THE SELVER REDUCTOR FOR ORGANEC COMPOUNDS LN GLACIAL ACETIC ACID Thesis {or ”19 Degres a; M. S. WCEIGAN STATE UNWEESETY Adam J. Gahn 1963 w 5 TH ESIS LIBRARY Michigan State University ABSTRACT THE SILVER REDUCTOR FOR ORGANIC COMPOUNDS IN GLACIAL ACETIC ACID by Adam J. Gahn Silver metal will not reduce the organic nitro compounds tested in glacial acetic acid. Quinone is reduced by silver due to formation of quinhydrone; however, this is a special case and not representative of most reducible organic compounds. Dissolved silver in acetic acid may be determined accurately and rapidly by potentiometric titration with standard chloride solution using the mercury-mercurous acetate electrode in acetic acid as reference electrode and clean silver metal as the indicator electrode. The accuracy and precision of this method compare favorably with the familiar gravimetric precipitation. The formal potential of the silver-silver(1) couple in acetic acid has not been determined. The silver species presentlin acetic aéid from 10'3 M to 10"5 M obeys the Nernst equation with an n value of one. THE SILVER REDUCTOR FOR ORGANIC COMPOUNDS IN GLACIAL ACETIC ACID BY Adam J. Gahn A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Chemistry 1963 ACKNOWLEDGMENT The author wishes to express his grati- tude to Dr. Kenneth G. Stone for his guidance and unfailing encouragement throughout the entire investigation and preparation of this thesis. **************** ii VITA Name: Adam Joseph Gahn Born: November 11, 1938 in Dayton, Ohio Academic Career: Kiser High School, Dayton, Ohio (1953-1957) Ohio Wesleyan University, Delaware, Ohio (1957-1961) Michigan State University, East Lansing, Michigan (1961- ) Degrees Held: A. B. in Chemistry, Ohio Wesleyan University (1961) iii TABLE OF CONTENTS Page INTRODUCTION O O O O O O O O O O O O O O O O O O O O O O O O O 1 EXPERIMENTAL O O O O O O O O O O O O O O O O O O O O O O O O 5 Chemicals......................... 6 StandardSolutions..................... 7 ApparatusUsed...................... 9 ApparatusPreparation.................. 9 ReferenceElectrode................ 9 Silver Reducing Column. . . . . . . . . . . . . . . 9 Indicating Electrode. . . . . . . . . . . . . . . . . 10 ReductionProcedures................... 10 IntheBeaker....................10 OntheColumn....................11 TitrationProcedures................... 11 Potentiometric Determination of Silver . . . . . . ll Titration of Hydrogen Ion Using an Indicator. . . . 12 DISCUSSION OF RESULTS. O O O O O O O O O O O O O O O O O O O 13 Reduction of Organic Compounds . . . . . . . . . . . . . 14 Organic Nitro Compounds. . . . . . . . . . . . . . 14 Quinone.......................14 Determination of Dissolved Silver . . . . . . . . . . . . 17 Standard Chloride Solution . . . . . . . . . . . . . 17 Standard Silver Solutions . . . . . . . . . . . . . . 17 Study of the Potential of the Silver Electrode in Glacial AceticAcid....................... 20 SUMMARY AND CONCLUSIONS. . . . . . . . . . . . . . . . . 28 LITERATURE CITED O O O O O O O O O O O O O O O O O O O O O O 30 iv TABLE II. III. IV. VI. VII. VIII. IX. LIST OF TABLES Volumetric Determination of Silver Oxidized by QuinoneO O O O O O O O O O O O O O O O O O O O O O O O O Analysis of Calcium Chloride in Acetic Acid . . . . . Analysis of AgClO4 from AgOAc (dissolved by excess Hc104)O O O O O O O O O O O O O O O O O O O O O O O O O O Titration of AgClO4 in Acetic Acid (effect of varied exceSSOfHCI-O4)ooocoo-9000000000000 Analysis of AgClO4 from AgOAc (dissolved by measured amount HC104) o o o o o o o o o o o o o o 0 Analysis of AgClO4 in Acetic Acid (from AgZO) . . . . Analysis of AgClO4 in Acetic Acid (from solid AgClO4) Comparison of Gravimetric and Volumetric Methods. Reaction of Redox Indicators in HOAc vs. Silver . . . . Conductance of AgClO4 Solution vs. Concentration . . Page l5 l8 19 19 21 21 22 22 25 27 LIST OF FIGURES FIGURE Page 1. Ultraviolet Spectra in Acetic Acid. . . . . . . . . . . . 16 2. Potentiometric Titration of Ag+ with Cl' . . . . . . . . 23 3. Electrode Potential vs. log Silver Concentration. . . . 