 71 72 73 75 82 85 86 18. 19. 20. 21. 22I 23I 24. A1. A2. PAGE Logarithm of overall quantum yield XE logarithm of uranium(IV) concentration . . . . . . . . . . . . . . . . . . . . . . . . 89 Logarithm of overall "oex(calc)"_ vs logarithm of uranium(IV) concentration . . . . . . . . . . . . . . . . . . . . . . . . 90 Logarithm of exchange rate y§_logarithm of hydrogen ion concentration I I I I I I I I I I I I I I I I I I I I I I I I 91 Logarithm of overall quantum yield vs logarithm of hydrogen ion concentration . . . . . . . . . . . . . . . . . . . . . 94 Absorbance 22 hydrogen ion concentration for exchange solutions of the hydrogen ion variation . . . . . . . . . . . 95 Logarithm of exchange rate y§_1ogarithm of 233U tracer concentration I I I I I I I I I I I I I I I I I I I I I I I I 97 Logarithm of exchange rate X§_logarithm of tartaric acid concentration . . . . . . . . . . . . . . . . . . . . . . . . 99 Arrangement of lamps, spherical mirror, and spectrOphotometer for lamp calibration. . . . . . . . . . . . . . . . . . . . . 123 Energy -v_§_ wavelengthI I I I I I I I I I I I I I I I I I I I I I 124 I. INTRODUCTION During the past two decades, considerable progress has been made in understanding the kinetics and mechanisms of electron-transfer reactions. This progress has been attributed to the availability of specific tracers, advances in instrumentation (for direct rate measure- ments in any time range) and experimental techniques, as well as to the development of satisfactory quantitative theory. In this connection, the elegant theoretical treatment(s) of electron—transfer reactions by R.A. Marcus1 is especially noteworthy. Marcus,1 Sutin,2 and Reynolds and Lumry3 have critically reviewed the existing theories, while Taube,4 Halpern,5 and Sutin6 have reviewed electron-transfer reactions in general. Electron-transfer reactions generally include (1) isotopic or electron exchange processes, in which no net chemical change occurs, as in 204T 3+ + 1 Tl+ :1 204 + 3+ Tl + T1 (1) and 55-59 55-59 Fe(CN)2- + Fe(CN)2- I: Fe(CN)2— + Fe(CN)2- (2) and (2) the more familiar oxidation-reduction reactions, in which chemical change is involved, as in Fe2+ + Ce4+ -—»-Fe3+ + Ce3+ (3) and 114+ + 1:13+ + 2H20 ——+ 1103+ + T1+ + 411+ (4) Exchange reactions constitute a relatively ”simple" class of reactions, since the reactants and products are identical, and therefore, the equilibrium constant K and standard free energy AF° may be assumed to be unity and zero, respectively. Their study has been of particular inter- est since they provide simpler models on which to base theoretical calculations. There are two well-established general mechanisms for electron- transfer processes: the so-called outer-sphere and inneresphere mechanisms. In the first of these, electron transfer occurs through an "extended" activated complex in which the primary coordination spheres of the reactant ions remain intact and unaltered with respect to the number and kind of ligands present; in the second, through a bridged activated complex in which the primary coordination spheres of the reactants are mutually linked by one or more bridging ligands. A dis- tinction between the two mechanisms is usually possible experimentally when (1) electron transfer between substitution—inert reactants is rapid (outer-sphere) or (2) the reactant complex of one metal ion and the product complex of the other are substitution inert, and the bridging ligand is incorporated into the substitution-inert product (inner- sphere). In other cases, it may be very difficult to distinguish between the two. Outer-sphere electron transfer occurs principally by a tunnel- ling process. According to (R.J.) Marcus, Zwolinski, and Eyring,7’8 tunnelling is a quantum-mechanical process in which the electron 3 "passes through" a potential-energy barrier rather than over it (in the classical sense). Conceptually, this barrier is the region of space occupied by water molecules of hydration separating the metal ions. A direct transfer through delocalized overlapping metal ion orbitals is also possible when a very close approach can be achieved in the activated complex (somewhat analogous to gas-phase exchange reactions, e.g., Ne- Ne2+). The rate constants of outer-sphere reactions, many of which are exchange reactions, cover a range of sixteen orders of magnitude, with 1 sec.1 for Co(NH3):+-C0(NH3)2+ and k > 108 for Fe(CN)2--Fe k < 10'8 gf (o~phenanthroline)3+. In general, complex ions containing unsaturated or large polarizable ligands (such as o-phenanthroline, bipyridyl, CN— or C1—) exchange rapidly and usually much faster than the corresponding aquo or ammine complexes. The pronounced rate dependence of certain outer-sphere reactions (Fe(CN)2-—Fe(CN)2- and MnOZéMn0:-) on specific + + cations‘(Cs+ > K > Na ) indicates that in some instances electron trans- fer can occur through a bridged outer-sphere activated complex, e.g., [Fe(CN)6...Cs...Fe(CN)2-]+ wherein the Cs+ ion is perhaps acting as a bridge to conduct the elec— tron, in addition to reducing the electrostatic repulsion between the anions. In inner-sphere reactions, electron transfer is preceded by the formation of a singly— or doubly-bridged intermediate. The function of the bridging ligand(s) may simply be (1) to reduce the electrostatic repulsion between the reactant ions and bring them close enough together to permit a direct exchange and/or, (2) to provide a continuous pathway 4 for electron "conduction" through 0 or n metal—ligand bonds. The second alternative is an example of double-exchange in which the reduc- tant loses an electron to the bridge as the bridge loses one to the oxidant (a concerted process). The use of vacant (but usually higher energy) orbitals of the bridging ligand for electron delocalization and migration has been called "superexchang ." Temporary reduction or oxidation of the bridge may result in chemical change in the bridge (e.g., cis-trans isomerization) when the removal or addition of one or more electrons produces a relatively long-lived metastable intermediate. The name chemical mechanism has been suggested for such reactions. The names of the preceding three processes have been suggested by Halpern and Orgel.9 Electron transfer can occur with or without the transfer of bridging ligand(s). However, the transfer of a bridge is incidental to the process and is strictly determined by the relative substitution labilities of the complex ions involved. The recognition of the inner- and outer-sphere processes has been made possible largely through the pioneer work of Taube and co-workers on the general system 2+ 2+ + 5m?” (5) 2+ Co(NI-13)5X + Cr 4 + 5H+ -—+ Cr(H20)5X2+ + Co where X - any of a large number (>100) of molecules or anions, e.g., halides, N3, NCS-, OH-, H20, (mono- and polyfunctional) organic acids, etc. Since Co(III) and Cr(III) complexes are substitution-inert, the~ original finding that Cl- was contained in the product,10 Cr(H20)5C12+, clearly demonstrated that electron transfer occurred through a bridged activated complex of the form 4++ [(H3N)5Co--Cl—-Cr(OH2)5 ] Extensive studies involving the systematic variation of the X group in reaction (5) has led to the discovery that (i) lone-pair electrons (as in N3) can participate in the bridging and transfer process, (ii) the series N; >> I- > Br- > C1. > F- (in the order of effectiveness as a bridging group) may be considered diagnostic of an inner-sphere mechan- ism, (iii) that rapid electron transfer can occur through a conjugated n-bonding system (conduction) in which the site attacked by Cr2+ may be "adjacent" to or "remote" from the point of attachment of Co(III), (iv) an unstable intermediate may be formed as a result of unorthodox coord- ination (S-bonded NCS-), plus others. There is at least one system11 studied thus far that proceeds by both general mechanisms; namely, Co(NH3)5X2+--Co(CN):-. The rate of the bridging mechanism path depends markedly on the identity of X, as is typical for inner-sphere mechanisms, whereas the outer-sphere path has a rate practically independent of X. Many of the special properties of electron-transfer reactions are a result of Franck-Condon restrictions. Applied originally to electronic absorption and emission processes, the Franck-Condon principle states, that electronic movement is so fast compared to nuclear movement (NIOOx) that the nuclear coordinates remain essentially unchanged during an electronic transition. This means that any rearrangement of the co- ordination and/or solvent shells (expansion, compression, asymmetric changes) which is necessary to establish the proper energy balance in. the activated complex must occur‘prigg to the actual electron-transfer act itself (for which AF - 01). Libby has recently considered 6 transition metal exchange12 and oxidation-reductionl3 reactions in light of this principle, and concludes that electron transfer occurs most readily when the activated complex assumes the most highly symmetrical configuration possible. Reactants which are normally symmetrical, therefore, would require relatively little energy for rearrangement (primarily of the solvent shells) whereas considerable energy and rearrangement (of both ligand and solvent shells) might be required for asymmetrical reactants. In this context, it is thought that the for- mation of a bridge(s) in a "symmetrical" activated complex can lower an otherwise appreciably higher activation energy. Sutin6 has pointed out that inner—sphere mechanisms appear to be preferred when rearrangement energies are large. Simultaneous two-electron transfers have been proposed for several transition elements, e.g., Tl, Sn, Sb, As, since their stable oxidation states differ by two electrons. Thus, a direct one-stage process, e.g., as in reaction (1), could occur in which two electrons are transferred from a single orbital on one of the reactants to a single orbital on the other. However, a direct two-electron transfer is difficult to distinguish kinetically from consecutive one-electron transfers. Reaction (4) is less likely to occur by a simultaneous two- electron transfer since the two electrons transferred from U(IV) come from different orbitals and pairing must occur at some stage.14 Various atom-transfer processes (H, 0, Cl) have been proposed and/or verified as alternative modes of "electron transfer." Dodson and Davidson;5 first proposed that electron transfer between aquo metal ions or related hydrolytic species (e.g., Fe2+-Fe3+ and Fe2+-Fe0H2+) may occur through transfer of a hydrogen atom betwben their hydration 7 spheres. With special reference to the Fe(II)-Fe(III) system, the evidence in support of such a mechanism is that the rates are much slower (1) in non-aqueous media (anhydrous alcohols and nitromethane) where H-atom transfer is less probable, and (2) in D 0 by a factor of two 2 which is consistent with, but not conclusive, that O-H bond formation and rupture occurs in the rate—determining step. Also, electron trans- fer has been demonstrated in ice16 and the activation energies of a number of other aquo metal ion systems are comparable (W10 kcal/mole), suggesting that a common mechanism involving water may be operative. Alternatively, Stranksl7 has suggested that electron transfer in these systems occurs gig a direct electron transfer or tunnelling process involving a hydrogen-bonded (outer-sphere) activated complex. Oxygen- atom transfer has been demonstrated, by means of 18O-labelling, in 2+ 2+ 18 several systems, e.g., Cr + U02 , oxygen-containing oxidizing agents.19 In this connection the kinetics and U(IV) with a number of of a number of actinide ion reactions have been interpreted in terms of (inner-sphere) oxygen-atom transfer, in addition to H-atom transfer and electron tunnelling as alternatives, (e.g., the Np(IV)-Np(VI) system). Newton and Rabideau20 have reviewed the kinetics and mechanisms of actinide electron-transfer reactions. Photoactivation and subsequent reaction of excited state species can lead to additional mechanistic possibilities. Photoexcitation may lead to (l) the catalysis of an existing thermal path or (2) a distinc- tively different reaction scheme(s) involving the (a) direct participa- tion of an excited state species and/or (b) free-radicals, produced, for example, as a result of photodecomposition of the solvent (or ligands). Uri21 has reviewed the photodecomposition of water by various ions from 8 which it appears that the following primary photooxidative process may have general validity for certain metal ions (given sufficiently ener- getic radiation), n+ Ex» M(n-l)+ M °H20 °OH + H+ (6) where M I Fe(III), Ce(IV), Tl(III),22 and Np(VI).23 A similar process involving H-atom photOproduction may be written for the photoreduction of H20 by various cations (Cr2+) and anions (halides). The recent photochemical studies of Co(III) and Cr(III) complexes by Adamson and co-workers24 have revealed that various types of excited states may have a distinctive chemistry which does not necessarily involve the traditional octahedral, square-planar, etc., geometries of complex ions and which is more understandable in terms of electronic structure and bonding than in terms of thermal reaction mechanisms. Adamson-states that the excited state reaction is not necessarily a rapid one, but it may be activated and it may be stereospecific. Only one photo-induced inorganic electron exchange system appears to have been studied, i.e., the Tl(I)—Tl(III) system. Exchange is thought to proceed through a chain-type mechanism (see p. 32 of the THEORETICAL section) involving Tl(II) and OH as transient intermediates as produced in a primary process corresponding to (6). Several quantitative theoretical treatments of electron-transfer reactions have appeared in the literature. While none of these are 25 and (R.A.) Marcus1 "complete" in themselves, the treatments of Hush have been applied most extensively. In general, they are equally applicable to electron-transfer reactions between (1) organic species, (2) metal complexes and organic molecules, and (3) nonemetallic 9 inorganic molecules as well as to reactions between complex ions. Thus far, the calculation of rate constants and free energies of acti- vation has been limited to the simpler outer-sphere processes, the agreement between calculated and experimentally observed rate constants being within a factor of 10 (for the very fast reactions), which corres- ponds to a AFT within N1.3 kcal/mole. The major factors considered in the calculation of these quan- tities are: (l) rearrangement free energies (ligand and solvation shells), (2) coulombic free energy, (3) the change-of-multiplicity con- tribution, and (4) the corrections for changes in ligand field effects. Reynolds and Lumry3 have critically reviewed all present (quantitative) theories and cite the further refinements needed. The purpose of the preceding section was to present a brief survey of the various types of processes that are thought to be involved in electron transfer reactions. The study of the catalytic effects of various anions on several electron transfer systems has been a major interest in this laboratory. In particular, kinetic studies of the 26,27 28,29 U(IV)rTl(III), U(IV)-U(VI), and Fe(II)-Fe(III)30 systems have been carried out in HClO in the presence of one or more of the follow- 4 ing dicarboxylic acids: oxalic, malic, malonic, succinic, maleic, fumaric, and (for all three systems) tartaric acid. Some of these acids caused inhibition of the rates, some were without effect, but for all the systems.studied tartaric acid caused the greatest acceleration of the rates. Consequently, the studies involving tartaric acid are the most detailed. The results of these studies indicate that the reactions are complex and involve several competitive paths. In the U(IV)-U(VI) exchange study, a marked photoacceleration of the rate was 10 observed in the presence of tartaric acid. Photoinduced exchange had been reported previously for this system in other acid media, but not in the presence of organic acids. A kinetic study of this photocatal- ysis seemed particularly interesting and desirable since a direct comparison of the photochemical and thermal reaction kinetics could then be made. Furthermore, some interesting mechanistic alternatives are possible (see p. 7). Therefore, this system was selected as my major research problem. However, the preliminary photolysis experiments revealed that tartaric acid (as are most carboxylic acids) is photo- oxidized by the uranyl ion, U02+, itself being reduced to U(IV). Con- sequently, this "exchange" system was abandoned in favor of the "pure" U(IV)-U(VI) system in HClO only. A knowledge of the kinetics and 4 mechanism(s) of this parent system is prerequisite to the understanding of any future and perhaps more complicated studies involving organic compounds. II. HISTORICAL This section is a review (1) of selected aspects of the chemistry of uranium ions in acidic solutions and (2) of the previous work on the uranium(IV)-uranium(VI) exchange system. The first part is intended as background material and will cover, briefly, the hydrolysis, spectra, photolysis, and electron transfer reactions of U(IV), U(V), and U(VI), principally in perchloric acid. Thermal and photo-induced exchange studies are discussed in the second part. An excellent review of the aqueous chemistry of uranium is contained in the text by Katz and analyt— 33 34 ' ical, and radiochemistry of uranium have been published recently. 3 Seaborg.31 Also, comprehensive reviews on the inorganic, A. Aqueous Chemistry of Uranium Uranium can exist in four well-defined oxidation ,states in aqueous solution, i.e., III, IV, V, and VI. In acid solution, the III and IV oxidation states exist as the hydrated cations, U3+ and U4+, whereas the higher oxidation states exist as the "-yl" ions, U0: and 2+ U02 . The potentials of the various oxidation states are shown in Figure 1. | 0.32 | 02+ 0.063 U0: 0.58 U4+ -0.631 U3+ -1.80 U (yellow) (brown?) (green) (red) U Figure 1. Formal reduction potentials of uranium in l LiHClO4 at 25°. 