26 vi INT RODUCT ION One of the most useful methods for the quantitative determin- ation of organic compounds containing a reducible function is the reduction of that function followed by a measurement of its disappear- ance or the appearance of the product reduced species. Metal reductors have long been used for the determination of inorganic species in aqueous solution but have not been generally applied to organic analysis. Keller demonstrated that both lead and zinc could be used to reduce nitro compounds, with the resulting amines and metal acetates being determined as bases in glacial acetic acid by radio-frequency titration with standard perchloric acid (7). Silver metal was proposed as a quantitative reductor of aqueous ferric ion as early as 1934 by Walden, Hammett, and Emonds (14). The subsequent extensions of this idea have been reviewed by Stegemann (12). Since the redox potential of silver metal is lowered by the presence of an excess of precipitating anion, for example, chloride in water, the existence of similar species in acetic acid suggested that silver might be used in a reducing column in glacial acetic acid. To study the electrode reactions in this nonaqueous system, a number of reference and indicator electrodes were considered. Conant and Hall (5) proposed an aqueous saturated calomel electrode as reference with contact to the acetic acid through a glass frit: Hg | ngClz(s), KCl (sat'd in H20), frit, HOAc Scaramo and Arnaldo (10) used mercury-mercurous acetate plated on gold and immersed in acetic acid: Au, Hg I HgOAc (s), HOAC The potentials of both of the above electrodes tended to drift with time. The potential of the former varied because of its liquid junction and the latter because of the thinness of the plated surface. Reproducible, drift—free electrodes were at best secondary standards. Kolthoff and Bruckenstein (4) proposed modification of the calomel electrode for use with acetic acid solvent: Hg, ngClz I NaCl, NaClO4, HOAc Schwabe (ll) assembled an electrode directly related to acetic acid in that the anion is acetate: Hg I Hg; (OAc); (s), NaOAc, HOAc Since chloride ion was to be determined in the study, the chloride-free mercury-mercurous acetate system using sodium perchlorate as electrolyte was chosen as a secondary reference. The original electrode of this type was proposed by Stone and Al-Qaraghuli (1). Hg I Hg; (OAc); (s) I) NaClO4 (sat'd in HOAc), HOAc The potential of this electrode still contains a junction potential but any variation has been minimized by the use of saturated solutions. Experimentally, the electrode has proven satisfactory. The choice of an indicator electrode for use in glacial acetic acid was based on consideration of the literature. Hills (6) reviewed the potentiometric measurement of hydrogen ion. One of the few redox systems reported in acetic acid is the detection of ceric ion by means of a platinum indicator electrode with glass as the reference electrode at constant pH by Rao and Vasudeva (9). Metallic indicator electrodes sensitive to either silver or halide ions in non-aqueous solvents are described by Bishop (3). Tomicek and other Czechoslovakian investigators succeeded in the potentiometric titration of chloride and bromide in acetic acid using a silver-silver chloride indicator electrode as summarized by Stock and Purdy (13). The standard electrode potential of the silver-silver chloride couple in glacial acetic acid is reported by Mukherjee to be +0.6180 volts versus a standard hydrogen on platinum reference (8). Again, a general review of indicator electrodes in acetic acid is presented by Hills (6). EXPERIMENTAL Chemicals The chemicals used in this investigation were not purified unless otherwise stated. Generally, analytical reagent or A.C.S. specifi- cation grades were used. The relatively impure nitro compounds could be analyzed in the process of the determination if necessary. The compounds used, their commercial source and labeled purity are as follows: Acetic acid, Mallinkrodt, analytical reagent and Allied (Baker and Adamson), A.C.S. Reagent Acetic anhydride, Matheson, Coleman, and Bell, A.C.S. Reagent Ammonium hydroxide, Mallinckrodt, Analytical Reagent Calcium chloride, Allied, Technical Crystal Violet, Eastman, Certified Reagent Erioglaucin, National Aniline, C.P. Hydrochloric acid, Mallinckrodt, Analytical Reagent Hydroquinone, Eastman, C.P. Lithium Chloride, Mallinckrodt, Analytical Reagent Mercurous Acetate, Amend, C.P. Mercury, Baker, Analyzed Reagent Methylene Blue, Matheson, Coleman, and Bell, Certified Reagent Neutral Red, Matheson, Coleman and Bell, C.P. Nitric acid, Mallinckrodt, Analytical Reagent ortho-Nitrobenzoic acid, Eastman, C.P. Nitrogen, Liquid Carbonic, Technical ortho-NitrOphenol, Eastman, C.P. para-Nitrophenol, Eastman, C.P. Perchloric acid, Baker, Analyzed Reagent Phenosafranine, Eastman, C.P. Potassium Acid Phthalate, Allied (Baker and Adamson), A.C.S. Reagent, Primary Standard Quinhydrone, Eastman, C.P. Quinone, Eastman, Practical Silver, G. F. Smith, Reagent Silver acetate, Baker, Purified Silver Nitrate, Baker, Analyzed Reagent Silver Oxide, Merck, Technical Silver Perchlorate, G. F. Smith, Reagent Sodium Acetate, anhydrous, Fisher, certified reagent Sodium Acetate, trihydrate, Baker, Analyzed Reagent Sodium chloride, Mallinckrodt, Analytical Reagent Sodium perchlorate, G. F. Smith, Anhydrous Reagent Zinc, Baker, to meet A.C.S. specifications. Standard Solutions Standard perchloric acid in acetic acid was prepared by diluting 8. 5 m1. of 72 per cent perchloric acid to about 1500 ml. with acetic acid and then adding 20 ml. of acetic anhydride with constant swirling. The solution was then diluted to exactly two liters with more acetic acid. The solution was standardized by reaction with accurately weighed amounts of dried potassium acid phthalate to the blue-green endpoint of crystal violet indicator (three drops 0. 1 per cent solution in ac etic acid). Standard sodium chloride solution was prepared by dissolving an accurately weighed amount of dried compound in water and diluting to one liter. Standard sodium acetate in acetic acid was prepared by first analyzing wéighe‘dsamp’les of sodium acetate trihydrate with previously prepared standard perchloric acid. Since the sodium acetate tri- hydrate appeared to be of primary standard quality, an accurately weighed amount was dissolved in acetic acid and diluted to exactly two liters. Later titrations of aliquots of this solution with standard perchloric acid agreed with the direct method of preparation. In all sodium acetate versus perchloric acid titrations, crystal violet was used as indicator. Standard silver in acetic acid was prepared by dissolving an accurately weighed amount of silver acetate in 70 ml. of acetic acid, adding an excess of 72 per cent perchloric acid (about 5 ml.) with constant swirling and diluting the resulting solution to 100 ml. The solu- tion was standardized by both potentiometric titration versus standard aqueous sodium chloride solution and by gravimetric precipitation of silver chloride. I Standard calcium chloride in glacial acetic acid was prepared by dissolving a weighed amount of the technical grade compound in about two liters of acetic acid. This solution was also standardized by potentiometric titration into an aliquot of the standard silver solution and by gravimetric precipitation by the addition of an excess of silver in the form of aqueous silver nitrate. Apparatus Us ed A Beckman Zeromatic pH meter equipped with a grounded solu- tion shield, a chloride-free mercury-mercurous acetate reference electrode (1), and a commercial silver electrode, Fisher Scientific Co. No. 9-312-37, was used for all potentiometric titrations. The silver electrode was used without special preparation except for the study of its potential as a function of dissolved silver concentration. Changes in acetic acid conductivity as a function of various salt concentrations were measured with an Industrial Instruments Corpora- tion Model R. C. conductance bridge. The constant of the cell used in this operation was about 0. l. A Beckman Model DK-2 spectrophotometer was used to measure the ultraviolet absorption spectra of various organic mixtures in glacial ac etic acid. Apparatus Preparation Reference Electrode The reference electrode used is essentially that described by Al-Qaraghuli and Stone (1) except that the jacketing solution contained saturated rather than 0. 