11 12 In the presence of other anions the values differ as a result of complex ion formation. The UNI/U3+ and U0§+/U0; attain equilibrium rapidly. The U03/U4+ and U0§+lU4+ potentials, however, potentials are reversible and involve oxygen transfer and are therefore irreversible. Nevertheless, these potentials are very reproducible and are hydrogen ion dependent. Solutions of uranium in the individual oxidation states (III-V) can be prepared by controlled-potential or chemical reduction (e.g., Zn 2+ solutions, or by dissolving a suitable salt (anhydrous 2 binary chloride) in solution. amalgam) of U0 Uranium(III) solutions are unstable and are oxidized by water (slowl ) and ox en (rapidl ). It has also been re orted3S that the Y Y8 Y P 4 tively unimportant to this study and, therefore, will not be considered U(III) ion slowly reduces C10 to Cl—. (The chemistry of U(III) is rela- further.) Uranium(IV) solutions remain essentially unchanged at 25° in the absence of oxygen. Even in the presence of oxygen and at moderate acidities, U(IV) is oxidized only slowly (mechanism discussed below). Uranium(V) is the least stable and most difficult oxidation state to observe in aqueous solution. Herasymenko36 was the first to observe 2+ to U0+. For a 2 2 long time his conclusions were considered doubtful until the polaro- U(V) in solution by the polarographic reduction of U0 graphic studies of Heal37and Harris and Kolthoff38 confirmed the existence of 00;. Uranium(V) exhibits the following characteristic disproportionation reaction: + 2+ + 4+ (7) + 2UO2 + 4H ::;U02 U + 2H20 13 An equilibrium constant of 1.05 x 109 was obtained by Nelson and Kraus39 in polarographic studies at 25° at an ionic strength of 2.0. Kraus and + co-workers also found that U02 is relatively stable in the pH range 2-4, where (1) approximately millimolar solutions can be prepared and (2) the disproportionation rate is negligibly slow.40(ln 24% HF solution U(V) 6 The kinetics and mechanism(s) of the U(V) disprOportionation exists as the UP ion and is very stable.41) reaction will be discussed under "Electron Transfer Reactions." Uranium(VI) is the most stable oxidation state in solution, despite its positive reduction potential (see Figure l). The high 2+ ion, which 2 behaves more like a 3+ ion in solution, as indicated by hydrolytic positive charge is stabilized by the formation of the U0 studies and the fact that the uranyl ion entrOpy is unusually low for a divalent ion: -17 e.u. y§.2 to 4 e.u. for many 2+ ions. It is now generally accepted that the uranyl ion has a symmetrical, linear struc- ture, O-U-O, which can be disturbed by strong local fields. Hydrolysis: The hydrolytic behavior of uranium ions has been studied very thoroughly, particularly in the case of U(VI). The degree of hydrolysis increases in the order U3+ < U0:+ < U4+ as deter- mined by the acid reaction of the respective salts in solution. The instabilities of the U3+ and U0: ions, as noted previously, makes hydrolytic investigations of these ions very difficult. Numerous investigations, based principally on pH measurements, indicate that the hydrolysis of U3+ is similar to that of the lanthanide(III) ions. The acid constant for U0: 10'3. is estimated to be significantly less than 14 The existence of U4+ as the unhydrolyzed species of U(IV) has been established unambiguously.42 Kraus and Nelson43 have shown that above a [H+] ~0.01, U4+ is hydrolyzed according to the simple monomeric reaction U4+ + H O -+ UOH3+ + +— + 2 H (8) with an acid constant K, at 25°, of 0.21 at zero ionic strength and 0.024 at an ionic strength of 2.00 (NaClOA). At lower acidities, further hydrolysis leads to polynuclear species of the general formula44 U(UOOH):+4. 3+ is the only species present in solution below a pH of m2.45 Recent equilibrium Uranium(VI) hydrolysis studies have shown that U0 ultracentrifugation46 and spectrophotometric work47 indicate that the principal hydrolyzed species of U0:+ in HClO4 at 25° are (U02)2(0H)§+ and (U02)3(0H):; however, an equally satisfactory fit of the data is obtained if the species U0 0H+ is included. Also, Hearne and White48 2 pr0posed the species U0 0H+ with a formation constant of 4 x 10-6 at 2 an ionic strength of 0.35. Recent kinetic studies of U02+ hydrolysis 2 49,50,51 by.relaxation methods have provided additional evidence in support of UOZOH+ as a major hydrolytic species. Sutton52 and MacInnes 53 2+ 2+ and Longsworth had previously proposed U205 and U308 as the major cationic species. Spectra: The absorption spectra of the U(IV) and U(VI) ions consist of relatively narrow bands in the ultraviolet, visible and near- infrared regions, as is characteristic of all actinide ion spectra. These bands arise from electronic transitions within the 5fn levels 15 and are only moderately influenced by ligand field effects (as compared to the greater perturbation of the spectral bands of the d transition metal ions). The ultraviolet and visible absorption spectra of U(IV) and U(VI) in l §_HC10 are illustrated in Figure 2. This figure is a reproduction 4 of the spectra which were experimentally observed in this study and which are discussed in the EXPERIMENTAL section on page 47. As regards U(IV) spectra in HClO two significant investigations have been re- 4: ported. Cohen and Carnall54 have measured the visible spectrum of U(IV) and U(III) both in HClO and DClO and have observed no discernible 4 4 changes either in intensity or peak positions. The ultraviolet absorp- tion spectra of U(IV) and U(III) in DClO are also reported. Kraus and 4 Nelson55 investigated the U(IV) absorption spectrum over the wavelength range 4000-11000 A. From a combination of hydrolytic and spectrophoto- metric data, they were able to deduce the absorption spectrum of U0H3+, which exhibits a maximum at ~6250 A (s :20). McKay and Woodhead,56 and Stewart57 report U(IV) spectra for HNO and HCl media, respectively. 3 The uranyl ion spectrum is one of the most extensively investi- gated of all molecular spectra. A recent monograph58 devoted to the spectrosc0py and photochemistry of uranyl compounds reviews most of the early work up to about 1962. Very recent papers by McGlynn and Smith59 and Bell and Biggers60 have been concerned with the theoretical inter— pretation of the spectra. By using SOphisticated computer—assisted resolution techniques, Bell and Biggers were able to resolve the complex overlapping uv and visible spectra (1795-5000 K) into 24 discrete bands, comprising 7 major absorption bands (or band groups). Two major bands, located in the visible region, are in accord with the two (vibrationally 16 N | oowN In H. moo.m " H ammoN " ”Iv-EH .mHN.o u H+NODH "Sauuommm mosmommuooam momm n H .z o.H u H .oo.H n H+mu "mnuooam sowunnomn< .mwvma mumuoanoume ea so“ dudes: mas «o annuommm Asowmmaamv mocmommuosam use AH>vasficmu= was A>Hvsswcmuz mo muuomam Goauauomn< .N muswfim N §u4w>i 085 020 00; 00—0 080 Son 080 80' 86¢ 08¢ 80¢ 8h» 8'» 8n 3 008 CONN <1 q: d O ———_—— de-JfiloTu/H/_J .\.l—\uos\./— — .\ ./ \r\ I I. a Ru 1.9 .l. i. .I I .98 .10a i. u w _. w I. _ 93 .1.3 v _ m ._ W 4 . _ _ __ .. w I. :Z:II:I:I w .X¥ lice“. .>:: . m izsitngzsecsfi.£2: a my efiuesaoeemgeaesfi ’ w... z. ,. .I 9*. nqoo 1. .. P—ub—Pthbpr—PPP-P—P-—hb—_~—-—-PFF+Fh~— he l7 perturbed) triplet excited states as previously proposed by McGlynn. The remaining five broad bands, which show no vibrational structure, were ascribed to ground state-singlet excited state transitions (*5 +-So). (Henceforth, * represents an excited state species only.) The fluorescence emission spectrum of the uranyl ion in aqueous solution consists of six bands arising from transitions from the first two excited levels to the five vibrational levels of the ground state. The lifetime of the emission process (T -+ S) is of the order of 10-4 to 10-5 seconds. The line spectrum illustrated in Figure 2 was prepared from the peak height intensity data of the authors. Uranyl ion fluorescence lifetimes and yields are affected by the viscosity of the medium (low viscosity -- strong quenching), temperature (primarily as it affects the viscosity), concentration of U0:+ (self- quenching), and a variety of inorganic (particularly halides), and organic quenching agents. (The perchlorate ion is "inactive" with respect to fluorescence quenching.) Practically all quenching ions are, in fact, reductants and are involved in reversible oxidation-reduction reactions, e.g., * .- U02+ + I —+ U0+ + I (9) 2 2 I + U0: -+ 1" + U0§+ (10) * UO§+ —+ U0§+ (11) Quenching (or selthuenching) may be ”chemical" or "physical" in nature, depending on how the energy lost for fluorescence is used. " . . . A distinction can be made between three mechanisms of quenching: quenching caused by association of the light-excited molecules with the quenching 18 molecule preceding the excitation (quenching by complexing), quenching by proximity of the excited and the quenching molecule (quenching by resonance transfer), and quenching by kinetic encounter of the quencher with the excited molecule. (In all three cases quenching can be either chemical or physical.)"61 A quantum yield for fluorescence of uranyl sulfate (10 g/i at 10°; complete absorption) was estimated by Vavilov and Levshin62 to be 0.28. No other fluorescence quantum yield data for U02+ 2 appear to be available. Photochemistry of U(VI): The photoactivity of uranyl compounds has been known for a long time. The recent monograph by Belford and Rabinowitch, mentioned previously, reviews chronologically the work up to about 1961. The majority of the systems studied pertain to uranyl— organic compound photoreactions (principally carboxylic acids), among which the uranyl-oxalic acid system (chemical actinometer) is perhaps 2+ with inorganic 2 compounds have been reported and, apparently, the U0§+-I_ system is the the best-known example. Relatively few reactions of U0 only one which has been studied quantitatively.63 Photolyses_induced by visible light are inherently slow (espe— 2+ cially non-chain reactions) since the molar absorptivities of free U02 average ~5;Mflcm-l above 3500 A. Consequently, secondary thermal reactions may develop and play an important part in the overall pro- cesses. Many uranyl reactions (especially with organic acids) proceed, however, by light absorption by complex ions which may have considerably higher molar absorptivities. The situation is more favorable for ultra- violet-induced reactions since the molar absorptivities of U0:+ increase l9 sharply with decreasing wavelength below N3500 A (see Figure 2). Photoactivated U0:+ can function as a true oxidant, resulting in reduction to U(IV), and/or as a sensitizer (or catalyst) for oxidation by other oxidants, particularly molecular oxygen, in which case U0§+ undergoes no net change: Failure to rigorously exclude oxygen from uranyl photochemical systems can result in the superposition of both modes of reaction. This is true of organic acid-U0:+ photoreactions which characteristically involve a combination of sensitized decomposi- tion (usually decarboxylation), direct photochemical oxidation, and, if oxygen is present, "auto-oxidation." Uranium(V) is thought to occur quite generally as an intermediate in uranyl photochemistry. Heidt and Moon64 have shown indirectly that U(V) occurs transiently in the photo—oxidation of various carbohydrates 2+ 2 I photolysis (2537 A light) was stopped indicated a second-order depen- and aqueous methanol by 00 The kinetics of U(IV) produced after the dance on the intermediate, in accordance with the U(V) disproportion— ation step —d[UO? /dt= kD[U0:]2[H+] . (12) Values of kD obtained for different substrates were the same and agree reasonably well with more recent data.66 The results of the U0§+~meth- anol study provided a confirmation of the steady-state (photostationary- state) hypothesis for a U(V) intermediate. Quantum yields for U(IV) 2+ production were ~0.14, and were not too strongly dependent upon U02 concentrations.65 Strong illumination of solutions (H280 and HClO 4 4 2+ 4+ of U02 and U causes a shift in the U(IV)—U(VI) reduction potential. , but not HCl) 20 This photoelectrochemical effect, known originally as the Becquerel effect, was interpreted by Heal and Thomas37 in terms of the displace- ment of the equilibrium U(IV) + U(VI) ::;2U(V) (13) to produce a higher (%10 times) steady-state concentration of U(V). This interpretation is corroborated by the results of Sobkowski67 who, in addition, revealed that the magnitude of the potential developed and rate at which the equilibrium potential is established is markedly dependent upon the nature of the electrode surface (smooth y§_platin- ized), which varies itself with the nature of the medium used. Electron Transfer Reactions: The oxidation of U(IV) by molecular oxygen in perchloric acid was investigated by Halpern and Smith.68 The kinetics of the suggested overall reaction 4+ 2+ + 2U + 02 + 2H20 :1 2U02 + 4H (14) conform to the rate law -d[U7), was found to be accelerated by oxalate ion and conformed to the following rate law at constant oxalate ion concentration: 1.7 0.5 R = k[U(IV)] [U(VI)] . (37) A three-step mechanism was proposed in which a U(IV)—U(V) exchange step is rate-determining. 2 2 U(IV) + 33v :2; 33u(1v) + U (38) Uranium(IV) and -(VI) were present as the ions [U(C204)4]4- and [U02(C204)2]2-, respectively. An analogous exchange step has not been reported for any other actinide system. Investigations at high HCl concentrations 83’“ have shown that the exchange rate is markedly accelerated in 6-10 M HCl. Deuterium en- richment caused a slight acceleration of the rate. A mechanism was proposed involving an activated complex composed of U(IV), U(VI), Cl-, and undissociated HCl (bridged). Benson and Brubaker29 examined the effects of several organic dicarboxylic acids on the exchange reaction in perchloric acid. The catalytic effect of these acids increased in thd order: malonic < maleic < malic << tartaric acid. The following three-term rate law was deduced for the exchange system in the presence of tartaric acid (H Tar). _ _ 2 5.7 x 10 4[u°+]2[uo§+] 7.3 x 10 5[u°+][uz1*ar] ‘:+ tu*1° [n+12 R- (39) 1.2 x 10'3[U°+][H2rar][uo§+] [n+12 + 27 Rates calculated by means of this expression agree well with those Obtained experimentally. The light-catalyzed exchange reported for this system was, apparently, partly the result of net photochemical reduction of U(VI) by tartaric acid. Exchange studies in the ethylene glycol—water, ethanol-water, and acetone-water solvent system387-89 indicate a marked dependence on the composition of the solvent. Wear92 has reviewed these systems and has attempted to write reasonable rate laws that will reproduce the observed rates of reaction. Photochemical Exchange Studies: No definite study of the photo- induced U(IV)-U(VI) exchange system has been carried out in any medium heretofore. In several of the above—mentioned studies,28’66’80’81’86 the exchange rate was observed to be markedly accelerated by light (primarily from tungsten lamps) and U(V) was presumed to be the active intermediate. However, the experiments were, for the most part, iso- lated ones and the light intensities involved were unknown. Betts' study in sulfuric acid80 provided the following rate law for the conditions of constant illumination and sulfate concentration (1.9 31): R _ k'[U(IV)]O'5[U(VI)O'5[H+]-O°30 (40) Photo-induced rates were ~20 times faster than the corresponding thermal rates. Poor agreement between an observed and calculated rate (assuming a U(V) disproportionation mechanism) lead Betts to conclude that U(V) disproportionation was insufficient, in itself, to account for the observed rate of,exchange. No detailed mechanism was presented. TheI 28 results of irradiations using low intensity monochromatic light (as supplied by a conventional spectrophotometer during normal Operation) pointed to U(VI) as the light sensitive ion. Exchange was catalyzed by 340 mu light (absorbed by U(VI) only) but not by 650 mu light (absorption by U(IV) only). [No definite conclusions can be made con- .cerning the U(IV) photoactivity since the intensity of light absorbed by each ion was not determined.] Masters and Schwartz66 obtained the first quantitative photo- chemical data for the U(IV)-U(VI) exchange system. Irradiation of a solution, containing 0.00132 M_U(IV), 0.00484 M_U(VI) and 0.24 M hydro- gen ion at ionic strength 2.0 with ultraviolet light (principally 2537 A light) caused the rate to be increased a lOOO-fold over the expected thermal rate. No change in reactant concentrations was detected and, an overall quantum yield of about 0.01 was estimated for the induced exchange. In order to explain the accelerative effect, it was assumed that photolysis brought about an increase in the steady-state concentra- tion of U(V) without causing a net reduction of the solute. Zielen, Sullivan and Cohen23 studied the photochemical reduction and autoreduction of Np(VI) in HClO4 at 25°. An "apparent" quantum yield for Np(V) formation of 0.032 t 0.011 was obtained for photolyses with 2537 A light. The-first order rate constant for the autoreduction of Np(VI) to Np(V) was measured as k - 3.1 i 0.2 x 10-9 sec-1. II I . THEORETICAL The rate of isotOpic exchange is mathematically described in terms of the first—order exchange law, the McKay equation93 ln(1 - F) - - “‘82P” . (41) The derivation of this equation and its modified forms, which take into account radiation—94 and separation-induced95 exchange and appreciable isotOpe effects,96 can be found elsewhere in the literature and will not be repeated here. As applied to the general isotopic exchange reaction * * AX + BX :: AX + BX (42) R is the rate at which X is being exchanged between the two species AX and BX, whose total concentrations are given by a and b. F is the fraction of exchange occurring in time, t. (X is an isotopic tracer introduced into the system in AX (or BX) in order to follow the rate of exchange). Exchange rates are evaluated from the slope of a plot of ln(1 - F) 32 t. Equation (41) is equally applicable to photochemical as well as thermal exchange systems, since it does not depend on the form of R, assuming that isotopic effects are negligible, and that there is no net change in the reactant concentrations. By varying the reaction condi- tions systematically, the dependence of R upon concentration, (the empirical rate law), temperature, and other experimental variables may be determined. The rate law for a simple bimolecular reaction is of the form: 29 30 R -= k[AX] [BX] (43) where k is the specific rate constant in Mil sec-l. Plausible thermal mechanisms are then based on and directly correlated with the empirical rate law so obtained. The interpretation of a photochemical exchange reaction mechanism, however, is based on the variation of the overall quantum yield for exchange, ¢ex’ with reactant concentrations and absorbed light intensi- ties. For exchange resulting from absorption of light by AX (see equation (42)), ¢ex is defined as * RI _ No. of ions (or molecules) of BX exchanged/ml-sec (44) No. of quanta absorbed by AX/ml-sec ¢ I ex Iabs where the numerator is the exchange rate, R' (now in terms of ions ex- changed/ml-sec) and is obtained as mentioned above. The denominator is the intensity of light of wavelength A absorbed by AX (labs). Primary quantum yields are of great theoretical importance but are difficult to estimate. True primary quantum yields are independent of reactant concentrations and absorbed intensities. It follows from the second law of photochemistry that the sum of all fl.of the primary quantum yields of the n different primary processes is equal to unity r‘I 2 ¢ - 1.00 . (45) i=1 i This includes the primary photOphysical processes of collisional deactiva- tion, fluorescence, radiationless transitions, and other similar processes. 