5 M sodium perchlorate in acetic acid. This change was made to facilitate preparation and gain reproducibility. Silver Reducing Column The silver reducing column was prepared in a typical Jones reductor column. Glass wool plugs were inserted above and below the granular silver metal packing to keep it in place and prevent fouling of the glass stop-cock. 10 Indicating Electrode Throughout most of the present work, the silver electrode was used just as it arrived. The sensitive surface, dull gray in appearance, was stored in glacial acetic acid between uses. Later, in the study of silver electrode potential, the surface of the electrode was cleaned by immersion in concentrated ammonium hydroxide followed by immersion in dilute nitric acid. .After a rinse in acetic acid, the electrode appeared to have the familiar flat white surface of clean silver metal. -Repeated washing as above did not change the behavior of the electrode except at very low dissolved silver concentrations as will be discussed later. Reduction Proc edures In the Beaker The beaker reduction procedure was devised in order to study efficiently the reducing properties of silver metal in glacial acetic acid. In this technique, a weighed amount of reducible organic compound was dissolved in about 40 ml of acetic acid in a 150 ml. beaker. The beaker was covered with a watchglass and a glass tube introduced beneath the surface of the solution to bubble dry nitrogen through the sample. After purging with nitrogen for fifteen minutes, approximately 0. 1 g. of granular metallic silver washed in dilute nitric acid was introduced to the solution and allowed to stand in contact with the solution under nitrogen purge until a constant potential between the silver and mercury- mercurous acetate electrodes was observed. The general procedure was varied at times by the addition of different amounts of standard perchloric acid solution just prior to the addition of the metallic silver. 11 On the C olumn The silver reducing column was used to reduce methylene blue by passing about O. 01 g of methylene blue hydrochloride in acetic acid plus excess perchloric acid through the column. The dye solution was eluted from the column at about one drop per second. .Precaution was taken so that at no time was the column allowed to run dry lest channelling of the column or oxidation of the reduced substance occur. Titration Proc edures Potentiometric Determination of Silver The silver ion formed by the oxidation of silver metal was titrated with standard calcium chloride in acetic acid. The reduction mixture was decanted from the metallic silver and the latter washed with more acetic acid. The total solution including the washings was placed in a 150 ml beaker, a Teflon coated magnetic stirrer bar and the electrodes were inserted, the nitrogen purge was continued. After noting the original potential, the addition of standard chloride was followed using the i700 millivolt scale of the pH meter. The (data obtained wereplotted on a graph showing the relationship of the potential as a function of the volume of standard chloride solution. The endpoint is assumed to be the mid-point of the potential jump, that is, the arithmetic mean of the potential curve before and after the equivalence point. Since the temperature of the laboratory did not vary more than two degrees C. from twenty-six degrees C. during the course of the titrations, it was felt that it was not necessary to thermostat the acetic acid solutions . 12 Titration of Hydrogen Ion Using an Indicator Hydrogen ion usually present in the reduction mixture as the result of the addition of perchloric acid, was determined by the addition of two to four drops 0. 1 per cent crystal violet solution and titration with standard sodium acetate. The yellow, acid crystal violet was titrated to the blue endpoint. When standard perchloric acid was used in the reverse procedure to titrate sodium acetate, the crystal violet color change was from blue to blue-green. That is, in both cases, the acid-base endpoint with crystal violet indicator in glacial acetic acid was taken as the blue to blue-green transition. DISCUSSION OF RESULTS 13 14 Reduction of Oganic Comppunds Organic Nitro Compounds Preliminary investigations were made to see whether silver metal in glacial acetic acid would actually reduce some of the organic compounds reported by Keller (7). It was anticipated that reduction of organic compounds would yield a corresponding oxidation of silver metal. Thus, if reduction occurred, its progress could be followed readily by potentiometric titration of dissolved silver with standard chloride. It was observed that in the presence of dissolved silver in acetic acid the mercury-mercurous acetate, silver cell yielded a positive potential. In the presence of dissolved chloride the potential was negative. The reduction of three organic nitro compounds with silver was attempted by the beaker procedure. Para-nitrophenol, ortho- nitrophenol, and ortho-nitrobenzoic