31 Primary quantum yields of photoinitiated exchange reactions may be determined in the following simple manner.97 For the exchange reaction in equation (42), in which exchange is initiated by light absorption by AX, the rate will be given by -€ad R °1Iabs ¢110(1 - 10 ) (46) (at low [AX]) where e is the molar absorptivity of AX, d the path length of light, a the [AX], and ID the incident light intensity of wavelength xI Expressing R in terms of the exchange half-life, tl/Z’ R a 0&693 as: b (47) 1/2 and combining it with equation (46), the exchange law may be rewritten 0.693. ab . - d . = ¢ 1 (1 - 10 ea ) (48) tl/2 +b 1 0 Hence, at known reactant concentrations, the primary quantum yield may be evaluated by determining the half-time as a function of the incident light intensity. However, overall quantum yields are generally more useful and often provide crucial information about the reaction system. A small ¢ex (<1 indicates that a chain mechanism is operative. A generalized kinetic treatment of photoinitiated exchange involving a chain-type reaction has been presented by Stranks and 32 98 Yandell. For the general exchange process * k A + B —+ A + B (42) +— (X's have been omitted for the sake of simplicity; labelling is still represented by *) occurring in liquid or gaseous phase, the following general mechanism is proposed (1) A + by —+ c + x (initiation) °1 (49) * * (ii) C + A ::_C + A (propagation) kl (50) n * (iii) C + B ::_B + C (propagation) k2 (51) followed by either (iv) C + X -+ A (termination) (52) or (v) C(+S) -+' A(+S') (termination) kt (53) Absorption of light or radiation (x, y, a, B) by species A generates the species C and X which can be either radicals or ions. For a two-electron exchange system in solution [e.g., the Tl(I)-T1(III) system], charge- transfer absorption can generate C in an oxidation state intermediate between the oxidation states of A and B. The rate of process (1) is equal to I where I s is the abs¢l ab absorbed intensity in.quanta liter-J'sec—1 and ¢1 (:1) is the primary quantum yield for the photolysis step itself. For liquid systems, where diffusion of geminate radicals is important, d1 is a measure of the efficiency with which a single absorbed quantum of light can produce the two species C and X which have escaped primary and secondary recombination. 33 The essential requirement of this mechanism, which leads to ¢ex>1’ is that C, as a chain carrier, must be capable of exchanging with 22£5.A and B at a rate much faster than its own rate of destruction, as in the sequence (ii) to (iv) or (v). The two alternative termination reactions (iv) and (v) are exam- ples of quadratic and linear termination reactions, both of which lead to very complex expressions for the rate and overall quantum yield, .¢ , for each of the two cases. For example, the rate expression for ex the quadratic case is of the form R - kla'kzb (ell 1/2 1: kla + kzb + (ktI abs (2 )1/2 abs°1) (54) k2°'°1Iabs + 1/2 kla + kzb + (ktIabs¢l) where a and b represent the total concentrations of the species A and B. The expression for e.‘ is obtained directly from (54) by the relationship 4, . (55) ex labs and is of the form ¢ . kla-kzb ( ¢1 )1/2 ex 1/2 k I kla + kzb + (ktlabeel) t abs kzb-el (56) + k + k b + (k I )1]2 1a 2 t aba¢1 The second term in (56) cannot exceed unity and arises from the non-chain 34 path, yig_reactions (1), (iii), and (iv). Quantum yields greater than unity arise from the first term in (56) and the contribution from this term depends on a balance between propagation and termination rates and the absorbed light intensity. The general conclusion is made that for a linear chain termination exchange reaction, ¢ex is independent of Iabs’ whereas for a quadratic -l/2 chain termination, equation (52). ¢ex iS Proportional to Iabs under appropriate conditions. IV. EXPERIMENTAL A. Preparation and Standardization of Reagents Starting materials were reagent grade chemicals and were used without further purification, except for tartaric acid, sodium per- chlorate monohydrate, and uranyl nitrate hexahydrate. Demineralized water was used in the preparation of all stock solutions. Such purified water, denoted hereafter simply as water, was obtained by passing distilled water through a mixed bed resin, Crystalab DEEMINIZER, Model CL-5. This water contained less than 0.5 parts per million ionic impurities (measured as NaCl). All glassware was scrupulously cleaned prior to use, especially the numerous items used in the preparation of exchange solutions. Ordi- nary cleaning with a detergent was followed by an overnight treatment with aqua regia, and subsequently, by successive rinsings with hot, distilled, and demineralized water. Class A volumetric ware (pipets, burets, and flasks) was employed in all (1) standardization procedures and (2) the preparations of solu- tions for actinometric and exchange experiments. When Class A volumetric apparatus was unavailable, but required, volumetric apparatus was cali- brated in accordance with the procedures outlined by the National Bureau of Standards.99 Sodium Hydroxide: Carbonate-free sodium hydroxide (0.3g) was prepared and standardized according to the procedure of Kolthoff and Sandell.loo'The sodium hydroxide solution was prepared in a polystyrene vessel by diluting a saturated solution with de-aerated water. The 35 36 vessel was fitted with a delivery assembly which enabled the solution to be dispensed without introducing atmospheric carbon dioxide into the vessel. Under these conditions, the titer of the standard solution changed only 0.1% during a two-year period. Sodium hydroxide solutions were standardized against primary standard potassium acid phthalate, which had been dried for two hours at 100° and using phenophthalein as the indicator. Perchloric Acid: Perchloric acid stock solutions were prepared by diluting Baker Analyzed (70-72%) perchloric acid to the required volume. Numerous perchloric acid stock solutions were prepared during the course of this investigation, but one particular stock solution (3.944 M) was used in preparing the majority of the exchange solutions. Perchloric acid stock solutions were standardized with sodium hydroxide and phenolphthalein was used as the indicator. Sodium Perchlorate: Sodium perchlorate was used to adjust the ionic strength to 2.00 M_in all exchange experiments. One major stock. solution was prepared from triply-recrystallized sodium perchlorate, and used specifically for this purpose. The recrystallization procedure employed was developed, principally, by Love.101 First, the starting material, reagent sodium perchlorate monohydrate from G. Frederick Smith Chemical Co., was dissolved in hot water to prepare a saturated solution. This solution was filtered through a fine fritted disc to remove dirt particles, placed on a hot plate and boiled until surface crystallization was observed (at ~142°). The crystals were dissolved in a minimum amount of water and then the solution was placed in an oven, thermostatted at 60°, where cooling and crystallization were allowed to take place 37 undisturbed during one day. (Crystallization from aqueous solutions of temperatures above 50° yields non-deliquescent prismatic crystals of anhydrous sodium perchlorate, whereas below 50° a deliquescent monohy- 102) The resulting crystals of anhydrous sodium perchlorate drate is formed. were collected in a coarse-fritted funnel, with care taken to keep out airborne dust, but were not washed because of their high solubility in water even at 0°. Three additional crops of crystals were collected by treating the resulting filtrate in the same way as the starting solution and repeating the steps just described. The once-recrystallized sodium perchlorate was recycled twice through the entire recrystallization pro- cedure, and a total of four crOps per cycle was collected in obtaining the final product. Sufficient purified product was obtained to prepare two liters of a 7.508 M_sodium perchlorate solution. To standardize the sodium perchlorate stock solution, one milli- liter aliquots were delivered into weighed porcelain crucibles and then evaporated to dryness in,a 160° oven. The crucibles containing the anhydrous sodium perchlorate were reweighed and the stock concentration was computed. Cerium(IV) in SulfuricyAcid: Standard solutions of cerium(IV) in sulfuric acid were prepared from the salt, (NH 103 4)4Ce(SO4)4:2H20, according to the procedure of Wilson and Wilson. This salt was obtained from the G. Frederick Smith Chemical Company. Cerium(IV) solutions were standardized against (1) primary stan- dard arsenic(III) oxide, with osmium(VIII) oxide as a catalyst and 104 ferroin as the indicator, or (2) sodium oxalate (NBS certified) at 70° by a potentiometric procedure by use of a saturated calomel and 38 platinum electrode set. Cerium(IV) solutions were used primarily for the determination of uranium in the respective stock solutions; the potentiometrically stan- dardized cerium(IV) was used in the actinometric studies which will be discussed later. Nitrggen Purification: Oxygen-free nitrogen was used as an inert cover gas in (1) all kinetic experiments and (2) during the preparation, transfer, and storage of all uranium(IV) stock solutions in order to prevent air oxidation of uranium(IV).lO6 The inert cover gas was prepared from Matheson pre-purified nitrogen (oxygen assay, 8 ppm) by passing it through a purification train, which consisted of the following components, in series, and which is schematically illustrated inFigure 3: (A) a tube furnace containing fine copper turnings at 450°, (B) a heated glass column packed with activated copper adsorbed on Fuller's earthlm (center at 175°), (C) duplicate gas scrubbing towers containing chromium(II) sulfate solutiora'os over Zn amalgam, (D) a gas scrubbing tower of demineralized water and, finally, (E) a gas scrubbing tower of 2 M sodium perchlorate (I - 2.00). Component (A) is the "rough," high-capacity oxygen getter, whereas components (B) and (C) are very efficient and high-capacity oxygen- removal units. Connections between components were made with thick-walled poly- vinyl chloride tubing, which is nearly impervious to oxygen.109 Detailed information on the purification scheme, including the 39 HEATED GLASS COLUMN (8) TUBE FURNACE (A) COLUMN ( PYREX; 2.75 in. DIAMI /INSULATION JACKET (PYREX) / FIBERGLASS STRIP (4, V4 in. WIDE I iIiIiIiIiI ”THERMOMETER (175°,CENTERI iaNlGHROME wuss (N0. 20,65 it use) ,cu/FULLER’S EARTH I I 1 II! I“: 'III' I assess \ ‘x \ \L m 'I Cu TURNINGS (450‘) 'III Iils IIHII III /GLASS WOOL /$CREEN SUPPORT (4 INOENTATIONSI gLEADS T0 Powessnm' BED RETAINING SCREEN (NICHROMEI U (W) STORAGE EXCHANGAENBOLUTION PHOTOLYSIS UNIT é mono, H20 orso,/2n (Hg) TRAP GAS sceussess: (E) (D) (C) Figure 3. Nitrogen purification train. 40 procedures for column construction and operation, preparation and in situ regeneration of getter materials, can be found elsewhere.27 Henceforth, any reference to nitrogen means oxygen-free nitrogen. Occasionally, "tank nitrogen" will be used to refer to pre-purified nitrogen. 238Uranium(VI) Perchlorate: Three types of uranium stock solutions were prepared and used during the course of the work: (1) 238uranium(VI) perchlorate, (2) 233U-enriched 238uranium(VI) perchlorate, and (3) 238uranium(IV) perchlorate stock solutions. Except where noted, any reference to exchange and uranium(VI) stock solutions automatically implies enrichment with 233U. 238Uranium(VI) perchlorate will be specified as such. No uranium(IV) stock solutions were enriched, there- fore, no isotopic designation is applicable. Specially purified 238uranium(VI) perchlorate was the starting solution used in preparing stock solutions of categories (2) and (3) above. 238Uranium(VI) perchlorate stock solutions were prepared by a following procedure similar to that used by Love,111 Benson,112 and Quinn .26 A hot solution (BS-95°) of recrystallized uranyl nitrate hexa- hydrate (99.9% assay, J.T. Baker Chemical Co.) was treated with a two-fold excess of 6% (w/w) hydrogen peroxide, to precipitate the gelatinous lemon-yellow uranium(VI) peroxide, U04-2H20. (U04-2H20 is °4H20 below 50°. A mixture of the two 4 results between 50 and 70°.)113’114 precipitated above 70°, U0 This precipitate was digested over- night at 80°, filtered, washed with water, and then dissolved in a minimum of hot 1 M perchloric acid. Dissolution was appreciably slow 41 and usually a full day was required to complete it. The resulting solu- tion was cooled, and then treated with dilute ammonium hydroxide to adjust the pH to N2. The UO4-2H20 was then reprecipitated by adding more H202, and subsequently recycled through the dissolution-precipita- tion procedure. A total of four complete cycles was carried out in order to obtain a high-purity product. The final.U04-2H20 precipitate (of considerably improved crystallinity) was filtered, washed thoroughly, and was initially dried for a week in an oven at 105-110°. During the oven drying, part of the U04°2H 0 had decomposed to give the orange 2 peroxide, U207.‘1]"S Consequently, the entire batch of U04-2H20 was con- verted to U207 by heating it in an oven at 165° for a week. Although U207 is slightly hygroscopic, no significant uptake of water was observed during the analytical weighings that followed. Uranium(VI) perchlorate stock solutions of the desired composition were prepared by dissolving weighed quantities of U207 in the stoichio— metric amount of perchloric acid in accordance with the following equation: + 411+ —+ 2U02+ 7 2 0 . (57) U 0 2 + 1/2 02 + 2H 2 Before making the final dilutions, the solutions were boiled vigorously for 2-3 hours to insure the complete destruction of any trace amounts of hydrogen peroxide. No detectable amounts of H202 were found when solu- tions were titrated with 0.1 N_cerium(IV) sulfate. Three liters of 0.2 M_uranium(VI) perchlorate was prepared in all. Two liters were used specifically for uranium(IV) preparations and had a slightly higher perchloric acid concentration. 42 233U-enriched 238Uranium(VI) Perchlorate: 233U-enriched uranium(VI) perchloric acid stock solutions were prepared by adding 10 or 25 ml of the tracer stock solution to the desired volume of the 238uranium(VI) stock solution described above. The tracer stock solution, which had 116 been prepared by Benson, contained 0.51 g of 233U03(97.3% isotopic purity) dissolved in 100 ml of solution 3.00 M_in HClO4 [1.74 x 10-331 233U02(C104)2]. The 233U had been separated from its daughter products, primarily 229Th, by anion-exchange chromatography in an 11 M_HC1 medium. Table 1 presents a comparison of the mode of decay, half-life, and specific activity for 233D and 238U. Table 1. Selected decay prOpertiesLl7for 233U and 238U. Specific Decay activity Nuclide mode Tl/Z’ Y (d/m/ug) Daughter, T1/2 233v o 1.62 x 105 2.103 x 104 229Th. 7.34 x 1037 238U a 4,51 x 109 0.739 234Th, 24.1 d Two principal stock solutions were prepared and used in making up exchange solutions. For preliminary and uranium(IV) variation experiments, a uranium(VI) stock was used having 0.01% 233U enrichment. This enrich— ment level was found to be unsatisfactory and therefore, a second stock, used for the H+, U(VI), and temperature variation experiments, was pre- pared with a 0.025% 233D enrichment. (Masters and Schwartz,66 and Bensonz8 used an isotopic enrichment of 2% 233U). Uranium(IV) Perchlorate: Uranium(IV) perchlorate stock solutions were prepared by the electrolytic reduction of uranium(VI) perchlorate 43 in perchloric acid solution. The procedure used was similar to that 26,27,28,77,78 described by several previous workers. The account given by Love27 is particularly comprehensive; hence, only a brief description of the procedure used will be presented here. Electrolyses were conducted in the dual-compartmented cell, illus- trated in Figure 4, part I, and a four-inch diameter mercury-pool cathode and a 1 cm2 platinum foil anode were used. The pool was reagent grade 119 mercury and had been cleaned just prior to use. The anode, fabricated from 22 gauge platinum wire and 4 mil foil, was placed in the anode compartment which contained perchloric acid of the same concentration as that in the final uranium(IV) solution. Typically, 800 ml of 0.1 M_uranium(VI) perchlorate, prepared from the 238uranium(VI) stock solution mentioned above, was electrolyzed at 0° for ~21 hrs at 0.25-0.3 A and 5-7 V. An Electro, Model D-612T, was used as the (filtered) dc power supply. Higher currents were avoided since (1) the perchlorate ion is reduced to chloride ion at or above 0.7 A3118 and (2) there is a greater tendency to produce the brownish- 27,28 black hydrous oxide, U0 -xH() frequently observed during the 2 2 ’ electrolysis (presumably resulting from localized depletion of acid). Solutions being electrolyzed were agitated by N sparging but were not 2 stirred mechanically. The electrolysis cell was maintained at 0° in an ice-salt bath to insure quantitative reduction of U(VI) to U(IV).77’78 Before beginning an electrolysis, the uranium(VI) solution was purged rapidly with N overnight in order to provide an oxygen-free 2 atmosphere for the electrolysis. (The entire apparatus, including tubing and transfer routes, had been thoroughly flushed out prior to the addition of the uranium(VI) solution). 44 .oumuoanouoe A>HvesfiGMH= mo ommuoum was cofiumumaonm .q musmfim woU (cathode) (60) Half-reaction (3) occurs only when the U(VI) has been reduced quantita- tively and is indicated by the appearance of the characteristic red color of the U3+ ion which is readily observed by viewing the transmitted light from a tungsten lamp placed directly behind the electrolysis cell. Electrolyses were terminated at the first appearance of the red color, since the U3+ ion is known to reduce slowly the perchlorate ion..'3'5’54 After electrolysis, oxygen (scrubbed and filtered) is bubbled through the solution briefly to re-oxidize U(III) to U(IV). The uranium(IV) solution is then transferred and filtered en route to the storage vessel (Figure 4, part II) where it was purged again and then stored under a positive-pressure inert atmosphere. When special pre- cautions are taken to eliminate the re—entry of air, uranium(IV) solutions can be stored for several months without detectable change. An average change of 5%/year was observed for the solutions considered herein (with most of the change occurring during the last six months). A precision 10 ml buret, part III of Figure 4, was installed as an integral part of the storage unit. Thus, solutions for exchange experiments and analysis could be prepared by dispensing a known volume of the stock solution directly into the make-up vessel without the need for an additional volume measurement. In this way, oxygen contamination of the prepared solution was greatly minimized and virtually eliminated in the case of the storage vessel. 