acid all failed to give positive cell potentials even after standing overnight in contact with metallic silver. Thus, no silver was oxidized to silver(I) and the results implied that silver would not reduce these compounds in acetic acid. Quinone Next, a more easily reduced compound was studied with the beaker reduction procedure. Quinone was apparently reduced and a substantial amount of dissolved silver detected and titrated as summarized in Table I. Upon calculation of the amount of dissolved silver titrated, it was apparent that about half as many m’illiequivalents of silver were oxidized as were added in the form of quinone. That is to say, because of the amount of dissolved silver titrated, approximately half of the quinone appeared to be reduced. 15 Table I. Volumetric Determination of Silver Oxidized by Quinone 4 Quinone Cl- No. of No. of meq. Ag g. meg. N ml. found 0.0157 0.291 0.0100 18.00 0.180 0.0405 0.749 0.1000 4.52 0.452 0.0373 0.690 0.1000 3.40 0.340 In an effort to explain the half-complete reduction of quinone, it was postulated that two molecules of quinone react with two atoms of silver and two hydrogen ions to yield one molecule of quinhydrone and two silver ions. The overall reaction might be written as the sum of two steps: ZAg ——>2Ag++2e .. + 2 Quinone + 2e + 2H -—> Quinhydrone + 2 Ag + 2 Quinone + 2H —-> 2 Ag+ + Quinhydrone Examination of the ultraviolet absorption spectra of known samples in acetic acid showed the spectrum of the reduction mixture to be more like that of quinhydrone than either quinone or hydro- quinone as shown in Figure 1. If the suggested reaction is correct, the reduction by silver metal would proceed because the formation of quinhydrone effectively removes product hydroquinone from the system. 16 .29.. 0:3... E «museum 3832:: 46.3th . own 15 59.983325 own omm oom owm com . q q q .- \.\\uM~_ u... - oo . o \ O 0.3555 Gofiodpom II... .\ \.. “28:de I.|.. Ms \... eoH . odonpfiafisd . . . . . x . onofifidoufirwm III C. s _ . .. om . I O. o.‘ O I‘. d. ~§DO No .0 ms. 0 . .. .z u A . om . o. to \~ Q L .. .. x . v n. O .. .. M .. I ow . w o. \II/ o 8‘. H m. \. / o s 0 1+ . \ I o x . 1. . \ z/ . s .. . .. om . E H \ [In-\Qo ~v. w . \ a». a .\ \s o A... .\. .. co .\ n. \.\..\ i. .l.\ \\\\ .oo\.\. 10F. ..\ ..\. A I on . \m.\... v.2... . 4 co . 17 Determination of Dissolved Silver Standard Chloride Solution Since calcium chloride appears to be one of the most soluble inorganic chloride salts in glacial acetic acid (2), standard solutions were prepared at approximate concentrations by weighing the salt and dissolving in acetic acid. The resulting solutions were then standardized by gravimetric precipitation of silver chloride with excess aqueous silver nitrate solution. That the standardizati’ons yielded results quite close to the concentrations anticipated by weighing out 8-mesh reagent grade anhydrous calcium chloride is substantiated by Table II. Standard Silver Solutions Dissolved silver solutions of known concentration in glacial acetic acid proved somewhat more difficult to prepare. First, as shown in Table III, known dissolved silver was prepared by solution of weighed silver acetate in acetic acid with excess perchloric acid. The result- ing solution was standardized gravimetrically via silver chloride which was precipitated by the addition of excess aqueous sodium chloride. Titration of the above silver solution potentiometrically showed no change in the amount of silver detected regardless of the amount of excess perchloric acid present (see Table IV). A second approach was the preparation of a neutral solution of silver in acetic acid by dissolving silver acetate in acetic acid containing a deficient amount of perchloric acid. Aliquots of this solution had to be drawn from the top so as not to disturb the undissolved silver acetate which settled to the bottom. The potentiometric titrations summarized l8 :2 .0 $2 .0 £5 38.0 £2 .0 $2 .o Smog. o i: .o E: .o $4. 335 E2 .0 $2 .o ammo.“ m :2 .o 82 .o .36 88.0 2.: .0 $2 .0 ago; a 32 .o E: .o 85 38.0 82 .o $2 .0 wmwog m 83 .0 SS .0 8.2 $86 $2 .0 82 .o $$$ N N2: .0 $8 .o co .2 $3 .0 $2 .0 82 .0 $$ .m 2 dofimnfih. Goflmfimfioonna .HE Z .12 OH 803 Z .ULMO .H.m.w nonrgdz ~68 522.882 “who. .Bm Gm... .m ~83 Eu... uses/w E 03.830 8223 no warms... .n 033. l9 sooo.o wom2.2 mooo.o oaoo.o wom2.2 maso.o asso.o wom2.2 mooo.o oooo.o ooam.22 mooo.o moeo.o oosm.22 mooo.o e58 u<0m< .05: .35 Essa o Hmflcouonm opofi—oofim awak— Smog .m 0.