46 Analysis of Uranium Stock Solutions: Uranium(IV), uranium(VI), and free perchloric acid concentrations in the various stock solutions were determined by titrimetric methods of analysis using 5-10 milli- equivalents of uranium and acid (free) in each triplicate analysis. Uranium(IV) solutions, made 2 M'in H S04, were titrated with 2 0.1 N_cerium(IV) sulfate at room temperature by using Fe(III) as a 2 catalyst and ferroin as the indicator.1 0 Reagent blanks were always less than 0.010 ml. Uranium(VI) solutions, also made 2 M_in H 804, were first passed 2 through a Jones reductor in which the uranium(VI) was reduced to a mix- ture of U(III) and U(IV).106 The reductor column effluent was sparged briefly with air to reoxidize the U(III) to U(IV), which was then titrated with cerium(IV) as above. Free acid concentrations, [Hg], in the uranium stock solutions ranged from 0.1-I'M.as HClO4 and were determined by the method of Ahrland.118 Appropriate volumes of a stock solution, U(IV) or U(VI), were passed through a cation-exchange column containing Dowex-SO X8 resin (strong cation exchanger) in the hydrogen ion form to liberate hydrogen 2+ 4+ ions in an amount equivalent to the U02 and U . After rinsing the column with water, the effluent was titrated with 0.1 N_sodium hydroxide and the number of free acid equivalents, nH+ determined from the 0’ following relationship: + + nHO - nHtotal - 4[U(IV)] - 2[U(VI)] . (61) On the basis of the hydrogen ion and uranium(IV) analyses, the loss of U(IV) in the stock solutions with time was accounted for quanti- tatively in terms of the following reaction: 47 04+ + 1/2 02 + H20 -—> 00:+ + 2H+ . " (62) This oxidative loss route was suspected, of course, but had not been confirmed previously. Results of cerium(IV) titrations of freshly prepared 238uranium(VI) stock solutions indicated that no significant oxidizable substances (H202) were present in the solutions to within the sensitivity of the method (0.2 microequivalents). Visible and ultraviolet absorption spectra (see Figure 2) of freshly prepared stock solutions were recorded over the wavelength range 2200-7200 A by means of a Cary Model 14 spectrOphotometer. Except as a54,60 noted below, agreement with published Spectr was excellent. Molar absorptivities computed for each of the major peaks (7) of the U(IV) spectrum were the same (within 1%) as those reported for DC10454 with the exception of the 2454 A peak. This discrepancy indicates that either (1) U(VI) was present in low concentration (undetected by titra- tion) and/or (2) U(IV) spectra in DClO y§_HC10 differ slightly in this 4 4 region. (Molar absorptivities for the wavelengths of the mercury-vapor emission spectrum are listed in Table Bl.) B. Exchange Experiments This section describes the procedures used in (1) preparing exchange solutions, (2) conducting exchange experiments and (3) process- ing the resultant solutions. Two types of exchange experiments were performed. Thermal experi— ments were performed in order to (l) duplicate and extend a portion of 28 the work of Benson, (2) determine any radiolytic dependence of the rate 48 (variation of 233U tracer level), and (3) determine the exchange rate for the condition of lowest acidity employed in the photochemical experiments. Photochemical experiments constituted the majority of the experiments performed and were concerned with the dependence of the exchange rate upon reactant concentrations [H+, U(IV), and U(VI)] and temperature. All exchange experiments were conducted at an ionic strength of 2.00 M (NaClO4), a temperature of 25.0° (except for the temperature variation) and under a protective atmosphere of nitrogen. Uranium(IV) is oxidized slowly by oxygen under thermal condi- tions; 68 however, the rate of oxidation is greatly accelerated under ultraviolet excitation, and, particularly, in the presence of high uranium(VI) concentrations. Consequently, special procedures were employed to exclude air completely from the exchange solutions and vessels. Preparation of Exchange Solutions: Exchange solutions for all thermal and most photochemical experiments were prepared as follows. The required volumes of labeled U(VI), NaClO4, (and tartaric acid, where appropriate) and HClqhstock solutions were dispensed into 50 or 100 ml volumetric flasks from bursts or pipets. A flush adaptor connected to a detachable extension by tubing, as shown in Figure 5, was inserted into each flask which was then mounted either singly or in series in a thermostated bath. Nitrogen was bubbled through the solutions vigor- ously for approximately twelve hours to completely displace the oxygen present. After de-aeration, a given flask was removed from the bath momentarily, the flush insert was removed, and the required amount of U(IV) stock solution was added from the in-line inert atmosphere burst. 49 .mcoausaom omamnoxo no>o ouocmmoEum cowouufie m mo monocouefime pom coaumnomlov men How msumumaa< .m ouawfim 32.9. m on :1 NZ- zo_mzw._.xm w4m---—-—— UNDERWATER STIRRING UNIT STIRRING BAR TEFLON SLEEVE (BRASS SHAFT) J FLEXIBLE CABLE‘STIRRING MOTOR Figure 6. Photolysis apparatus. 53 .muuasonflo voumfioommm was mama .m madman 8 so 8> 3 “545 2322.2 012an? “on”. on so as on Minnow n24 _ \J . a G 9 .Som m «To $5224 on 03mg mosnnaomm I .. _ wa00m $24... agar—bum monm w><>>ln_.5...._ 54 This calibration was checked independently by means of a procedure employing the 0.15 M potassium ferrioxalate actinometer (see section entitled "Chemical Actinometry") in combination with two filters (Pyrex #7740 and Corning #3391) which allowed intensity data to be obtained for the following three spectral regions: A<2730 A, 2730 A 4085 A. The results of the calibration and actinometric check are listed in Table 2. More details are presented in Appendix A. Table 2. Emission intensities of a mercury-vapor lamp, Model LO 735A-7 (Hanovia). NBS-lampyComparison Chemical Actinometryb Wavelengtha Relative Intensity Distribution Absorbed Intensities A Energy Quanta (%) (quanta/sec) 2537 100.0 100.0 (88.1) 4.10 x 1015 (86%) 3126-32 1.69 2.09 (1.84) 3650 1.71 2.11 (1.86) ______4.4.51 x 1014 (142) 4047-78 1.06 1.69 (1.49) 4358 1.68 2.89 (2.54) 2.2 x 1014 (52) 5461 , 2.23 4.79 (4.22) 8The 5770-90 A region is omitted since neither the exchange nor actinometer solutions absorb appreciably in this region. bThe 0.15 M actinometer (15 mm depth of solution) is N110 times more sensitive for 4358 A radiation than for 5461 A radiation which is N6 times more sensitive than for 5770- 90 A radiation; hence, the last value given (2. 2 x 1014) is essentially for 4358 A light. The calibrated lamp was not used for any of the photolyses. Details of the calibration can be found in Appendix A. Constant light output was achieved by (l) regulating the power input to 1.00 A and (2) controlling the temperature Of the lamp envelope. Under these conditions, the voltage (and consequently wattage), and N2 55 airflow could be varied appreciably without changing the spectral dis- tribution or output. These observations are consistent with those of Heidt and Boy1e3124who studied the effect of several experimental para- meters on the output of the 2537 A radiation. Constant power input was facilitated by regulating (i0.2%) the 115 Vac line supply by means of a Sorenson (Model 1000 S) ac voltage regulator. Temperature control was accomplished primarily by mounting the lamp in a water-jacketed housing maintained at 25.0°. In the housing, which was fabricated to close tolerances from an inner 34/45 8 joint, the lamp was supported by a circular "shoulder" on which the edge of the lamp base rested. There was virtually no "play" in the lamp once it was seated. Six interchangeable photolysis vessels were fabricated from inner 34/45 I joints. When fitted onto the housing, the distance, L, between the outer tip of the lamp envelope and the bottom, inside surface of the photolysis vessel was reproducibly 6.10 i 0.05 in. The lamp-to- solution distance is estimated at about 5.4 in when there is 5 m1 of solution in the vessel. Both housing and vessels were made of Pyrex glass and were painted black on the outside; the housing also had an undercoat of silver paint. Solutions undergoing photolysis were stirred continuously by a stirring bar ("peanut") which was magnetically activated by an under- water stirring unit. The underwater unit, as shown in Figure 6, was powered externally by a flexible cable attached to a standard stirring motor (also see Figure 8). A nitrogen atmosphere was maintained over the solutions.at all _times during photolysis. After passing through a scrubbing tower of S6 2 §_sodium perchlorate (same ionic strength as the exchange solutions) at the temperature of the bath, the nitrogen was metered in through a flowmeter at a fixed, reproducible rate. It exited through the extremely narrow, annular clearances at the top of the housing and served to fur— ther cool the lamp envelope. A schematic drawing of the complete photolysis apparatus as arranged in the bath is shown in Figure 8. The temperature of the photolyte remained constant during photol- ysis. This point was checked out specifically during an extended (60 minutes) photolysis of an exchange solution [#P(7)]. The post-photolysis temperature measured was identical to the bath temperature to within 0.05°, the limit of sensitivity of the thermometer used. The following operational procedure was used in conducting the exchange solution photolyses; (l) A photolysis vessel, containing only a stirring bar, was -ginserted in place. The whole assembly was lowered to a pre- set position (by a mechanical stap) at which the water level of the bath wasjust§below the side—arm I joint. (2) The lamp was turned on and allowed to warm up for at least a half-hour before a series of photolyses were begun. The stirring unit and circulating pump were turned on immediately after the lamp. -(3) The entire apparatus was flushed for 5-10 minutes with a rapid flow of tank nitrogen yi§_a by-paes. Nitrogen was then re-rOuted into the flush route for the photolysis. (4) After stable operating conditions were attained (primarily the lamp current), 5 ml of the photolyte was pipetted (jetted) into the vessel through the side-arm. A timer was PHOTOLYSIS UNIT MECHANICAL STOP 75 gal. BATH (CIRCULATING PUMP AND COOLING COILS ARE NOT SHOWN.) ,anmgpzaaw «aaaauafigy ' 57 II no VOLTAGE REGULATOR THERMOREGULATOR 4 RECTIFIER AND FULL-WAVE BRIDGE I VARIABLE RESISTOR .w/r/m - 3mg *\ r ST RA E - NoCIO4(2MI $2331 q\ \scsusssa g' I "" Natoz-FREE) I _/ TANK "z Figure 8. Photolysis apparatus and associated equipment. 58 tripped and the time recorded when half the sample had been delivered. The lamp current was checked frequently, but ad- justments were seldom necessary. The remainder of the unphotolyzed exchange sample (10 ml initially) was used to measure the solution absorbance at 6500 A by using a Beckman DU spectrophotometer. (5) Upon completion of the photolysis, which usually lasted from one to twelve hours, the sample was removed from the light path (quenching the exchange) and the lamp remained on for the next photolysis. The solution absorbance was remeasured at 6500 A and compared with the initial measurement. In general, only slight losses (<3%) of uranium(IV) were ex- perienced. Photolyses were repeated (or the respective samples discarded) in which the change in uranium(IV) concen- tration exceeded 5%. The photolyzed solution was then processed as described below. Essentially the same procedure was followed in photolyzing acti- nometer solutions. These along with other pertinent items will be discussed in the section entitled "Chemical Actinometry." Processing of the Exchange Solutions: The progress of an exchange was followed by (l) separating uranium(IV) from uranium(VI), (2) prepar- ing counting samples, and, subsequently, (3) determining, by o-counting, the rate of grow-in of 233U in the originally unlabeled U(IV) species. Several procedures discussed in the following three subsections (Separa- tion, Preparation of Counting Samples, and Counting Techniques) were adapted from the work of Benson.28 A flow diagram of the overall pro- cessing procedure is given in Figure 9. 59 EXCHANGE SOLUTION (a) THERMAL AND PHOTOCHEMICAL (b) 5ml ALIQUOTS I I “ZERO-TIME” SEPARATION "INFINITE-TIME" SAME AS SOLVENT EXTRACTION: (o) DILUTION WITH omemAL sowno" = 5ml o.Iu TTA/BENZENE # HCIO 2min (CONTACT TIME) (b) ELECTROREDUCTION ORGANIC I AQUEOUS = URANIUM RECOVERY I WASH (2-3x) RE-EXTRACTION 5m| 0.5 M HCIO A Sml 3.0 u HCI 0.5 min (CONTACT TIME) 2 min (CONTACT TIME) AOUEOUS [ ORGANIC AQUEOUS I ORGANIC WASTE ‘_, 1.5 ml -—-- WASTE 3.5 ml I I PREPARATION OF COUNTING SAMPLES (a) TRIPLICATE 0.5 ml ALIOUOTS (b) EVAPORATION UNDER HEAT LAMP (o) BECKMAN DU SPECTROPHOTOMETER (c) AOO H20 RE-EVAPORATE UNDER c-58.0 u" cm" AT 6500 A NH3 ATMOSPHERE FOR um!) IN 3 u HCI (6) HEAT TO 550'C IN FURNACE U(IV) ANALYSIS ‘—> URANIUM I —’ RECOVERY a-COUNTING (a) WINDOWLESS FLOW COUNTERS (b) (0.000 COUNTS/SAMPLE Figure 9. Flow diagram of exchange solution processing. 60 Separation: The separation procedure used was essentially the same as that of Masters and Schwartz.66 The exchange solution (5 ml) was delivered into a 30 ml separatory funnel containing an equal volume of 0.1 g 2-thenoy1trifluoroacetone (TTA) solution in benzene. The separatory funnel was shaken vigorously for two minutes and the lower, aqueous phase drained Off and discarded. The uranium(IV) was extracted 125 into the benzene phase as a 1:4 neutral molecule, U(TTA)4: 4+ + _ _., 63 U (aq) + 1. TTA H(org) ‘_ U(TTA)4(org) + 4H (aq) ( ) Five milliliters of a 0.5 fl_perchloric acid wash solution was added and the funnel shaken vigorously for one-half minute. The aqueous layer was drained off and discarded. Normally, two such washes were adequate; however, for solutions at high uranium(VI) concentration, a third wash was found necessary, since traces of uranium(VI) in the wash could be detected by spectrophotometry. Finally, uranium(IV) was re-extracted into the aqueous phase, as a chlorocomplex, by vigorously contacting the benzene layer with 5 ml Of 3.0 fl hydrochloric acid for two minutes. This final aqueous extract normally contained 65-752 of the uranium(IV) and virtually none Of the uranium(VI) originally present in the aliquot. [U(VI) is not extracted at a pH <3.12fi Three 0.5 ml aliquots were taken and used to prepare triplicate counting samples (see below) and the remainder of the solution was used for the spectrophotometric deter- mination Of the uranium(IV) concentration. Exchange solutions at acidities <1 §_were quenched immediately before separation by adding acid to adjust the perchloric acid concen- tration to 1‘g, In order to minimize the differences in the total amount Of uranium extracted, less volume Of exchange solution was used in the 61 separations for certain solutions of the uranium(IV) variation. The actual volume of solution used was determined usually from the results of a preliminary extraction. Alternatively, estimation could have been made from available solvent extraction datanlzs’127 No exchange was induced by the separation procedure(s) used. For thermal experiments, the first sample withdrawn was designated as the "zero time" sample. The separation time was taken as the time the sample was delivered into the separatory funnel. For photochemical experiments, the unphotolyzed solution was used as the "zero time" sample and was processed coincident with the first photolysis of the "run" sequence. Occasionally, a second "zero time" sample was processed toward the end of the experiment. For photochemical experiments, separation times corresponded to the time a photolysis was terminated. "Infinite" or "complete exchange" rates could not be calculated accurately, since the separation was not quantitative or precisely reproducible. Therefore, special samples were prepared for each exchange experiment, taking advantage of the fact that the specific activity Of uranium is the same in both oxidation states when the exchange is complete. "Complete exchange" samples were Obtained by electrolyzing an aliquot of an exchange solution to convert all the uranium present to the U(IV) oxidation state. Electrolyses were carried out in a 150 ml beaker by means of a mercury-pool cathode and platinum anode and at a potential of 6 Vdc and a current of 0.2-0.3 A. The Operating conditions were very similar to those described for the preparation of uranium(IV) stock solutions, except no inert atmosphere was provided. The "synthesized" solutions were processed as described above. 62 The uranium(IV) in 3 E hydrochloric acid solution from above was placed in a one centimeter quartz cell and the absorbance, AS, measured at a wavelength of 6500 A, by means Of a Beckman DU spectrophotometer. ' 128 Solutions of uranium(IV) in 3 fl_hydrochloric acid Obey Beer's law and exhibit a molar absorptivity, E, of 58 £71 cm-l. (Using a solution Of 0.1231 §_U(IV) in 3.0 N HCl, the molar absorptivity was found to be 58.0.) The uranium(IV) concentration, C, in moles/liter can be deter- mined from the following relationship: 0 = AS/El (64) where 2 is the cell path length in centimeters, and As’ E, and C are as indicated above. The uranium(IV) concentrations, or at least absorbances, were needed for the determination of specific activities. Preparation of CoUntingSamples: The triplicate 0.5 m1 aliquots taken for counting sample preparation were delivered onto 30 mm watch- glasses, in earlier experiments, and then onto quartz discs one inch in diameter in later experiments. The edges of the sample mounts were ringed with a ceramic wax pencil to prevent sample losses through creepage during drying. The samples were evaporated to dryness rapidly under a heat lamp and then transferred to a muffle furnace. When the temperature reached 500°, the furnace was turned off; the samples were allowed to cool and then removed from the oven. When possible, all samples for a given experiment were heated at the same time. For the most part, adherent, uniform coatings (thickness and distribution) of U0 were Obtained by using this procedure. Considerably better coatings 3 were Obtained by incorporating into the procedure, a technique in use at 63 Argonne National Laboratory.129 As before, samples were evaporated to dryness under a heat lamp. After a short cooling period, $0.5 ml of water was added to redissolve the residue and then the sample was re- evaporated under a heat lamp, but under an atmosphere of ammonia. These samples were heated in the furnace as above. Approximately 0.4 mg of uranium was deposited on each sample mounting (density thickness of m.08 mg/cmz). At this level, self- m absorption of o-particles is significant. However, differences in self- absorption from sample to sample were virtually negligible, since the amount of uranium(IV) extracted in each separation of a given experiment I was relatively constant. Counting Techniques: Samples were counted for o-particles by use Of the proportional region in one of two different windowless, flow, counting systems. A manual system incorporating a detector (Radiation Instrument Development Laboratory, Model 2-7) with an external pre- amplifier, and a glow-tube scaler (Baird Atomics, Inc., Model 309) was used initially. Samples from five experiments, P(24)-P(29), were counted by use of a Nuclear Chicago automatic counting system, complete with its associated detector, pre-amplifier (input sensitivity at 10), scaler (Model 8160), and sample changer (Model 1042). The input line voltage was regulatedVDH .OOfiumfium> A>Hvasflsmus Mow coaumuucmosoo A>Hva=flcmuo mw.oocmnuomn< J93: 2.253 m m c N .HH magmas TASTE em Noam Iw :.~. I ”:23 x 35 \ v. \. \. \. SQ + QB; H cam I...< \\o \\ \ N. vv 33stsossv“v 72 .< ammm I A ..o.mN I a m: oo.~ I H moo.a I ~+mi m 0 mIOH x caw.m I HA>HVDL .sofimfluma, 3365.38“: now coaumuuaooooo Caeswamuo ImIN mononuomna. .NH ounwfim “mo.xaeifiic2:d 00. ca om ow om om oe om ON 0. o _IEo.-=:.mH_.mmvuw K o. 08.0 + ER 3 ..mmv .I. m4 \ ON m2... ommdaomlhmdms 30stsossv‘5v 0v As, ABSORBANCE 73 35 30 25 20 15 IO / A,= I 5.44 z 0.03) XIO3[K3Fe(C204I3]+O.31' e =(5.44 10.03m03 M'Icm" Figure 13. 12 Absorbance X§_potassium ferrioxalate (K3[Fe(C204)3]) 24 36 43 [K3 Fe(CZO4I3] I M I I0“) concentration. _ 7. = 0. [H2804] — 0.1 g, T 25.0 , A 2537 GO A. 72 74 aid of precision 9 mm solution cell spacers. Each actinometer solution used in the absorbance measurements was subsequently photolyzed for 5 minutes and the absorbed quanta/ml-sec computed. A graph of the quanta absorbed (of 2537 A radiation)/ml-sec ‘35 the A8 for the actinometer solutions, shown in Figure 14, was con- structed from which quanta/ml-sec for the corresponding exchange solution absorbances could be obtained graphically. These intensity data were used in computing the apparent quantum yields for exchange. 75 .2 OH x 00.0 Ou m- «Ion x 00.0 I chHusaom wumamxofluumm Eoflmmmuoo mo mocmnuomnm MM ommldi AGOfiumemu < nmmmv wmnuommm mucosa 1: musmwm O 23.33 _Hnlvouo..uu_ no. .0 853334 00 ow Om ON 0. HmAqomovmalmM .< mmmm um _ _ _ _ 6.322.: +1 use 5232:. 2: to: «$3... .3 nous—5.030 3.3008 .0 00:3... "+1.:>3.A>:3 0 +1: .231 I 0.0? 2).: '15— l_II_ 0.9 (“-0”) nee—Iw/pquOGQO OIuono V. RESULTS A. Light Intensities and Overall Quantum Yields Incident light intensities for each Of the lamps used were com- puted from the actinometric and lamp calibration data in accordance with the following relationship: = flag/.129 (69) I O ’ 3 Z¢1D1f1 ' where I0 is the total incident light intensity in quanta/sec (for A :_5461 A, nFe2+/sec is the number of Fe2+ ions produced/sec during a five-minute photolysis and is computed from a measured absorbance, As’ using equation.(68), 01 and fi are the quantum yield for Fe2+ production by light of l1 and the fraction of that light absorbed by a 1.-cm path length of the 6.0 x 10"3 M_actinometer solution, 5 and D1 is the fraction Of the lamp output as A1 (NBS lamp calibration data, Table 2). The denominator in equation (69) is equal to 1.18 and is essentially an average quantum yield, weighted in accordance with the lamp spectral distribution and modified to correct for any incomplete absorption of xi. The results for three lamps are listed in Table 3. A value of 1.23 was used in computing the incident intensity for lamp III. This value is based on the fractions of light absorbed and quantum yields for the 0.15 M ferrioxalate actinometer, but is higher principally because 5461 A radiation was not considered in the computation. Since lamp deterioration does not materially affect the spectrum of a properly cleaned quartz mercury arc}36 it was assumed that all lamps 76 77 Table 3. Total incident intensities for lamps I, II, and III. Incident AS nFe2+/sec Intensities, Io Lamps (daily av) (X 10'15) (quanta/sec x 10-15) I or II (new) 0.870 r 0.039 4.92 i 0.22 4.17 I (aged) 0.522 t 0.013 2.95 i 0.07 2.50 If?” Ila 0.545 1 0.050 3.08 i 0.28 2.61 m 0.895 5.06 4.11 ’1 involved, new or aged, exhibited the same relative spectral distribution and, consequently, that the incident intensities were directly propor- E? tional to the gross actinometric yields (nFe2+/sec). Moreover, lamp I (new) was assumed to have the same output as lamps II or III, which were observed to be identical within experimental error. Lamp 11a is the same as lamp II, except that the output had been reduced temporarily by a mineral coating which had encrusted the lamp tip. It is believed that P(25) is the only experiment affected by the lower intensity. Overall quantum yields for exchange, ¢ex’ were determined by two different procedures. The first procedure consisted of computing ¢ex directly from the experimentally Observed exchange rates, R, and cor— responding absorbed intensity data in accordance with Claesson's e133 137 principl and the following general expression for quantum yields: _ No. of ions exchanged ex NO. Of quanta absorbed ¢ (per unit time and volume) (44) MOre specifically, I- I/ , 78 (R x 10-8.M/sec)(0.0052)(6.023 x 1023 ions/g-ion) q>ex - (L)(quanta absorbed/ml-sec x 1014)(5 ml) ' (70) Values of the quanta absorbed (Of 2537 A radiation)/m1-sec for the various exchange conditions were interpolated from the graph illustrated in Figure 14 (see also p. 74) and, depending upon the lamp used, were adjusted for differences in the lamp outputs by means of the correction factor, L. The computed ¢ex values are tabulated with the rate data Of _1=§ ‘s each concentration variation study. Although the quantum yields are based on the absorption of 2537 A light, they do not apply exclusively I 1a.. to this wavelength, since the actinometer and exchange solutions absorb the longer wavelength light (>3130 A) to slightly different extents. But since the 2537 A radiation comprised 90-96% of the total light intensity absorbed by either the actinometer or exchange solutions, it may be assumed that the ¢ex values apply equally well to either the total absorbed intensity (all wavelengths) or to the 2537 A radiation. The least reliable quantum yields are those for the U(IV) variation, wherein the fraction of the 4358 and 5461 A radiation absorbed increases with progressively higher [U(IV)]. The second procedure for Obtaining overall quantum yields involved the use of galculated (as opposed to experimentally observed) absorbed intensities. From a knowledge of the molar absorptivities, E, for each uranium species at each wavelength of light involved (see Table Bl in Appendix B), the total incident light intensity, 10’ and the lamp spec- tral distribution (Di values mentioned above), absorbed intensities (for each uranium species) were calculated by use of equations derived from the Beer-Lambert law.138 The fraction F, of the incident light 79 intensity, Io of wavelength Ai’ absorbed by a solution of path length d, containing two absorbing species [U(IV) and U(VI)] is given by: F I 1 - T I 1 - 10_d(€lcl + EZCZ), (71) and C C are the molar absorp- 1’ e2 1’ 2 tivities and concentrations of the two absorbing species, respectively. where T is the transmittance, E The distribution of the absorbed light, I8 = FIO, between the two absorbing species is given by 6:. 'Q ll (elcl) . a (ElCl + EZCZ) I f1, etc. for £2, (72) where f1 and f2 are the fractions of the light, Ia’ that is absorbed by each species. A summary of the solution and ion absorption data is presented in Table 82 in Appendix B. By using the above series of equations, the intensity of light Of A1 absorbed by each uranium ion was calculated for each exchange condi- tion [of the U(IV) and U(VI) variations] and for each of the six major wavelengths absorbed, with d - 1.00. The ¢ex values, designated as oex(calc), were then calculated as before, but by use of the total cal- culated light intensity absorbed by each exchange solution. These results are also listed in the appropriate concentration variation section. Correlations Of the exchange rate with the light intensity absorbed by each uranium ion(s) will be mentioned later. 80 B. Calculation Of Exchange Results 93 The logarithmic form of the McKay equation, as applied to the U(IV)-U(VI) exchange system, is as follows: 1U(IV)] + [U(VIA 1 l - F = —R t . 73 r“ ) (U(IV))[U(VI)] ( ) T:— F represents the fraction of exchange in time t, and is equal to the following, expressed in terms of specific activities;139 F a (s - So)/S0° - So) . (74) E” S is the specific activity (counts per minute per absorbance unit) of the U(IV) fraction of the sample photolyzed for (or removed at) time t, So the U(IV) specific activity at "zero time," and Sm the specific activity at the "complete exchange." Specific activities were calculated from the averaged sample activities (triplicate set) and their corresponding absorbances and were, in turn, used to calculate a series of fractions of exchange. Writing the McKay equation in the form of (73), the slope would be equal to -R{[U(IV)] + [U(VI)]/[U(IV)][U(VI)]} and the intercept would be zero in the absence of induced exchange. Exchange rates were computed from the most probable slope, as determined by a linear least-squares treatment of the data, and use of the formulas given by Youdenil'40 'The require- ment that the error in x (time) be small as compared with the error in y, or 1n (1 - F), was clearly met. The standard deviations O in the slope and intercept were computed 81 by using standard formulas}4O The standard deviation in the slope was used to determine the standard deviation in the rate. A comparison of the least-squares intercept values and the associated standard deviations indicated that no exchange had been induced by the separation methods since all values were within :3% of the theoretical value. Bensoéfil'and 66 Masters and Schwartz also reported no induced exchange. The data were tested for the rejection of deviant points accord- . 'lm.4'|‘ .117 ing to a 20-rejection criterion. If the absolute difference between the least-squares value of ln (1 - F) and the experimental value exceeded 20, the point was rejected. A new slope and intercept were then recal- E; culated, based on the remaining data, with only one revision permitted. A The McKay equation for each experiment was hand plotted to certify visually that linear graphs were obtained and, therefore, that the system conformed to the exchange law. Some typical graphs for experi- ments at different hydrogen ion concentrations are presented in Figure 15. Occasionally, curvature (tailing) was detected at longer photolysis times, but this always coincided with change in the [U(IV)]. Consequent- ly, a second restriction was imposed on the data. Data for any photolysis in which the change in [U(IV)] exceeded 5% were discarded. Computations were carried out by means of a computer program, written in FORTRAN IV and executed on a Control Data Corporation 3600 Computer. The program and a sample of the input and output data are presented in Appendix C. Uranium(yl) Concentration Dependence: The effect of uranium(VI) concentration on the exchange rate was evaluated over a lS-fold concen- tration range: 7.40 x 10-3 to 0.111 M9 The data are summarized in 82 .ovoo Hmucoafiuooxo EUH3 soon» so osonm HH+mH moo.mm n H mm.oo.m n H “NIOH x on.~ u maH>vDH ”NIOH x om.a n HA>HVDH .oooovcooop OOH cowouvzn osu Mom oawu.mm Am I HVOH mo unmouw Hmowoha .ma ouswfim ANIo— x 335:: as; or O— n 0 I‘ d! dl _ d I— d u d - A4 I! 4 u - .wwau I 8.: 1 co I. (.0Av Aunmfio I. . I as... I Go I. I. (.55v 1. $5.0 I QO 3.: I. mAu n. u I r H I . q p L _ _ . _ _ _I _ . _ _ . _ LAM— 83 Table 4. Figure 16 shows the graphs of reaction order, log R.X§_log [U(VI)], which were obtained. Data for experiments P(l7)-P(21), employ- ing lamp I, constitute the lower curve in set B, whereas those for experiments P(26)-P(28), employing lamp II, are for the upper plot, which has a least-squares slope of -1.37 i 0.21. It is apparent that the exchange rate exhibits a non-linear and inverse dependence on [U(VI)]. At low [U(VI)] an approximate zero-order dependence is indicated, while IF“ at higher concentrations a decreasing order is in effect. Experiments P(26)-P(28) were performed last and solely for the purpose Of verifying the results of the first set. A log-log plot of ¢ex z§_[U(VI)] is . illustrated in Figure 17. It is observed that the rate data conform to E” a single curve (at high [U(VI)]), having a least-squares slope of —1.31 i 0.17, when absorbed intensities are taken into account. ¢ex(calc), based on calculated absorbed intensities, are plotted on the same curve showing the agreement between the two sets of date listed in Table 4. The appearance of this curve is similar to that in Figure 16 (set B, lower curve), since the intensity of the light absorbed (all A) was nearly constant. Uranium(IV) Concentration Dependence: The effect of the uranium(IV) concentration on the exchange rate was evaluated over the range 0.00598 to 0.0837 M, at a constant [U(VI)] - 2.76 x 10-2. The data are summarized in Table 5 and graphed in Figure 16 (set A) along with the U(VI) data for comparative purposes. A positive non-linear order was obtained as indicated in the upper curve (solid circles). A least- squares straight-line through these same points would have a slope of 0.42 i 0.04. If one included P(6), the order would be 0.33 i 0.11. 84 Table 4. Dependence of exchange rate and overall quantum yields on uranium(VI) concentration. [U(IV)] = 5.87 x 10' ; [H+] = 1.00; I = 2.00 M; T = 25.0°; P(17-21), lamp I; P(26-28), lamp II. [U(VI)] R Experiment (M;x 102) (Msec‘l x 108) ¢ex(overall) x 102 Observed Calculated P(17) 0.740 1.95 i 0.14 3.55 2.53 ,__ P(18) 1.47 2.05 i 0.14 3.18 2.61 j P(19) 2.96 1.79 i 0.03 2.40 2.26 I P(20) 5.92 1.61 i 0.37 2.02 2.01 P(21) 8.87 1.09 i 0.10 1.37 1.36 P(26) 7.44 1.94 1 0.18 1.46 1.51 a“ P(27) 9.66 1.45 i 0.11 1.09 1.12 sis” P(28) 11.14 1.10 i 0.12 0.826 0.85 85 . Ioa xlmmfi u "353 60.39, "m ANIOH x oR.N I HAH>VDH .66a08> HA>HV:_ H< moo.m~ I a ”z oo.~ I H moo.a I H+:_ .mcojmuuooooou SEEDNOOAO new $553.28.; mo Enuwumwoa MN mums owcmsoxm mo ESUHumwOA HAH>VDH as HESS».— ..o 00.0 5.0 000.0 4 q — q — d - q q I— d a — u — q —q — q — e — q - e — a II 0.0 r _ . . VIII. '1 0.. .enasa: . “Hess: . snaI-enfim . F (on r 1 .o.a I [of .l 0.0 T. 06 I OK I 0.0 I 06 . _A_._P_._ _ _. _ A _ . ___._»_t-._ I. .oa shaman hours" I) u 86 ..o.m~ I a A: oo.~ I H moo.s I H+m_ ”NIOH x o~.a I HA>HVDH .aowumuucmoaoo 3383.25.25 mo Enuwumwoa WIN Bum?» Educmoo HHOUOPO mo Enuwumwoq .ma 0.3me fie 93a 0N0 0rd no.0 5.0 000.0 N000 q — fij _ q _ _ _ _ — _ _ _ _ _ 3.03396225200 0 (wood 02:02.0 4 I XO¢ 1 No.0 2.0.3.7818; 1 no.0 I cod l I no.0 00.0 1 87 om.H I H+mim Ham.o mao.o ow.m o~.o Aqv No.0 H ow.m mao.o naq.o qm.m HN.¢ on No.a H om.m mm.w Amvm mam.o mom.o mm.m mm.m ~m.H H mm.¢ mm.n vam mam.o an.o 38.m oH.m 83.o A ea.8 mm.~ Amvm mm.H mo.a oo.m ma.m o~.o H mw.m 0N.H Aqvm I I I I km.o A mm.H mam.o «have ~3.H oN.H mm.H oo.~ mH.o A oe.~ mam.o ARCH ARV on Ame HSV Amv ANV AHV A935 Ac+m+HC woumasoamu UOPHOmno Awoa x .7000 my ANoH x .3 unoauooxm m m “06H Hw AH HHm 06H HNV a HA>HVDH AvmuoHOOHOOV :.AHHmuo>ovxoe: moa x AHHNHOPOVROG a . s .AUHAAIUAH 66506658 I H .4 Hoam I H .m wmmq I mH .m HMHN I HH “H aamH m.o.m~ I a mm_oo.~ I H moo.H I _+m_ a IoH x oa.m I HAH>V:_ .aOHumuuaoosoo A>Hvaswomua do moaoa» aauomso HHOMOPO was many 0 awnoxo mo ouflovflonon .m OHAOH 88 It should be pointed out that the condition of highest [U(IV)] = 0.0837, P(9), is based only on four experimental points. Originally there were six points; however, two values had been discarded because of known determinate errors. Had these points been retained, the R would be 5.30, instead of 7.80, x 10-8 M sec—1, and the lower curve (dashed) would apply. Also, the standard deviation in the R would be 6% higher. The somewhat greater scatter in the data of this variation, especially at high [U(IV)], is related to the inherent error associated with a slow exchange rate and a low tracer level. The tracer level used in this series was 0.01% (of the [U(VIM);1§ 0.025% for most other experi- ments. Log-log plots of overall quantum yields (calculated and Observed) '35 [U(IV)] are shown in Figure 18. These curves are based on the averages of the values in columns (4) and (5) in Table 5. Curve P(9)-l is based on R - 7.80 x 10-8 M sec-l (excluding P(6) as before), and P(9)-2 on R - 5.30 x 10-81M_sec-l. A least-squares line through the points considered for P(9)-l has a lepe of 0.41 i 0.04. "Quantum yields" were also computed on the basis of the total light intensity absorbed by the U(IV) ion for the wavelengths 2537, 4358, and 5461 A. One set is based on the sum of all three wavelengths; a second set is based on the sum of the latter two. These data are also listed in Table 5, columns (6) and (7), and graphed in B and A of Figure 19. At the very most, only qualitative significance may be attached to these "quantum yields." Hydrogen Ion Concentration Dependence: The effect of the hydro- gen ion concentration on the exchange rate is illustrated in Figure 20. F P" .H SE 1.0.2 I H. mm oo.~ I H 39H I FE ANIoH fig I 2:33 .GOHumuucooaoo A>Hv83§mus mo ELUHumwOH MN paowm Esuamso HHOIHOPO mo Esaunumwoq .wH 0.3me 89 :5 male 9.0 8.0 6.0 . mood _ _ _ _ . _ H H _ _ _ _ _ _ _ _ 6228.80 $2 :3 3w 8:380 \ IN \ \ In 1 In 10 1» 1m _L P p H _ H H H _F . b . b . m (ZOI x) "0 9O \. MIR... tel; .. . 30.14.41 Isl-II, .50 .o M>H0>Huoommou .HIoom.m wloa x Aom.m pom ow.mv u m 86 comma wmcHH "NI .HIHavH .H>HV: an empoomnm mmHUHmamHaH uanH we saw was I 3me II #0 .GOHumuuomoooo A>Hvsdfiamus mo anufiumwoa m> :Aoamov e: Hamum>o mo enuflumwoq .oa mpswflm Hay m>_:d m0 000 000.0 . _ q — H — a u q _ _ _ _ _ _ _ _ _ — _ _ u — _ _ _ H F0 I «3%. on? . Bow 2 .8 6an I T .1 HI LO... n 0 n m. D I 4 m... r NIAva // l ’ 1 ’10, L .r. 1de I Naomi. women 4 c8 ..on .I. I. .< .I.0H ' I 1 .HH H — H H H HHH H. b H H H HHH HH _:J H. H AVN 91 .H 9:3 3.0.2 I H mm 8H I H ”NIOH x SH I 2003 ”NIH: x 8H I 23:: .cowumuucmocou OOH ammouwhs .uHo Ezufiumwoa MN Oumu 098033 00 ESUHumwOA .om muswflm :3 TH.”— 00 0.... 0.0 0.» 0.~ 0.. 0.0 0.0 0.0 «.0 H0. .1 — — — fi 1 d d u d I d d OF $30 I L0.~ 8 . W .l [on I O o. I I06 I 2500 00.0:0I0:0e H r :6 .2 3.3 8.05365 (6.8 0.. I (0.0 I. T IOK T am—vh [0.0 I 0:200:03 an 0:00:00 330 G 0.0 II 03:0 0.0. 00:03:03 5 08.03230 .02 0.4.0 3:0 0 — H — P _ H H H H L H H H 92 The data are tabulated in Table 6. The [H+] was systematically varied over a lO-fold concentration range, 1.80-0.179 M, in the experiments P(lO)-P(l6), while the [U(IV)] and [U(VI)] were maintained constant at 2 and 2.76 x 10-2 M respectively. The ionic strength and 1.20 x 10' temperature were maintained at 2.00 M_and 25.0° respectively. As indi— cated in Figure 20, the reaction order graph exhibits curvature in the high acid region (30.7 M acid). A reaction order of -0.65 i 0.05 would mm? be obtained from a linear least-squares slope of a line including all solid-circled points. The result for the thermal experiment, T(l6), has been plotted on the same graph since thermal exchange is competitive 5" under the conditions of the lowest [H+] employed, i.e., at [H+] - 0.179. ¥ The log of overall ¢ex 33 log of [H+] are plotted in Figure 21. The whole curve could be slightly low by a constant factor, since the measured solution absorbances were higher than the calculated values for the 1 M acid region. Calculations indicate that at a [H+] - 1.00 M, 2+ 2 ion absorbs >99% of the incident 2537 K [experiment P(12)] the U0 radiation. Calculation of oex(calc) for the [H+] variation was not possible, since molar absorptivity data for U(IV) is not available for the ultra- violet region as a function of acidity. However, the gross changes in absorbance for the low acidity region in Figure 22 may be attributed primarily to the absorption of 2537 A light by the U(IV) species, 2+ 2 have been reported45 to be constant (i2-3%) over the entire spectrum for presumably UOH3+, since the molar absorptivities for the U0 ion (1) the pH range 0.1 to 1.5 and (2) are known to be nearly invariant (<12 change) over the acidity range, 0.125 to 1.00 M above 3400 A542 Pre- sumably this holds true for the lower wavelength region also. 93 Table 6. Dependence of exchange rate and overall quantum yield on hydrogen ion concentration. 2.76 x 10‘2; I = 2.00 y; T = 25.0°; lamp I. [U(IV)] = 1.20 x 10'2; [U(VI)] = + [H ] R ¢ex(overall) Experiment (M) (M_sec"l x 108) (x 102) P(10) 1.80 2.00 i 0.24 1.63 P(ll) 1.50 2.17 i 0.12 1.70 P(12) 1.00 2.49 i 0.07 2.03 P(l3) 0.750 3.01 t 0.07 2.43 P(l4) 0.500 4.40 i 0.12 3.55 P(15) 0.250 6.85 i 0.25 5.51 P(16) 0.179 11.4 i 0.5 9.12 T(l6)a' 0.179 1.70 i 0.18 a Thermal experiment 94 .H nsna m.o.m~ n a «m oo.~ u H moo.H n H+m_ ”muoa x on.~ u HAH>VDH ”muoa x om.a u HA>HVDH .cowumuuamoaoo :Ofl come—v.3 mo anufiumwoa wfl “0.3.: 553226 Hanson/0 mo Enufiumwoq :3 6.3me nflv T. 1”— 0.~ 0.. 0.0 n~.0 «.0 a _ . . _ 4 _ _ a _ _ a _ 8.: 2:323... 3 933:8 $5.. a :25 0533—230 5 notoEueoo .oz 0.6 .4 ON 1 V (ZOtX)”'¢ J. L r ___p___ _— 95 .m ammm n K moo.mm n a 1: oo.~ u H mmuoa x om.~ n HAH>VDH “N-OH x o~.H n HA>HVD_ .GOfiumHumar Goa cowoubmn 93 mo mooflunaom mwcmnoxm pow cofiumuucmoooo Goa comes»: ImM mononuomoz. .NN munwfim as Ta o.