“:th ., 1 J l ooo.o omo.o cod .0 omH .0 com .o 3110A ‘Ietiuaiod 27 former, quinhydrone, the formal potential of silver metal must be too high for straightforward reductions. In an aqueous chloride system, silver reduces many substances due to the lowering of the silver ion activity by precipitation of insoluble silver chloride. It would appear from this investigation that none of the silver salts present in glacial acetic acid are insoluble enough to have analogous effect. Thus, reduction via the silver reductor is observed only when there is an effective means of removing the product materials from the reaction. In a final attempt to explain behavior of dissolved silver in acetic acid, the conductance of a series of solutions was measured as a function of the silver concentration. However, because of the low conductivity of acetic acid solutions, most of the measurements were necessarily made near the end of the conductance bridge scale where readings are known to be erratic. Therefore, from the data in Table X, one may conclude only that conductance decreases as the dissolved silver concentration decreases. Table X. Conductance of AgClO4 Solution vs. Concentration 1:. === Molar AgClO4 Observed Conductance Concentration in ohms"l x 106 1.60 x10"2 7.40 9.00 x10"3 2.90 2.75 x10'3 1.60 8.18 x10'4 0.70 4.58 x10'4 0.66 3.90 x10"'5 0.18 1.27 x10'5 0.10 SUMMARY AND C ONCLUSIONS 28 29 Silver metal will not reduce the organic nitro compounds tested in glacial acetic acid. Quinone is reduced by silver due to the form- ation of quinhydrone; however, this is a special case and not repre- sentative of most reducible organic compounds. Dissolved silver in acetic acid can be determined accurately and rapidly by potentiometric titration with standard chloride solution using the mercury-mercurous acetate electrode in acetic acid as reference electrode and clean silver metal as the indicator electrode. The accuracy and precision of this method compare favorably with the familiar gravimetric precipitation of silver chloride. . Further, use of the method is indicated whenever the solution being titrated develops a color which may mask chloride indicators. While a formal potential for the silver-silver(I) couple in acetic acid cannot be readily determined, the silver specie present from 10'3 M to 10"5 M would appear to be monomeric. In that region, the potential of the silver electrode obeys the Nernst equation with an 11 value of one. Finally it must be said that many questions remain to be answered with regard to the silver system in glacial acetic acid. Thus, one might propose to determine the true nature of the monomeric silver species, whether it is silver ion or an ion pair. Further work would certainly investigate the reducing properties of silver metal in chloride medium to see if the potential is lowered in a method precisely analogue to the Walden reductor. Finally, the determination of silver using complexing agents other than chloride, such as iodide, bromide, thiocyanate, or cyanide, might be undertaken. LITERATURE CITED 30 10.. 11. 12. 13. 14. 31 Al- Qaraghuli, N. and Stone, K. C., Anal. Chem. El, 1448 (1959). Audrieth, L. and Kleinberg, J., Non Aqueous Solvents, John Wiley and Sons, Inc., New York, 1952, p. 149. Bishop, E., Analystll, 672-84 (1952). Bruckenstein, S. and Kolthoff, I. M., J. Am. Chem. Soc. 18, 2974 (1956). . Conant, J. B. and Hall, N. F., J. Am. Chem. Soc. .42, 3047 (1927). Hills, G. J. Reference Electrodes: Theory and Practice, Edited by D. J. Ives and J. Janz, Academic Press, New York, 1961, pp. 433-463. Keller, H. E. , Reduction of Nitro Compounds with Metal Reducing Columns in Glacial Acetic Acid, M. S. Thesis, Michigan State University, 1959. Mukherjee, L. M., J. Am. Chem. Soc., 12, 4040 (1957). Rao, G. P. and Vasudeva, A. R., Z. Anal. Chem. 187, 96 (1962). Scarano, E. and Arnaldo, C., Anal. Chim. Acta _1_2_, 292-9 (1955). Schwabe, K., Naturwissenschaften 44, 350 (1957). Stegemann, K. , Neuere Mas sanalytische Methoden: Die chemische Analyse, Band 33, p. 163, “Metals as Reducing Agents. " Stock, J. T. and Purdy, W. R., Chem. Revs. 21, 1159 (1957). Walden, G. H., Hammett, L. P., and Emonds, S. M., J. Am. Chem. Soc. _5_6_, 350 (1934). mama! LIBRARY .JUN 31744191]:- llHll|H||||llllllllhllllllllllltlllll‘lllHlllHllll‘lllIHll 31293 02446 7312