~ m... 0.. n0 0 u q - q u u u d % — u q d 0 d 1 q q d Nw if I T L :2: .2 u l 5 09.2.0 «iv 056334 If] fiT lf1 lnr I . c3332; .23 2 02.32.; «4 E 3032: sonvsossv‘sv .l 1: UrTlU iiiil. T 96 Radiolysis and Temperature Dependencies: The possibility of a radiolytic effect on the exchange rate was evaluated by varying the [2330] in four thermal experiments over the range, (2.16 to 13.2) x 10-6. The other invariant experimental conditions were: [U(IV)] - 0.0250, [U(VI)] - 0.0274, [n+1 = 1:00, [Tartaric acid] - 0.130, I = 2.00 M, and the temperature at 25.0°. The data are listed in Table 7. The log-log plot shown in Figure 23 indicates an order of 0.10 i 0.17. Al- though a different 233U-U(VI) stock solution was used in T-4, one is not justified in discarding this experiment. However, it is conceivable that this point is deviant and that the actual radiolytic dependence is some- what more pronounced than indicated above. The level employed in T-l was the same as that used in the solutions of the U(IV) variation. Table 7. Dependence of exchan e rate on the 233U concentration. [U(IV)] - 2.50 x 10‘ ; [U(VI)] - 2.74 x 10-2; [n+1 - 1.00; [Tartaric acid] - 0.130; I - 2.00 M; T - 25.0°. Experiment [233U(VI)] R (thermal) (M x 106 (M sec.l x 109) T-l 4.52 4.08 i 0.38 T-2 8.83 4.96 i 0.31 T-3 13.2 6.82 i 0.43 T-4 2.16 5.60 i 0.22 The effect of temperature on the exchange rate was evaluated at [the temperatures 14.2°, 25.0°. and 32.0° for the following conditions: 3, I - 2.00 g, and [n+1 - 1.00. [U(IV)] - 5.87 x 10'3, [U(VI)] - 7.60 x 10' The data are listed in Table.8. A marked increase in the rate was observed when the temperature was increased from 14.2 to 25.0°. The rate for P(25) is lower presumably because of the lower incident 97 .muomefiuomxm Hmaumsu moo.mm n H mm.oo.m n H mooé n _+:H moma.o u _nnun nannnnnni 1 non x «N.~ u HAH>VDH m OH x om.~ u HA>HVDH N NI .GOflumuucmoooo “momma :mmm wo Ecuflumwoa mm.mumu owcmcoxm mo snuwumwoq .mm munmflm I a :09 x s: H n3”— n_. nu— mm m" .5 mu m c. n N _nmfl. —-—-—-—d1—d—-—q—q—q— q — u — d — q — 1| .l n J a I. .v .1 c. \1 . . W I O i. In. n .. C i 0.? I 11 . in x . o 2.390.235 - m r 1 r. .110 I 1m ..LLLLLLLLLL ._ . _ p _ . _ o. 98 Table 8. Dependence of exchange rate on temperature. + [U(IV)] = 5.87 x 10- ; [U(VI)] = 7.60 x 10-3; [H 1 = 1.00; I = 2.00 M; P(23,24), lamp II; P(25), lamp IIa. Temperature R Experiment (°C) (Msec-l x 108) P(24) 14.2 1.41 i 0.08 P(23) 25.0 2.95 i 0.08 P(25) 32.0 2.45 i 0.20 intensity involved (lamp Ila). In view of the apparent complexity of the exchange reaction, as indicated by the fractional order dependencies, the determination of an activation energy was not even attempted. Tartaric Acid Concentration Dependence (Thermal): The effect of the concentration of tartaric acid, [HzTar], on the thermal exchange rate was evaluated over the 30-fold concentration range, 0.00910 to 0.260 M, Ten experiments (plus five duplicates) were performed under conditions identical with those of Benson?9 i.e., [U(IV)] - 0.0250 I - 2.00 M [U(VI)] - 0.0274 T - 25.0° [H+] - 1.00 (and 0.85) [HzTar] - varied in an effort to extend part of his work and therefore better define the limits of his tartaric acid dependence study. The results are shown in Table 9 and graphed in Figure 24 from which an order of 1.19 i 0.03 was obtained (for all plotted data) from the slope of the log-log plot. Slight deviation from linearity is dis- cernible for the low limit of the variation. A value of 1.28 i 0.03 is obtained for the lepe based on the data points for the seven highest tartaric acid concentrations. 99 ‘000 I T ITTIII] 1 11111111] I 111111 [Ill 1111 500 I l T l '00 1'11111 111111 1 I 1 R1! 00:41:10”) on o 1 l Slope -1.19 1’ 0.0 (all points) Slope-1.201 0.03 10 (7 points) 11111] 111111 I l T l , 1 11111111 1 111111111 1 1 11111 (1001 DIM CAD to [Tartaric Acid](!1 Figure 24. Logarithm of exchange rate gs logarithm of tartaric acid concentration. [U(IV)] = 2.50 x 10’2; [U(VI)] = 2.74 x 10' I = 2.00 M; T = 25.0°; thermal experiments. 2; [11+] = 1.00; 100 Table 9. Dependence of exchange rate on tartaric acid concentration. [U(IV)] . 0.0250; [U(VI)] = 0.0274; [n+1 = 1.00; 1 - 2.00 M; and T = 25.0°; thermal experiments. [Tartaric Acid] R Experiment (M_x 10) ' (E.SeC-l x 1010) 2 0.0910 2.04 3 0.130 2.81 4 0.390 9.28 5 0.650 18.1 6 0.910 24.8 73 1.30 54.0 8 1.69 57.0 9 1.95 74.4 10 2.21 84.3 11 2.60 106.0 a[11+] - 0.850 VI. DISCUSSION A. Discussion of Errors and Reproducibility of Results Photochemical Rate Data: As indicated by the standard deviation in the exchange rates, the data for a given experiment are fairly con- sistent, 0(av) - 10%. The uncertainties in the data arise from two main sources of error: (1) change in the [U(IV)] during photolysis, result- ing from presumably, photocatalyzed oxidation of U(IV) by oxygen, and (2) gradual change in the output of lamp I (main one) through aging. In general, the net effect of a U(IV) loss would be slightly lower rates than would be found with no loss of U(IV). Slightly higher rates could be observed for the high [U(VI)] conditions, however. In any case, the uncertainty should not exceed %5%. Although no definite trends in the data are discernible to indicate a major effect by (oxidative) loss of U(IV), a mechanism involving oxygen—catalyzed exchange is still a possibility. A second source of error becomes evident on examining the rates of comparable experiments as listed in Table 10. Table 10. Exchange rates for comparable experiments. [11+] - 1.00; 1 = 2.00 M; T = 25.0°; lamp 1. [U(IV)] [U(VI)] 3; Order in which Experiment (M x 102) (M x 102) (M sec-1 x 108) performed P(4) 1.20 2.76 3.87 l P(12) 1.20 2.76 2.49 64% of P(4) 3 P(7) 0.598 2.76 2.46 2 P(19) 0.587 2.96 1.79 73% of P(7) 4 101 102 The agreement is poor for both sets of experiments, but slightly better for the second set when the differences in U(IV) and U(VI) concentrations are taken into account. The decreasing trend of R.with order of perfor- mance suggests that (1) the lamp output (10) had gradually diminished over this time period through aging and, consequently, that (2) the U(IV) and hydrogen ion dependences might be somewhat more pronounced than was observed since the experiments, P(4 to 21), were performed in consecutive numerical order. However, no gross effects were apparent from the McKay plots, all of which were linear except where noted previously (page 81). This uncertainty does not apply to the experiments of the U(VI) varia- tion, however, since the lamp outputs were periodically monitored by means of chemical actinometry. Good reproducibility is apparent for these experiments for which a log-log graph of ¢ex !§_[U(VI)] conforms to a single curve (see Figure 17). The two sets of experiments were per- formed at different times by using different lamps and, for P(26-28), in random order. The incident light (A <3650) was completely absorbed in all solu— tions and usually within a very thin layer of solution. The question of whether R was considered and specifically with regard to local I Roverall the U(VI) variation. Since the solutions were stirred moderately rapidly and the exchange rates were very slow, any possible differences in the local 32 overall rates were judged to be small.143 Thermal Exchangg Results: The thermal exchange rate observed for T(l6), [H+] - 0.179, is 1.70 x 10-8 M sec-1. This rate agrees satis— factorily with 1.23 and 1.6 x 10-8 Msec-1 as computed from the rate laws of Masters and Schwartz, equation (34), and Rona, equation (30), l 103 respectively. A rate constant of 2.5 x 10.5 M sec—l, as reported by Stranks and Wilkins,144 for Rona's work was used in computing the latter value. The observed photochemical rate is approximately 7 times the cor— responding thermal exchange rate. Calculations of thermal rates for higher acidities by using either of the thermal rate expressions which have been reported, indicated that the thermal contribution to the gross rate would be essentially small for the other conditions studied. For example, at a [H+] = 1.00, the thermal rate would be slower by at least a factor of 102. For the condition of highest [U(IV)], P(9), the thermal rate is calculated to be ~1/20 that of T(l6), assuming a second-order dependence on U(IV). T(16) is the only thermal experiment performed in which tartaric acid was not present. The agreement between comparable thermal and photochemical (one) exchange rates obtained by Benson and those of the present study is very poor. In all cases, the rates obtained by Benson are higher by at least a factor of 2 and often by a full order of magnitude. (This holds true for the above—mentioned thermal condition). All attempts to duplicate the results of his tartaric study were unsuccessful; furthermore, a different reaction order with respect to tartaric acid was obtained; namely, 1.19 i 0.03 y§_0.89 i 0.11 as obtained by Benson. It also should be pointed out that the rates obtained by Benson were consist- ently higher than those of Rona for comparable conditions. The exact reasons for the discrepancies are not known, but could include one or more of the following in the suspected order of importance: (1) uncer- tainties in the measurement of the acidities of exchange solutions,145 (2) 233U radiolytic effects, (3) contamination of stock solutions by 104 trace catalytic impurities. With respect to a possible radiolytic ex- planation, Benson reported using a 2% enrichment of 233U. If this figure is correct, it would mean that the tracer level used in his ex— periments was 'blO2 higher than in the comparable experiments reported 'herein. As noted previously, the exchange rate is accelerated slightly ‘with increasing tracer concentration. B. Interpretation of Photochemical Data The following empirical rate law is established from the experi- mental results for conditions of constant incident light intensity: 1 R = k [U(IV)]0'41[U(VI)]-l'4 [H+]-0.65 (74) The fractional exchange orders shown are only approximate and apply to a limited portion of the region investigated, since all of the order graphs exhibited curvature. The presence of fractional orders in this expres- sion indicates that the exchange system is not simple and that probably several exchange paths are Operative. It is obvious from the form of the rate law that the exchange is not simply the result of photoacceler- ation of an existing thermal path. The relative unimportance of radiolysis products in the exchange process(es) is illustrated as follows. From a knowledge of the tracer 233U (2.103 X 104 level normally employed, the specific activity of d/m/ug, Table l), and the energy of its a-particles (4.8 Mev), the con- centrations of the radiolytically produced species, (0H + H)/2 sec can be computed by using a value of C(OH + H) of N4,146 where G is the con- ventional radiation yield or molecules produced per 100 eV of absorbed energy. For the U(IV) variation, the rate of production of 0H and H 105 12 12 inculd be N1 X 10- Msec”l as compared with (2.5 and 10) X 10- M_sec- for the H+ and U(VI) variations (for the highest [U(VI)] employed). IImplicit in the above calculation is the assumption that the y-rays asso- ciated with the a-decay of 233U do not make a significant contribution to the radiolysis of the solution. The rate of photoactivation of uranyl l7 * ions (U0:+ ) would be N8 X 10 ions/1 sec and would correspond to ml X 10-6 mole of excited state species/2 sec , or a factor of 105 - 106 greater than the radiolytic production. 0n the basis of the above, it is doubtful that the steady-state concentrations of the radiolytic pro- ducts are high enough to compete effectively with the concurrent photo- chemical process(es). The following relationships could be important in the overall exchange process(es). Excitation and de-excitation of U02+ occur in solution according 2 to the processes: * U0:+ + hv -$ U0:+ (excitation and fluorescence) (75) k 2 and 2+* 2+ k3 2+ U02 + U02 -+ 2U02 (self-quenching) (76) 2+* 2+ where U02 and U02 represent the excited and ground states of the uranyl ion, respectively, and k1, k2, and k3 are the appropriate specific rate constants. From Vavilov's self-quenching experiments on uranyl (sulfate) solutions it appearsthatk3 s 99 k2.58 Under conditions of constant absorbed intensity, Ia’ (and for the moment assuming no subse- * quent reaction of U0:+ ) the steady-state (photostationary-state) con- * centration of U0:+ would be given by 106 I * “103+ 1.: a 2+ , <77) k2 + k [U0 ] 2+ 3 2 -(e[U0 ‘where Ia - Io[l — 10 2 ]d)], and as indicated, would be dependent 'upon the bulk [UO§+]. (lo, a, and d have been described previously.) This self—quenching mechanism could qualitatively explain the inhibitory * effect of U0:+ on the exchange rate, if U0:+ initiates exchange. * Photodecomposition of U0:+ could generate U(V) and an OH radical according to the process it 00%+ ~H20 ——+ 00; + OH + H+ (78) which could involve coordinated or free H20, although the former seems more reasonable. This process or a closely related one involving OH-, instead of H20, has been proposed for the photooxidation of H2 mate (uranyl-sensitized)152 or could be inferred from the work of Gordon and Taube on the U(V)-catalyzed oxygen exchange between U02+ and H 180. 2 2 47 8 and Np(VI)23 are involved in analo- 0 by bro- Iron(III),1 T1(III),22 Ce(IV),14 gous process(es), as mentioned previously. Although reaction (78) is feasible on energetic grounds (N117 kcal/Einstein for 2537 A radiation), the back-reaction (reverse of (78), which corresponds to primary and secondary recombination, would have to be very efficient since no net change in U0:+ or formation of 02, through 0H recombination and subse- quent decomposition, are observed in irradiations of U02+ solutions 2 alone. Competition for U(V) and/or 0H would result, however, when suit- 149 - 150 able species are present. Neither H20 nor the C104 ion are photo- decomposed by light of A >2000 K. Uranium(V) could also be produced by a photoinitiated process as in 107 U(IV) + um)" I: 2U(V) (79) where the U(IV) species is probably UOH3+ rather than U4+, in view of the 2+* 2 (a triplet-excited state) or a species derived directly from it, e.g., as * observed inverse dependence on H+. The U(VI) species might be U0 in photohydrolysis * 00:+ 4120 —-> 002011+ + 11+ (80) 2+_ 2 H20 exchange, and conceivably could result from the recombination reac- This hydrolytic species is thought to be involved in the intrinsic U0 tion in (78). In turn, the resultant U(V) would interact by one or more of the following steps which would lead to exchange: 2+ 75 9 (1) formation of a complex intermediate with U02 + 233 2+ 3+ 233 + 2+ U02 + UO2 ——* [U204 ] -+ U02 + U02 (81) (ii) disproportionation39 (essentially the reverse of equation (14) . + + 2+ 4+ 2U02 + 4H —+ U02 + U + H20 (7) (iii) exchange with U(IV), as gostulated for the U(IV)—U(VI) system in oxalate media. 6 2 2 33U(V) + U(IV) gum + 33U(IV). (33) No evidence exists to support reaction (38) in any actinide system in acid perchlorate media; therefore, it is considered to be of little sig- nificance here. The complex intermediate in equation (81) is formed to a signifi- cant extent even in.1‘M_HC104. At progressively higher [U0§+], the 108 increased formation of this complex would decrease any free [U(V)], thereby inhibiting the disproportionation reaction (7) and, hence, the observed exchange rate (assuming exchange proceeds through a U(V) inter- mediate). An exchange path initiated by, or as a result of, photoactivated U(IV) could provide a partial explanation of the positive U(IV) depen- dence.‘ Although U(IV) has not been well-established as a photoactive species,151 the following evidence supports such a hypothesis. The ex- change rate increases (but perhaps at a decreasing rate), (see Figures 18 and 19) as the fraction of light absorbed by U(IV) increases. Also, enhanced exchange rates were observed for preliminary experiments using filtered light153 (>4910 A, wherein the increase in the rate is clearly related to the absorption (most likely of 5461 A radiation) by U(IV), 3+ doesn't absorb above N4800 A (see Figure 2). In this context, if U0:+ exchange conditions (an estimate of 28% has been made for uranyl sulfate since the U0 fluorescence yields were at all appreciable under the solutionséz), a significant fraction of the emitted light, A >4700 A, would be reabsorbed by U(IV). Exchange initiated by photoactivated U(IV) could occur, conceivably, as a result of one of the photohydrolysis steps represented in equations (82) and (83) 04+ + 1120 1‘" UOH3+ + 11+ (82) U0H3+ + H20 -h—"— Mon):+ + 11+ (83) followed by exchange with U(VI) or less likely U(V) as represented by equations (79) or (81), respectively. The increased absorption of 2537 A light by U(IV) as a function of decreasing acidity, as noted previously for solutions of the H+ variation is consistent with (83). A negative 109 second or third order dependence on [H+] would have to be obtained ex— perimentally in order for exchange to proceed through and be consistent with the path represented by equation (79) and involving U(OH)§+ or UOH3+, respectively and UO§+. Since (1) an inverse first-order dependence ap- proximately on hydrogen ion was observed, and (2) this mechanism does not account for the pronounced inverse dependence on U(VI), neither of the latter possibilities are likely to constitute a major step involving ex- change photoinduced by U(IV). Also, if the observed exchange rate R were determined solely by the light absorbed by U(IV), 1&4, which it is not, ¢ex values near or >1 would be observed (see Table 5, column 4). As additional alternatives of exchange involving U(IV), exchange could be catalyzed conceivably by photochemical oxidation of U(IV) by oxygen or by trace concentrations of radiolytic products, since the thermal oxidation of U(IV) apparently proceeds by a chain mechanism in- volving U(V) and H02 as chain carriers.68 The photoinduced exchange could involve reaction (78) or (79) as the initial step, followed by ex- change through (81) and the propagation steps + 2+ - U02 + 02 + H20 -—* U02 + H02 + 0H (17) and 3+ + + H02 + UOH + H20 -+ U02 + H202 + 2H (18) etc. as in the thermal route (page 21). Since slight U(IV) loss occurred in‘many experiments, this or an equivalent oxygen-catalyzed path un- doubtedly contributes to the overall observed exchange. Exchange proceeding primarily through reaction (79), as a rate- determining step, is a definite possibility since (1) U(IV) would be expected to exhibit a positive and perhaps non-linear exchange order, 110 since at the higher [U(IV)], U(IV) could act as an inner-filter (for 4358 and 2537 A light), and (2) the inverse dependence on U(VI) can be ac- counted for (under constant Ia) in terms of the self-quenching and U(V)- U(VI) complexation effects as noted above. Evidence for a positive (and perhaps linear) dependence of R upon I8 is provided by the results of the two independent sets of experiments of the U(VI) variation (coincidence of the ¢ex curves at the higher [U(VI)]). The major shortcoming of this mechanism is in accounting for the observed H+ dependence. If reaction (79) is of the form UOH3+ + 00 011" —> 200+ + 211+ (84) 2 +— 2 or * UOH3+ + 002+ + H 0 ———+ 200+ + 311+ (85) 2 2 +— 2 and is assumed to proceed through the same activated complex as the ther- mal exchange,66 i.e., (H0-U-0-UO§+)+, then the predicted dependences would be at least third and fourth-order, respectively. However, if the reaction proceeded through a different activated complex, involving a lower H+ dependence, better qualitative agreement with the observed de- pendence would be realized. An exchange mechanism based on the photoinitiation step (78) appears to offer the best general qualitative explanation of the experi- mental results. A summary of the processes that could be involved is as follows: * (i) U0§+°H20 23+ UO§+ -H20 (excitation-fluorescence) (75) * (ii) 00:+ + no:+ —-> 200? (self-quenching) (76) * (iii) 002+ ~11 0 -—-> 00"” + 2 2 2 + OH + H (photoinitiation, 01) (78) 111 233UO2+ 3+ 233 + 2+ + (iv) 002 + 2 —-+ [U204 ] ——> U02 + U02 (81) (v) 2330072L + U(IV) —-> 233U(IV) + 00‘2L (86) (vi) 0033+ + OH —+ 00‘; + 211+ (87) (vii) 00‘2L + 0H + 11+ ——+ 00:+ + H20 (88) + + (viii) 2UO2 + H ——+ products (rate-determining) (7) Steps (1 to iii) may occur as shown or absorption of 2537 A radiation may cause (iii) to occur directly, thereby eliminating the necessity of the two preliminary steps (and therefore the self-quenching feature) as kin— etic steps. The inhibition of the U(VI) variation would then depend only on step (iv), the formation of the U(V)—U(VI) complex. Step (v) occurs slowly, if at all; it is only included to point out that ¢ex >1 would not be expected, even though a chain-mechanism is proposed (since U0: would 3+ and U(IV) faster than its own rate of destruction, i.e., through the rate-determining step (viii)). The magnitude of the not exchange with U0 ¢ex might then be an indication of the importance of this path. Steps (vi and vii) would be necessary so that no net change in U(IV) or U(VI) would be observed with time. The mechanism accounts for a first or second-order H+ dependence, 3 positive U(IV) dependence (but perhaps not as pronounced as observed) since U(IV) would be expected to compete more effectively with the sec- ondary recombination processes(es) (step vii) for the 0H radical at higher [U(IV)]. The low quantum yields observed are (1) understandable in terms of the back-reaction (88), plus the competitive processes of fluorescence emission and self-quenching, and (2) in good agreement with 66 the previously reported value of 0.01. The fact that Np(VI) is photo- reduced in an analogous photoinitiation step and that the observed 112 overall ¢ex is about the same order of magnitude (= 0.032) lends addi- tional plausibility to the pr0posed mechanism. VII. SUMMARY The kinetics of the uranium(IV)-uranium(VI) electron exchange reaction were investigated in aqueous perchoric acid solutions under con- ditions of constant incident light intensity. A low-pressure mercury- vapor lamp, Model LO 735A-7 (Hanovia), was used as the light source which emitted principally 2537 A radiation. The exchange was inhibited by uranium(VI) and hydrogen ion, but accelerated by uranium(IV) and by in- creasing the temperature. Non—linear order graphs were obtained for uranium(IV), uranium(VI), and hydrogen ion, having the approximate orders of 0.41, -l.4, and -0.65, respectively. Overall quantum yields for ex- change ranged from 0.01 to 0.1, based on the absorption of light by uranium(VI), and were determined by means of potassium ferrioxalate actinometry. Plausible exchange mechanisms are discussed in terms of a uranium (V) intermediate as produced by one or more of the following steps: (1) U(IV) + 0 -+ U(V) +'H0 2 2 (2) U(IV) + U(VI) :2: 2U(V) 2 (3) UO2 +412) 1‘3» 00’2" + OH + 11+ A mechanism based on step (3) as the principal exchange path is in rea- sonable qualitative agreement with the experimental results. 113 LIST OF REFERENCES 10. ll. 12. 13. 14. 15. 16. 17. LIST OF REFERENCES Marcus, R.A., Ann. Rev. Phys. Chem., 12, 155 (1964). Sutin, N., Ann. Rev. Nucl. Sci., 12, 285 (1962). Reynolds, W.L. and R.W. Lumry, "Mechanisms of Electron Transfer," The Ronald Press Company, New York, 1966. Taube, H., in "Advances in Inorganic Chemistry and Radiochemistry," H.J. Emeleus and A.G. Sharpe (eds.), Vol. 1, Academic Press, New York, 1959, pp. 1-53. Halpern, J., Quart. Rev. (London), 12, 207 (1961). Sutin, N., Ann. Rev. Phys. Chem., 11, 119 (1966). Marcus, R.J., B.J. Zwolinski, and H. Eyring, J. Phys. Chem., 58, 432 (1954). Zwolinski, B.J., R.J. Marcus, and H. Eyring, Chem. Rev., 55, 157 (1955). Halpern, J. and L.E. Orgel, Discussions Faraday Soc., 32, 32 (1960). Taube, H., H. Meyers, and R.C. Rich, J. Am. Chem. Soc., 15, 4118 (1953). Candlin, J.P., J. Halpern, and S. Nakamura, J. Am. Chem. Soc., 85, 2517 (1963). Libby, W.F., J. Phys. Chem.,.ég, 863 (1952). Libby, W.F., J. Chem. Phys., 38, 420 (1963). Sykes, A.G., "Kinetics of Inorganic Reactions," Pergamon Press Inc., New York, 1966, p. 120. Dodson, R.W. and N. Davidson, J. Phys. Chem., 56, 866 (1952). Horne. R.A., J. Inorg. Nucl. Chem.,.gg, 1139 (1963). Stranks, D.R., in J. Lewis and R.C. Wilkens (eds.), "Modern Coord- ination Chemistry," Intersceince, New York, 1960, p. 78. 114 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 115 Gordon, G., Inorg. Chem., 2, 1277 (1963). Gordon, G., and H. Taube, ER$Q°9.1» 69 (1962). Newton, T.W. and S.W. Rabideau, J. Phys. Chem., 63, 365 (1959). Uri, N., Chem. Rev., 59, 413 (1952). Stranks, D.R. and J.R. Yandell, in "Exchange Reactions," (Proceed- ings of the Symposium on Exchange Reactions held at Brookhaven National Laboratory, Upton, New York, May 31—June 4, 1965), International Atomic Energy Agency, Vienna, Austria, 1965, p. 83. Zielen, A.J., J.C. Sullivan, and D. Cohen, J. Inorg. Nucl. Chem., .1, 378 (1958). Adamson, A.w., Record Chem. Progr. (Kresge-Hooker Sci. Lib.), Mg, 191 (1968). Hush, N.S., Trans. Faraday Soc., 51, 557 (1961). Quinn, L.P., Ph.D. Thesis, Michigan State University (1961). Love, C.M., Ph.D. Thesis, Michigan State University (1964). Benson, E.P.,Jr., Ph.D. Thesis, Michigan State University (1963). Benson, E.P., Jr. and C.H. Brubaker, Jr., in "Proceedings of the Symposium on Coordination Chemistry," (VIII International Conference on Coordination Chemistry, Tihany, Hungary, 1964), M.T. Beck (ed.), Akademiai Kiado, Budapest, 1965, pp. 427- 435. McAuley, A. and C.H. Brubaker, Jr., Inorg. Chem., 3, 273 (1964). Katz, J.J. and G.T. Seaborg, "The Chemistry of the Actinide Ele- ments," John Wiley and Sons, Inc., New York, 1957, Chapter V, pp 0 94-203 s Katz, J.J. and E. Rabinowitz, eds., "Chemistry of Uranium," Col- lected Papers, Book 1 and 2, Atomic Energy Publication, TID- 5290, 1958. Booman, G.L. and J.E. Rein, in I.M. Kolthoff and P.J. Elving (eds.), "Treatise on Analytical Chemistry," Part II; Vol. 9, Inter- science, New York, 1962, pp. 1-188. Grindler, J.E., "The Radiochemistry of Uranium," NAS-NS-3050, Clearinghouse for Federal Scientific and Technical Informa- tion, U.S. Dept. of Commerce, Washington 25, D.C., pp. 1-350. Lawrence, R.W., J. Am. Chem. Soc., 26, 776 (1934). 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54.. 55. 56. 57. 116 Herasymenko, P., Trans. Faraday Soc., 34, 267 (1928). Heal, H.G., Trans. Faraday Soc., £3, 1 (1949); ibid. idem., 43, 11 (1949). Harris, W.E., and I.M. Kolthoff, J. Am. Chem. Soc., 31, 1484 (1945); ibid. idem., 33, 1175 (1946); ibid. idem., 33, 446 (1947). Nelson, F. and R.A. Kraus, J. Am. Chem. Soc. 13, 2157 (1951). Kraus, R.A., F. Nelson, and G.L. Johnson, ibid., 13, 2510 (1949); Kraus, R.A. and F. Nelson, ibid. 13, 2517 (1949). Asprey, L.B. and R.A. Penneman, Inorg. Chem., 3, 727 (1964). Betts, R.H. and R.M. Leigh, Can. J. Research, B28, 514 (1950). Kraus, R.A. and F. Nelson, J. Am. Chem. Soc., 13, 3901 (1950). Sillén, L.G. and S. Hietanen, Acta Chem. Scand., 39, 1531 (1956). Ahrland,S., Acts Chem. Scand., 3, 374 (1949). Rush, R.M., J.S. Johnson, and R.A. Kraus, Inorg. Chem., 3, 378 (1962). Rush, R.M. and J.S. Johnson, J.Am. Chem. Soc., 31, 821 (1963). Hearne, J.A. and A.G. White, J. Chem. Soc., 2168 (1957). Whittaker, M.P., E.M. Eyring, and E. Dibble, J. Phys. Chem., 32, 2319 (1965). Hurwitz, P.A. and 0. Atkinson, ibid.,.13, 4142 (1967). Cole, D.L., E.M. Eyring, D.T. Rampton, A. Silzars, and R.P. Jensen, ibid., 13, 2771 (1967). Sutton, 1., J. Chem. Soc., 8275 (1949). MacInnes, D.A., and L.G. Longsworth, "U.S. Atomic Energy Commis- sion," MDDC-9ll (1942). Cohen, D. and W.T. Carnall, J. Phys. Chem., 4, 1933 (1960). Kraus, R.A. and F. Nelson, J. Am. Chem. Soc., 13, 3901 (1950); ibid. idem., 3721 (1955). McKay, R.A.C. and J.L. Woodhead, J. Chem. Soc., 717 (1964); see also McKay, H.A.C. and D. Scargill, J. Inorg. Nucl. Chem., 39, 3095 (1968). Stewart, D.C., "Absorption Spectra of Lanthanide and Actinide Rare Earths, II." ANL-4812(AECD-3351), Argonne (1952). 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 117 Rabinowitch, E. and R. Linn Belford, "Spectroscopy and Photo— chemistry of Uranyl Compounds," The Macmillan Company, New York, 1964. McGlynn, S.P. and J.R. Smith, J. Mol. Spectroscopy, 3, 164 (1961). Bell, J.T. and R.E. Biggers, J. Mol. Spectroscopy, 33, 247 (1965); ibid.'idem.,'33, 262 (1967); ibid. idem., 33, 312 (1968). See also Bell, J.T., J. Inorg. Nucl. Chem., 33 (1969). See reference 58, p. 223. See reference 58, p. 217. See reference 58, pp. 229-334. Heidt, L.J. and K.A. Moon, J. Am. Chem. Soc., 13, 5803 (1953). Heidt, L.J., 3333.,.Z3, 5962 (1954). Masters, B.J. and L.L. Schwartz, J. Am. Chem. Soc, 33, 2620 (1961). Sobkowski, J., J. Inorg. Nucl. Chem., 31, 2351 (1965). Halpern, J. and J.C. Smith, Can. J. Chem., 32, 1419 (1956). Kern, D.M.H. and E.F. Orlemann, J. Am. Chem. Soc., 13, 2102 (1949). Koryta, J. and J. Koutecky, Chem. Listy,.g3, 1605 (1954). Heidt, L.J., J. Am. Chem. Soc., 13, 5962 (1954). Imai, H., Bull. Chem. Soc. (Japan), 33, 873 (1957). Duke, F.R. and R.C. Pinkerton, J. Am. Chem. Soc., 13, 2361 (1951). Sobkowski, J., J. Inorg. Nucl. Chem., 31, 2351 (1965). Newton, T.W. and F.B. Baker, Inorg. Chem., 4, 1166 (1965). Newton, T.W. and F.B. Baker, Inorg. Chem., 3, S69 (1964) and references cited therein. Gordon, G. and H. Taube, J. Inorg. Nucl. Chem., 33, 272 (1961). Gordon, G. and H. Taube, J. Inorg. Nucl. Chem., 33, 189 (1961). Masters, B.J. and H.K. Perkins, data presented before the Division of Inorganic Chemistry at the 138th Meeting of the American Chemical Society, New York, N.Y., September, 1960. Betta, R.H., Can. J. Research, 263, 702 (1948). 81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94. ' 95. 96. 97. 98. 118 BHchmann, K. and K.H. Lieser, Ber. Bunsenges. Phys. Chem., 33, 522 (1965)(Ger); a summary appears in "Exchange Reactions," (Proceedings of the Symposium on Exchange Reactions, held at Brookhaven National Laboratory, Upton, New York, May 31-June 4, 1965), International Atomic Energy Agency, Vienna, Austria, 1965, pp. 38-39. Rona, E., J. Am. Chem. Soc., 13, 4339 (1950). Kakihana, H. and H. Tomiyasu, in "Exchange Reactions," (Proceed- ings of the Symposium on Exchange Reactions, held at Brook- haven National Laboratory, Upton, New York, May 31-June 4, 1965) International Atomic Energy Agency, Vienna, Austria, 1965, p. 121. Tomiyasu, H., H. Fukutomi, and H. Kakihana, J. Inorg. Nucl. Chem., 30, 2501 (1968). Shimokawa, J. and G. Nishio, J. Nucl. Sci. Tech., 3, (7) 221 (1964). Grinberg, A.A., and D.N. Bykhovskii, Radiokhimiya, g, 234 (1962). Melton, S.C., A. Indelli and E.S. Amis, J. Inorg. Nucl. Chem., 31, 317 (1961). Ibid. idem., 1.7.» 325 (1961). Mathews, D.M., A. Hefley and E.S. Amis, J. Phys. Chem.L§3, 1236 (1959); Indelli, A. and E.S. Amis, J. Am. Che. Soc., 33, 4180 (1959). King, E.L., "U.S. Atomic Energy Commission," MDDC-813 (1947). Hindman, J.C., J.C. Sullivan and D. Cohen, J. Am. Chem. Soc., 33, 2316 (1959). Wear, J.O., J. Inorg. Nucl. Chem., 33, 1445 (1963). McKay, H.A.C., Nature, 148, 997 (1438). Challenger, G.E. and B.J. Masters, J. Am. Chem. Soc., 13, 3012 (1956). See reference 2, p. 30. Harris, C.M., Trans. Faraday Soc., £3, 137 (1952). Stranks, D.R. and R.C. Wilkins, Chem. Rev., 31, 758 (1957). Stranks, D.R. and J.R. Yandell, in "Exchange Reactions," (Proceed- ings of the Symposium on Exchange Reactions, held at Brook- haven National Laboratory, Upton, New York, May 31—June 4, 1965), International Atomic Energy Agency, Vienna, Austria, 1965, p. 83. 99. 100. 101. 102. 103 O 104. 105. 106. 107. 108. 109. 110. 111. 112. 113. 114. 115. 116. 117. 118. 119. 119 "Testing of Glass Volumetric Apparatus," National Bureau of Stan- dards Circular 602, U.S. Government Printing Office, 1959. Kolthoff, I.M. and E.B. Sandell, "Textbook of Quantitative Inor- ganic Analysis," 3rd Ed., The Macmillan Co., New York, 1952, pp. 526-529. See reference 27, p. 54-55. Mellor, J.W., "A Comprehensive Treatise on Inorganic and Theo- retical Chemistry," Vol. II, Longmans, Green and Co., London, 1922, p. 395. Wilson, C. and D. Wilson,(eds.),"Comprehensive Analytical Chemis- try," Vol. IB, Elsevier Pub. Co., Amsterdam,1959-l962, p. 245. See reference 100, p. 583. Willard, H.H. and P. Young, J. Am. Chem. Soc., 39, 1322 (1928). Lundell, G.E.F. and H.B. Knowles, J. Am. Chem. Soc., 31, 2637 (1925). Meyer, F.R. and G.T. Ronge, Angew. Chem., 33, 637 (1939). Dodd, R.E. and P.L. Robinson, "Experimental Inorganic Chemistry," Elsevier Publishing Co., Amsterdam, 1954, pp. 166-167. J.P. Mickel Associates, Detroit, Michigan, unpublished data. See reference 27, pp. 46-49. See reference 27, p. 37. See reference 28, p. 21. Sato,‘T., Naturwissenschaften, 33, 668 (1961); (see C.A. 56:9683g); I. Kobayashi, Rika Gaku Kenkyusho Hokoku 31, 349 (1961); (see C.A. 56:11165d). Silverman, L. and R.A. Sallach, J. Phys. Chem. 33, 370-371 (1961). Boggs, J.E. and M. El-Chehabi, J. Am. Chem. Soc., 12, 4258 (1957). (See also Doklady Akad. Nauk SSSR, 138, 1345 (1961); C.A. 56:111641). See reference 28, p. 20; reference 29, p. 427. Schirmer, W. and N. wachter, Actinide Rev., 3_(No. 2)128 (1968). Ahrland, S. and R. Larsson, Acts Chem. Scand., 3, 137 (1954). Hodgman, C.D., ed., "Handbook of Chemistry and Physics," 38th Ed., Chemical Rubber, Cleveland, 1956, p. 3040. 120. 121. 122. 123. 124. 125. 126. 127. 128. 129. 130. 131. 132. 133. 134. 135. 136. 137. 138. 120 Willard, H.H., N.H. Furman and C.E. Bricker, "Elements of Quan- titative Analysis; Theory and Practice," 4th Ed., Van Nostrand, Princeton, New Jersey, 1956, p. 243. Adamson, A.W., private communication. Engelhard Hanovia, Inc., "Hanovia Low Pressure Quartz Mercury- Vapor Lamp, Model LO 735A-7," Data File EH-322, Newark, New Jersey. DuRon, B., Lamp Division, Engelhard Hanovia, Inc., private communi- cation (9/68).; ' Heidt, L.J. and H.B. Boyles, J. Am. Chem. Soc., 33, 5728 (1951). Betts, R.H. and R. M. Leigh, Can. J. Research, B28, 514 (1950). Hyde, E.K., Proceedings, International Conference on the Peaceful Uses of Atomic Energy, United Nations, N.Y., Vol. 7, (1956), p. 281. Day, R.A., Jr., R.N. Wilhite, and F.D. Hamilton, J. Am. Chem. Soc., .11, 3180 (1955). See reference 28, p. 38. Mewherter, J.L., Knowles Atomic Power Laboratory, unpublished results. Overman, R.T. and H.M. Clark, "Radioisotope Techniques," McGraw- Hill Book Co., Inc., New York, 1960, p. 114. Calvert, J.G. and J.N. Pitts, Jr., "Photochemistry," John Wiley and Sons, Inc., New York, 1966, p. 783. Hatchard, C.G. and C.A. Parker, Proc. Roy. Soc. (London), A235, 518 (1956); see also C.A. Parker, Proc. Roy. Soc. (London), A220, 104 (1953). Claesson, I.M., Arkiv Kemi,.3Q,40 (1956); see also D.H. Volman and J.C. Chen, J. Am. Chem. Soc., 33, 4141 (1959). See reference 131, p. 797. See reference 131, p. 784. Anderson, W.T., Hanovia Lamp Division, Engelhard Hanovia, Inc., Newark, N.J., product literature entitled, "The Quartz- Mercury Arc." See5reference 131, p. 15. Willard, H.H., and L.L. Merritt, Jr. and J.A. Dean, "Instrumental Methods of Analysis," D. Van Nostrand Co., Inc., 4th ed., Princeton, NeJe, 1965, p. 76-770 139. 140. 141. 142. 143. 144. 145. 146. 147. 148. 149. 150. 151. 152. 153. 154. 155. 156. 157. 121 Prestwood, R. and A.C. Wahl, J. Am. Chem. Soc., 33, 3137 (1949). Youden, W.J., "Statistical Methods for Chemists," John Wiley and Sons, Inc., New York, 1951, pp. 40-43. See reference 28, p. 43. Bell, J.T.,Oak Ridge National Laboratory, unpublished data. Heidt, L.J., private communication. Stranks, D.R. and R.G. Wilkins, Chem. Rev., _Z,749 (1957). See reference 27, p. 73. Hochanadel, C.J., private communication. See reference 21, p. 416. See reference 21, p. 417. Barret, J. and J.H. Baxendale, Trans. Faraday Soc., 33, 37 (1960). Pearson, G.S., in "Advances in Inorganic and Radiochemistry," H.J. Emeleus and A.G. Sharpe (eds.), Vol. 8, Academic Press, New York, 1966, p. 208. Dainton, F.S. and D.G.L. James, J. Chim. Phys., 33, C17 (1951). See reference 58, p. 244. Brubaker, C.H., Jr., private communication. Stair, R., R.G. Johnston, and E.W. Halbach, J. Research, National Bureau of Standards, 64A, 291 (1960). Ibid. idem., 64A, 295 (1960). "Optimum Spectrophotometer Parameters Application Report AR l4-2," Applied Physics Corporation, Monrovia, California, 1964. Instruction Manual, Cary Recording SpectrOphotometer, Model 14, Applied Physics Corporation, Monrovia, California. APPENDICES APPENDIX A RELATIVE CALIBRATION OF A LOW-PRESSURE MERCURY-VAPOR LAMP (MODEL LO 735A-7, HANOVIA) APPENDIX A RELATIVE CALIBRATION OF A LOW-PRESSURE MERCURY-VAPOR LAMP (MODEL LO 735A-7, HANOVIA) A low-pressure mercury-vapor lamp, identical to the type used in the photolysis experiments, was calibrated by comparison with a tung- sten ribbon-filament lamp having a quartz window. The tungsten lamp, a secondary standard of spectral radiance (uW/ster-nm-mmz), was supplied by the National Bureau of Standards as lamp No. U-202 and had been calibrated against a blackbody. Figure Al shows the geometrical arrangement of the lamps, spherical mirror, and Cary Model 14 spectrOphotometer used in the calibration. The spherical mirror was mounted on a pivot which allowed the lamps to be switched alternately into the optical path without changing the position of either lamp. The lamps were operated in strict accordance with their specifications which are as follows: the NBS lamp at 35A at 6Vac, and the mercury-vapor lamp at 1.0A at 36 Vdc. For the mercury-vapor lamp all other Operating conditions were the same as described in the Experimental section of the thesis. The 115 Vac primary power supplies for the lamps were regulated by means of Sola transformers. The emission spectrum of the tungsten lamp was recorded by means of the Cary 14 spectrophotometer. Figure A2 shows a composite graph of this spectrum, W, and the absolute output (spectral radiance), A, of the standard tungsten lamp. The scale of the ordinate is in terms of rela- tive energy for curve W, but in spectral radiance for curve A. In recording this spectrum, the slit width was changed at 3100 A from 122 123 SPHERICAL MIRROR (3.5" diom) Aluminized front surface Focal pt. of mirror and entrance lens of instrument MTungsten Lamp (N 88) - / ”9 W” “m" Shield from direct illumination in Housing @ 25° _— CARY MODEL 14 1 SPECTROPHOTOMETER 1 1 Figure A1. Arrangement of lamps, spherical mirror, and spectrophotom— eter for lamp calibration. 124 10.0 : 1 1 1 l r- . C - ‘. ” TUNGSTEN LAMP, w * (CARY RESPONSE) . (RELATIVE ENERGY) 1.0 E- ° -—-l P TUNGSTEN LAMP, u-202 I - SPECTRAL RADIANCE ~ - -l L -120 g 0.1‘ . ‘-° 7'“ In 1- . _1 ( l— "' 0.5 . R. SPECTRAL RESPONSE 4 CORRECTION CURVE __,,. 0.01 L— 1 0.1 I Z _ ./ -i 0901' 1 l l l J 1 2500 . 5000 3500 4000 4 5000 5500 6000. IMMELENGTH (1 Figure A2. Energy X§_wavelength. For NBS tungsten lamp: A is the absolute output (spectral radiance), W the response on Cary spectrophotometer (relative energy). 125 0.030 to 0.300 mm in order to Obtain the entire region shown. A factor of 100 was taken into account in computing the relative energies for the lower wavelength region since the energy passing through the slits is proportional to the square of the slit width. As for the plots of the two sets of data, some uncertainty (perhaps 5%) arises from the fact that the lower region was not a perfect continuation of the upper wave- length region. Presumably, this uncertainty is associated with the reproducibility and accuracy of the Slit settings. Spectral response correction factors, characteristic of the com- posite effect Of the grating, prism, and phototube were obtained from the ratios, Ai/wi’ of the two curves at each wavelength A of the mercury-vapor spectrum. The spectral data and calculated ratios are listed in Table A1. Curve R in Figure A2 is the spectral response correction curve for the entire region investigated (2500-6000 A). Table A1. The spectral radiance and Cary response for the standard tungsten lamp (U-202) and the spectral response factors for a Cary 14 spectrophotometer. Tuggsten lamp U-202 Spectral response Wavelength Spectral radiance Cary response Correction factors A, A uW/ster-nm-mm2 (relative energy) Ai/wi 2537 1.50 x 10"2 1.80 x 10'2 0.833 3126-32 3.75 x 10'1 1.09 0.343 3650 2.15 8.90 0.242 4047 5.51 2.50 x 101 0.221 4358 9.80 4.54 x 101 0.216 5461 4.12 x 101 8.63 x 101 0.477 5770-90 5.21 x 101 7.53 x 10 0.692 126 With respect to the mercury-vapor lamp, the maximum signal response, T, of each of the emission lines was recorded essentially manually and the values, as relative energies, are listed in Table A2. A complete and undistorted spectrum could not be recorded even at the slowest instrument scanning speed (1/2 A/sec) because of the inability of the pen to keep up with the signal for the most intense lines. Ideally, a mercury-vapor lamp should be calibrated against a standard lamp of the same type (i.e., low-pressure lamp with low-pressure lamp, etc.). In such cases, a relative calibration is obtained directly as the normalized ratios of the corresponding outputs, since the spectral band widths are essentially the same. In general, "in experiments where [similar] sources of radiation are being compared, no knowledge of the spectral reflectance of the auxiliary mirrors, the spectrometer_trans- mission characteristics, or the spectral sensitivity of the detector is required. Furthermore,when the same auxiliary optics are employed no measure need be taken of the spectrometer slit widths, or slit areas, provided the slit is fully and uniformly filled in both cases." The foregoing is strictly valid only when the same type of spectra are being compared (continuum y§_continuum, etc.), since the energy passed through the slits is proportional to the square of the slit width in the case of a continuum, but only proportional to the slit width for a line spectrum. For the geometry employed, the entrance slit was fully and uni- formly filled for both lamps. The calibration of a mercury-vapor lamp (line spectrum) against a tungsten-lamp (continuum) is not as straightforward a procedure since the spectral interval being investigated differs significantly (1) from one band to another (for the mercury-vapor spectrum) and (2) for the two 127 types of spectra being considered. Hence, the resolution of the measur- ing instrument must be taken into account. For a Cary Model 14 spectrOphotometer the spectral band width (resolution) is given by: Spectral band width (SBW) , 3. -- D°S + c + L where D is the reciprocal dispersion in A/mm S is the slit width in mm C is the Slit curvature mismatch in A L is the Rayleigh diffraction limit in A Graphs of these quantities 3§_wavelength (all are a function of wave- length) may be found in two Applied Physics Corporation publications. In short, each of the three right-hand terms compensates for the reduc- tion in intensity because the incident energy is distributed over a wider wavelength span. The Observed response (T) for each band was corrected in accordance with the following relationship which takes into account the instrument resolution and spectral response factors: A T(—-1-)SBW = 61 ”1 where I is the integrated intensity (in arbitrary energy units) of a given band having the center wavelength A. a is a prOportionality constant for the Cary response and the absolute output graphs in Figure A2. -The quantities T, SBW, A, and H have been described previously. The 128 relative band intensities (energy) were converted into relative band intensities (quanta) by applying the factor, A, A/2537 A; both sets of data were then normalized with respect to the 2537 A line. The Cary resolution data and relative spectral intensity data for the mercury- vapor lamp are given in Table A2. The reliability of the calculated spectral band width (SBW) data was checked for the 2537 A band for three different slit widths (1.0, 2.0, and 3.0 mm). For each slit width used, the experimentally deter- mined SBW was 5—6% higher than the calculated value (i.e., using the Applied Physics data). It was assumed that the deviation for the other slit settings would be in the same direction and about the same order of magnitude, since a plot of the square root of the instrument response at 5150 A (plateau region) by using the tungsten lamp y§_slit setting was linear over the region 0.030 to 0.005 mm. 129 $5.6 mm.m mw.H mm.w omm.o ooa.o HmH.o w.mm down mm.~ mo.H mm.H w.ma cum.o «NH.o mmo.o w.mm wmme mo.H oo.H mw.o 0.0H mmm.o oHH.o mHH.o w.~m mqoq HH.N HN.H N¢.H m.NH moq.o ooa.o mH~.o w.om omom ao.~ mo.a oq.H oo.m omm.o «wo.o o~m.o w.o~ «mam o.ooa o.ooa w.~w m.oma omo.o Hoo.o mnq.o «.mH nmmm mucosa xwuoom an H M .3mm M .A N .m aa\w .0 w .4 moauamOOOOa o>fiumamm monommmm AsOfiusaommmv Anmmuammmv gunman“: aOfimuommfin nmumOHo>m3 mama uomm>lmusouoz :MMMWOmmmm OOHWHMHWMHQ ounwmwmno Hmoounaoom A3885 7.3.2 3 Hoooz .aemH uomn>l>usuuoe a you dump muHmOOuOH O>Humaou mom muouoemumn noqusaomou «H Homo: humo .N< OHAOH APPENDIX B LIGHT ABSORPTION DATA FOR URANIUM(IV)-URANIUM(VI) SOLUTIONS APPENDIX B LIGHT ABSORPTION DATA FOR URANIUM(IV)-URANIUM(VI) SOLUTIONS Table Bl. Molar absorptivities of uranium(IV) and uranium(VI) for the wavelengths of the mercury-vapor emission spectrum. Sodium perchlorate-perchloric acid media, T N 25°. Columns (1) and (2): [H+] - 0.974, I - 1.00 M, Column (3): [n+1 - 1.00. 1 - 3.00 g, Molar absorptivity, E(M-1cm-¥) Wavelength Uranium(IV) Uranium(VI) K (1)“ <2>“'f (3)":f 2537 7.2 (5.07)c 423 (435)d 3130 <0.08c 47.8 3656 - N ceanaaoo >9 venuoups « no OH .>u«acaun« usuwu no neuuueum s am new uceuue> uo eusenuosn< s s< moo.o asoo.o mooo.oe nooo.oa ono.o mica . an.“ n .Asava. ado.o soo.o m.nm sna.° n~o.o n.aa ~n~.o o~s.o m.oo on~.o oo.a o.ooa na.n oo.a o.ooa n.no «H.Ha Amuvm aso.o non.o N.Hm oo~.o ooa.o s.oa ~no.o asn.o no~.o coo.a ~o.s ooo.~ o.ooa o.~s ano.o Aa~vm ooo.o Nos.o o.os o-.o aao.o ~.os ~on.c oon.o ana.o coo.a on.n ooo.~ s.oo s.~n one.“ Ao~oa moo.o asn.o o.oa o-.o ans.o m.oo oon.o ~nn.o mma.o ooo.a n~.s ooo.~ o.ooa o.mn saw.” RHNVE omo.o was.o o.~s ona.o ooo.o o.ma mon.o an~.o n~a.o soo.o mm.~ ooo.~ o.oo ~.n~ oaa.n .oave oao.o non.o o.kn oao.o oan.o o.oo oo~.o oma.o ~.sa noo.o ~oo.o as.» ooo.a n.0o e.ua ona.~ Agave aso.o o-.o m.on noo.o na~.o a.os ooa.o cao.o «.mo ano.o son.o soa.o ooo.~ n.oo ms.o oao.a Amado soo.o o o ama.o G.SN -o.o aaa.o o.~a ono.o am.o o.as oao.o ann.o o.sa nan.o aso.o H.ao -.n nona.o Anson “use: on Aso.nv Ana.ov x~a.~v Am.sso Annoy oa>ao noos< undo: oe sao.o “ma.o omo.o a~n.a ~o.~a Nioa . ooa.~ . .A~>O=_ Ros.o ms.a Hos.o o.~a aao.o ~n5.o o.s~ amo.o Hoa.o 6.0a hoo.o «mo.o mn.o soo.o oo.a as.n o~o.o «an.a Ass. sa~.o sea. Ns~.o H.Ao soo.o amn.o mm.a «soo.o h~a.o oa ooo.o ~mm.o “so.o Hoo.o oo.a n~.o nono.o oom.o Assn nas.° oo.a «ma.o ~.oa soo.o aas.o m.ma «so.o ana.o as.“ noo.o ~na.o mn.o noo.o oo.a no.~ non.o coo.n flood «No.0 nNswo son.o m.oa aa~.o okn.o nn.m “Ho.o o~H.o m~.m ~oo.o ~no.o ma.o ~oo.o oo.a oo.a «Na.c ~on.~ Anon oon.o noon ~a~.o ~oe.o n.~o ana.o Nom.o om.o snoo.o s-.o ao.a Hoo.o ~ns.o mao.o Hoo.o oo.a on.o aoo.o ooa.a Anon “>Hoa an 5 a< um >u. a< um >N n< u..— >N e< am >N ad em 5 e< Awe." x .3 sown—Jew unsuue> _uaaaua>a ensesuuu as.aav Ao.aac . A~.oav Amo.qwc Aao.quv Aao.nev oaSa» ooae undo: oaa.o ooa.o ~oo.o mao.o sao.o no.n Analog noon .ASaonouoa ”cannon“ noon ammo “nos omon onam annw « .4 .guuaoaoS-s .mOOH enfiomun mam mdoaunaom mmnmsoxo >9 wonuomnm usmfia mo mdosuomum .Nm OHQMH 1L32 G .>on .coa a nooano> xoaoaw .AH>v= + A>an mooaw .AH>V= is aw .A>Hv: “no noononnn Roancooca semis Hobos .ooosss . > “so oo.a - o .ooansaon no soooo xaao>aoooonon .maosn .mmms .mnlasos .omom .oman .amnw . n.s.m.~.a. o.oa sa.s mm.n NH.o n oH.a sa.aa AmNVE n.6a we." am.m aa.o n ms.a “mo.s leave n.4m sk.n mm.n ma.o n sa.a was.a Romeo m.na «m.n Ns.~ 03.0 n no.3 sew.m Aa~vn n.oa m4.m as.~ am.o n Ho.a oas.n Ao~ve m.sa ~m.~ mm.~ mo.o n os.a wma.~ leave m.mo oa.~ om.~ sa.o n mo.~ ass.a “wave a.ma am.a ~m.~ sa.o n ma.a nsmn.o lance 2H>02 An.mav A~o.a n an.nv wm~.o .mwm.o a.~m ma.a oam.o s.m~ mo.o wa.o n ow.s Nam.m Amen ~mo.o moo.o n.6a «a.o ~oo.o oooonoeon ow.m ma.o n os.~ wan.o Reva os~.o 0mm.o m.o~ no.4 mam.o a.oa no.6 ~m.a n sm.s oma.n loco .mma.o ~a~.o a.ma ca.a soa.o o.m~ so.m oo.o n 65.6 ~mm.~ Amen mao.o maa.o H.oa mm.o saa.o ~.w~ sm.n o~.o n am.m ooa.a Assn A>Hvs o.n. o.m.aa a. s.m.~‘ ‘ «as a and 4 Has Amos x we “Non x m0 ooaosaom unnaoo> A213 x Alcoa C Amaloa x Hlose C 3v «0 30 mo >OHWN A213 x alone 5 ~5mequ Amaloa x Minus 3 m mundane: owosnuxm .4on .4me >me .N >us Camuw AoHv Adv Amy ARV on Amv Aqv Amv va flay .OOAOAmcouOH monuomnm OOOOHOOHOO :OH3 moumu owomnoxm mnfiumamnuoo OH new: moon mo xumaasm .mm nanny APPENDIX C COMPUTER PROGRAM AND SAMPLE OF INPUT-OUTPUT DATA 00 APPENDIX C COMPUTER PROGRAM AND SAMPLE OF INPUT-OUTPUT DATA COOP, ll499,HOESCHELE,,'G FTN,L,E,G. PROGRAM RATES DIMENSION X(100), Y(100), D(100), CPM(100), lABS(lOO),EN(100), COMP(200), 22(100) READ 11, NRUN . NRUN-NO. OF RUNS FOR WHICH DATA HAS BEEN SUBMITTED 11 FORMAT (12) 1 READ 5, (COMP(I), I-I,12) COMP(1)-IDENTIFICAT10N OF THE DATA FOR A GIVEN EXCHANGE RUN 5 F0RMAT(18A4) READ 100, A4, A6, BKG A4 AND A61ARE THE CONCENTRATIONS OF U(IV) AND U(VI), RESPECTIVELY BKG-BACKGROUND ACTIVITY IN COUNTS/MINUTE 100 F0RMAT(3F10.0) READ 100, CO, ABO, ENO CO, ABO, ENG-COUNTs/MINUTES,AESORDANCE, AND NORMALIZATION FACTOR FOR THE ZERO-TIME SAMPLE, RESP. READ 100, CINF, ABINF, ENINF CINF, ABINF, ENINF-COUNTs/MINUTE, ABSORBANCE, AND NORMALIZATION FACTOR FOR THE INFINITE-TIME SAMPLE, RESP. READ 101, J, (CPM(I), ABS(I), EN(I),X(I), I-1, J) CPM(I), ABS(I), EN(I), X(I), - COUNTS/MINUTE, ABSORBANCE NORMALIZATION FACTOR, AND CORRESPONDING TIME(MINUTES) OF A GIVEN SAMPLE(I) 101 FORMAT (13/(4F10.0)) BKG-BKG/60.0 PRINT 200 PRINT 5, (COMP(I), 1-1,12) PRINT 102, A4, A6, BKG PRINT 102, co, ABO, ENO PRINT 102, CINF, ABINF, ENINF 102 FORMAT (3F13.6) PRINT 103, J, (GPM(I), ABS(I), EN(I), X(I)- I-1,J) 103 FORMAT (13/(4F13,6)) CALCULATION OF SPECIFIC ACTIVITIES, SO+HINF 00-00/60.0 CINF-CINF/60.0 S0-((CO-BKG)*ENO)/ABO HINF-((CINF-BKG)*ENINF)/ABINF CALCULATION OF LN(l-F) VALUES DO 78 1-1, J CONVERSION OF MINUTES TO SECONDS X(I)-X(I)*60.0 CPM(I)-CPM(I)/60.0 z-((CPM(I)-BKG)*EN(I))/ABS(I) 133 78 70 120 122 123. 130 23 25 140 400 200 205 134 z-1.0-(z-S0)/(HINF-SO) ZZX(I)-Z 22X(I) - l-F FOR SAMPLE(I) Y(I)-LOGF(Z) LOGF(Z) CORRESPONDS TO THE LN(l-F)FOR SAMPLE(I)---LN(1-F) VALUES ARE THE Y TERMS IN THE LEAST SQUARES ROUTINE CONTINUE K-J BEGIN LEAST SQUARES ROUTINE SUMXY-0.0 SUMx=0.0 SUMY-0.0 SUMX2-0.0 DO 70 1-1, J SUMXY-SUMXY+Y(I)*X(I) SUMX-SUMX+X(I) SUMY-SUMY+Y(I) SUsz -x<1)**2 + SUsz Z-K SLOPE-(Z*SUMXY-SUMX*SUMY)/(Z*SUMX2-SUMX**2) B-(SUMY*SUMX2-SUMXY*SUMX)/(Z*SUMX2-SUMX**2) DO 120 1-1, J CALCULATION OF STANDARD DEVIATIONS, D(I)-SLOPE*X(I)+8-Y(I)~ DO 123 I-l, J IF(X(I)) 122, 122, 123 D(I)-0.o CONTINUE SUMD2-0.0 DO 130 I-1, J SUMDZ-SUMD2+D(I)**2 STDEv-SQRTF(SUMD2/(z-2.)) CONST-2.o OPTDEv-CONST*STDEV IF(J-K) 23, 23, 400 D0 140, 1-1, J IF(ABSF(D(I))-OPTDEV) 140, 25, 25 Y(I)-0.0 X(I)-0.0 K-K-l CONTINUE IF (J-K) 400, 400, 2 SDEvs-STDEV/(SQRTF(SUMX2-(SUMX**2)/Z)) SDEVI-STDEV*SQRTF(SUMX2/(Z*SUMX2-SUMX**2)) CALCULATION 0FHALF-TIME,THALF THALF- -(LOGF(2.0))/8L0PE OMF-EXPF(8) PRINT 200 FORMAT (1H1. ///) PRINT 205 FORMAT(10x, 31HMCKAY DATA FOR J.D. HOESCHELE/) PRINT 210, (COMP(I), 1-1,12) 210 220 230 231 240 250 260 271 270 280 289 "290 300 500 135 FORMAT(10X,12A4///) CALCULATION OF THE RATE OF EXCHANGE, RXG Ax-(A4*A6)/(A4+A6) PRINT 220, SLOPE F0RMAT(10X, 8HSLOPE - 812.4/) PRINT 230, B F0RMAT(10X, lZHINTERCEPT = E12.4/) PRINT 231, OMF F0RMAT(10X, 14HAT T=0, 1-F = E12.4/) RXG--SL0PE*AX PRINT 240, RXG F0RMAT(10X, 27HRATE 0F EXCHANGE, M/SEC. = E12.4/) PRINT 250, THALF F0RMAT(10X, 18HHALF TIME, SEC. - E12.4/) PRINT 260, STDEV F0RMAT(10X, 24HSTD. DEV. FOR SINGLE Y= E12.4/) PRINT 270, SDEVS SDRXG=AX*ADEVS PRINT 271, SDRXG FORMAT(10X, 19HVARIATION IN RXG- E12.4/) FORMAT(10X, 21HVARIATION IN SLOPE- E12.4/) PRINT 280, SDEVI FORMAT(10X, 25HSTD. DEV. FOR INTERCEPT- E12.4///) PRINT 289 FORMAT(10X, 47H TIME, SECONDS LN(1-F) l-F /) DO 300 1-1, J PRINT 290, X(I), Y(I), 22X(I) FORMAT(10X, 3E16.4) CONTINUE NRUN-NRUN-I IF(NRUN) 500, 500, 1 CONTINUE END EXECUTE . 136 INPUT DATA P(5)-U(IV) VARIATION (H+)=1.00, I=2.00 M, T=25.0 0.023920 402.500000 4205.600000 403.500000 414.200000 576.700000 511.500000 644.200000 657.400000 835.600000 0. .472000 0. 0 0000000 027640 389000 .428000 .448000 .450000 .464000 .468000 .428000 .500000 0.466667 1.021000 1.000000 .010400 .005100 .017300 .000600 .999500 .003500 .997800 CJF‘CJF‘P‘P‘P‘ OUTPUT DATA 46.000000 80.000000 180.000000 250.000000 285.000000 325.000000 400.300000 P(5)-U(IV) VARIATION (H+)=l.00, I=2.00 M, T=25.0 SLOPE - -3.5954-006 INTERCEPT - 7.2309-003 AT T-o, l-F - 1.0073+000 RATE OF EXCHANGE, M/SEC. - 4.6104-008 HALF TIME, SEC. - 1.9279+005 FOR SINGLE y- 1.2698-002 FOR SLOPE- 6.6914-007 STD. DEV. FOR RXG- 8.5804-009 FOR INTERCEPT- 1.0185—002 STD. DEV. STD. DEV. STD. DEV. TIME, SECONDS 2.7600+003 4.8000+003 l.0800+004 l.5000+004 l.7100+004 l.9500+004 2.4018+004 LN(l-F) -7.7208-003 -5.6919-003 -4.4307-002 -2.3700-002 -5.2296-002 -6.9388-002 -8.4172-002 l-F 9.9231-001 9.9432-001 9.5666-001 9.7658-001 9.4905-001 9.3296-001 9.1927-001 APPENDIX D ORIGINAL KINETIC DATA APPENDIX D ORIGINAL KINETIC DATA Table D1. Dependence of exchan e rate on concentration of uranium(IV). [U(VI)] = 2.76 x 10‘ ; [H+] = 1.00; I = 2.00 E; T - 25.0°; lamp I. Experiment l-F t(min) -2 P(4) [U(IV)] . 1.20 x 10 0.998 30.0 R - 3.87 t 0.20 x 10'8 Msec’l 0.976 66.0 Intercept - 0.998 t 0.004 0.972 100.0 0.945 180.0 0.885 410.0 0.858 565.0 _2 PO) [U(IV)] - 2.39 x 10 0.992 46.0 R - 4.76 i 0.46 x 10’8 Msec’1 0.994 80.0 Intercept - 1.005 t 0.005 0.957 180.0 0.949 285.0 0.933 325.0 0.919 400.3 P(6) . ‘ [U(VI)] - 4.37 t 1.32 0.998 60.0 R.- 4.37 i 1.32 x 10-8‘M_secfl 0.961 130.0 Intercept - 0.992 t 0.011 0.953 222.0 0.957 300.0 0.933 460.0 137 138 Table D1 (cont'd.) Experiment l-F t(min) -3 P(7) [U(IV)] - 5.98 x 10 0.980 75.1 R - 2.46 i 0.15 x 10’8 Msec'1 0.939 150.0 Intercept - 0.997 t 0.007 0.943 185.0 0.935 250.0 0.897 350.0 0.858 525.0 0.855 530.0 0.812 650.0 _3 P(8) [U(IV)] - 5.98 x 10 1.031 90.0 R.- 1.85 i 0.57 x 10’8 _Msec’l 1.035 180.0 Intercept - 1.083 r 0.03 1.027 225.0 1.068 375.0 0.943 525.0 0.891 794.0 -2 P(9) [U(IV)] - 8.37 x 10 0.998 125.0 R - 7.80 r 0.98 x 10'8 Msec'l (4 pts) 0.956 300.0 Intercept - 1.028 r 0.016 0.951 400.0 0.894 600.0 (0.898) (750.0) (0.908) (800.0) 139 Table D2. Dependence of exchange rate on uranium(VI) concentration. [U(IV)] = 5.87 X 10' ; [H+] = 1.00; I - 2.00 M, T = 25.0°; P(17-21), lamp 1; P(26-28), lamp II. Experiment l-F t(min) _3 P(17) [U(VI)] - 7.40 x 10 0.947 110.0 R - 1.95 r 0.14 x 10"8 _M sec’1 0.935 175.0 Intercept . 0.985 r 0.010 0.892 272.0 0.866 320.0 0.869 350.3 0.852 400.0 0.816 585.1 0.781 614.8 _2 P(18) [U(VI)] - 1.47 x 10 0.988 50.3 R.- 2.05 i 0.14 x 10'8_M_sec‘1 0.961 115.3 Intercept - 0.999 t 0.008 0.952 150.0 0.940 200.2 0.909 335.0 0.889 475.0 0.812 625.0 0.799 780.0 ~ _2 P(19) [U(VI)] - 2.96 x 10 0.994 25.0 R- 1.76 i 0.03 x 10'8 M sec’1 0.992 50.0 Intercept - 1.002 t 0.001 0.986 75.2 0.976 110.0 0.968 160.0 0.948 265.0 0.901 495.1 0.888 541.1 0.886 550.3 140 Table D2 (cont'd.) Experiment l-F t(min) _2 P(20) [U(VI)] - 5.92 x 10 0.992 25.2 R- 1.61 r 0.37 x 10‘8 M sec"1 0.984 60.2 Intercept - 0.992 t 0.012 0.975 100.1 0.942 225.0 0.960 275.0 0.932 359.2 0.891 400.0 0.927 500.0 _2 P(21) [U(VI)] - 8.87 x 10 0.998 20.0 R - 1.09 t 0.10 x 10'8 M sec"1 0.994 50.1 Intercept - 0.9962 i 0.004 0.983 75.0 0.978 151.3 0.970 200.1 0.969 300.0 0.935 410.0 0.932 510.0 0.940 550.5 0.921 701.2 _2 P(26) [U(VI)] - 7.44 x 10 0.998 20.2 R - 1.94.: 0.18 x 10"8 M sec"1 0.992 65.0 Intercept - 1.002 t 0.004 0.982 100.0 0.972 140.0 0.957 181.2 0.959 200.0 0.946 250.0 0.946 305.0 141 Table D2 (cont'd.) Experiment l-F t(min) _2 P(27) [U(VI)] - 9.66 x 10 0.993 36.3 R - 1.45 i 0.11 x 10"8 Msec'l 0.987 60.0 Intercept - 0.996 i 0.003 0.983 95.0 0.966 155.0 0.967 205.0 0.956 250.1 0.955 300.0 0.940 360.0 ' -1 P(28) [U(VI)] - 1.11 x 10 0.980 60.0 R - 1.10 r 0.12 x 10’8 M nec'l 0.985 100.0 Intercept - 0.993 r 0.004 0.984 130.0 0.964 205.0 0.964 250.0 0.948 405.0 0.939 460.0 “‘1 142 Table D3. Dependence of exchan e rate on hydrogen ion concentration. [U(IV)] - 1.20 X 10' ; [U(VI)] - 2.76 X 10‘2; I - 2.00 M, T - 25.0°; lamp 1. Experiment l-F t(min) + P(10) [H 1 - 1.80 0.986 110.0 R.- 2.00 t 0.24 x 10’8 Msec‘l 0.978 ' 210.0 Intercept - 1.006 i 0.009 0.979 275.0 0.941 360.0 0.948 450.0 0.914 565.0 0.931 678.0 0.891 775.0 0.894 836.0 + P(ll) [H 1 - 1.50 0.985 60.0 R - 2.17 r 0.12 x 10’8 M eec‘l 0.979 130.0 Intercept - 0.999 r 0.005 0.973 226.0 0.954 325.0 0.917 480.0 0.934 600.0 0.891 715.0 0.892 - 770.0 0.876 835.0 + P(12) [H 1 - 1.00 0.990 58.0 R- 2.49 i 0.07 x 10'8 M sec"1 0.987 100.0 Intercept - 1.003 r 0.002 ‘ 0.964 250.0 ' 0.950 275.2 0.933 415.0 0.914 501.5 0.898 600.0 0.864 840.0 Table D3 (cont'd.) 143 Experiment l-F t(min) + P(l3) [a 1 . 0.750 0.981 60.0 R - 3.01 1 0.07 x 10'8 _rg_sec‘l 0.970 100.1 Intercept - 0.988 t 0.003 0.948 180.2 0.927 280.1 0.911 350.0 0.889 485.3 0.860 650.8 0.858 740.1 0.841 770.4 0.812 900.0 + P(14) [H 1 - 0.500 0.979 50.3 R - 4.40 .+. 0.12 x 10'8 1»; sec” 0.968 108.0 Intercept - 0.997 t 0.005 0.875 400.0 0.813 670.0 0.793 700.9 0.776 800.0 + 9(15) [u 1 - 0.250 0.986 25.0 R.- 6.85 t 0.25 x 10"8 g_eec' 0.957 61.0 Intercept - 0.997 t 0.006 0.937 156.0 0.871 258.0 0.836 350.7 0.803 450.8 0.730 625.5 Table D3 (cont'd.) 144 Experiment l-F t(min) + P(16) [a 1 - 0.179 0.932 54.0 R - 11.4 i 0.5 x 10'8 msec’1 0.925 80.0 Intercept - 0.984 t 0.010 0.929 100.0 0.835 180.0 0.803 257.0 0.768 300.0 0.699 400.0 0.648 525.0 + T(16) (thermal) [H 1 - 0.179 0.964 50.2 R - 1.70 i 0.18 x 10'8 gsec'1 0.965 100.4 Intercept - 0.972 t 0.008 0.952 239.9 0.952 300.4 0.917 360.0 0.877 650.0 0.815 1514.0 145 Table D4. Dependence of exchange rate on temperature. [U(IV)] - 5.87 x 10- , [U(VI)] . 7. 60 x 10-3; [H+ 1= 1.00; I - 2.00 g; P(23,24), lamp II; P(25), lamp IIa. Experiment l-F t(min) P(23) T - 25.0° 0.992 20.0 R.- 2.95 r 0.08 x 10-8‘§_sec-l 0.978 52.8 Intercept - 1.005 t 0.005 0.967 100.0 0.911 175.2 0.873 250.0 0.824 356.0 0.790 460.0 0.726 615.0 P(24) T - 14.23° 0.996 30.3 R,- 1.41 r 0.08 x 10'81_4_z-3ec'l 0.987 60.9 Intercept - 1.000 t 0.003 0.972 100.0 0.960 135.5 0.951 190.2 0.941 235.5 0.932 300.0 0.911 355.0 9(25) T - 32.0° 0.974 25.5 R - 2.45+ 0. 20 x 10'8M _sec 1 0.982 55.0 Intercept - 1.000 t 0.007- 0.975 80.0 0.943 125.0 0.915 185.0 0.906 225.0 0.907 250.5 0.848 350.0 146 Table D5. Dependence of exchange rate on the 233U concentration. [U(IV)] = 2.50 x 10-2; [U(VI)] = 2.74 x 10-2; [n+1 = 1.00; [Tartaric acid] = 0.130; I - 2.00 g; T - 25.0°. Thermal experiments. Experiment l—F t(min) R(l) [2330(v111 - 4.52 x 10'6 0.969 ' 931.0‘ R - 4.00 t 0.38 x 10'9 Piece“1 0.940 1423.0 Intercept = 0.977 t 0.010 0.957 2481.0 0.907 3901.0 0.910 4377.0 0.845 5753.0 0.833 7203.0 0.837 9606.0 0.817 10096.0 0.802 11642.0 0.758 12521.0 233U