I u I Al: HI ‘ .l I, ‘ x I W ‘1‘ \ a I tlH | _1 I .mdm CHRONinIENTEOME'FRfC ANALYSES OF SOME DILUTE SOLUTIQNfi IN AQUEOUS MEDIUM Timfia gov We Degas OE M. S. MICHEGAN STATE UNEVERSIW Frederick Milier Mong 1957 C.2, v“ LIBRARY Michigan State University MICHIGAN STATE UNIVERSITY M EAST LANSING, MIJHIGAN CHRONOPOTENTIOMETRIC ANALYSIS OF SOME DILUTE SOLUTIONS IN AQUEOUS MEDIUM By Frederick Miller Mong A THESIS Submitted to the College of Science and Arts of Michigan State University of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Chemistry 1957 d-—;J-enr e—{mf ACKNOWLEDGMENT The author wishes to express his sincere gratitude to Dr..Andrew Timnick for his encouragement and direction throughout the course of the investigation and preparation of this thesis. vv l\I\I\I\/V\IV\J\I\I\I_ ‘I\“I\")I\"’I€"n"r\"i\"r{"n‘r\_rn\"'n"n ii VITA Name: Frederick Miller Mong Born: July 25, 1933 in Franklin, Pennsylvania Academic Career: Franklin High School, 19h7-l951 Grove City College, Grove City, Pennsylvania, 1951-1955 Michigan State University East Lansing, Michigan, 1955-1957 Degrees Held: B. S. -- Grove City College, 1955 iii CHRONOPOTENTIOMETRIC ANALYSIS OF SOME DILUTE SOLUTIONS IN AQUEOUS MEDIUM By Frederick Miller Mong AN ABSTRACT Submitted to the College of Science and Arts of Michigan State University of Agriculture and Applied Science in partial fulfillment of the requirements i for the degree of MASTER OF SCIENCE Department of Chemistry Year 1957 ' 0 Approved W ABSTRACT The instrumentation and analytical application of chronopotenti-- ometry to some dilute aqueous solutions were investigated. Design and construction of a glass cell, construction of several platinum electrodes and construction and calibration of a constant current supply were carried out. The instrumentation assembly was tested with various concentrations of lead ion in 0.1 M potassium nitrate as the supporting electrolyte. A close check on the geometrically measured area of the platinum electrode and the experimentally obtained area was observed. The use of a mercury plated platinum electrode in chronopotenti- ometry was introduced. The presence of plated mercury on a platinum electrode shifted the hydrogen overvoltage 0.3 to O.h volts in the negative direction. The shift was sufficient so that several additional ions such as antimony and zinc could be determined with this solid electrode. Calibration curves were obtained for antimony (III) and bismuth (III) separately, in l N nitric acid using a mercury plated platinum electrode. No break could be obtained for arsenic (III) over the range studied or with the various supporting electrolyte tested. A mean deviation of i 1.70% for 2;[%13| was obtained for bismuth (III) over the concentration range of 3 x 10”5 to 1.x 10-3 moles per liter. For antimony (III), ZJEiZZ-decreased as concentration of this ion decreased from 2 x 10'.3 to 6 x 10"4 moles per liter. Mixtures containing antimony (III) and bismuth (III) were resolved. Arsenic (III) did not interfere in this resolution. Chronopotentiograms for 0.001.M thallium (I), 0.001 M copper (II) and 0.001 M cadmium (II) each, in 0.1 M potassium chloride as the supporting electrolyte were obtained using a bright platinum electrode. vi TABLE OF CONTENTS I. INmODUCTIONOO00.0.0000...OOIOOOOCOIOOOOOIOIOOIO0.0.0.0.0... 1 II. HISTORICAL AND THEORETICAL BACKGROUND ........... . ....... .... III. EXPERIMENTAL................... ........ ......... ..... ....... 3 ’ 9 Chemicals and Solutions.................................. 9 Apparatus................................................ 9 Cell.................................................. 9 Electrodes............................................ ll Constant Current Supply............................... 11 Calibration of Glassware.............................. 13 ’Arrangement of.Apparatus.............................. 13 Procedures............................................... 15 Current Supply Calibration............................ 15 Preliminary Tests with Lead Solutions................. 15 Use of a Mercury-Plated Platinum E1ectrode............ 25 Determination of a Proper Supporting Electrolyte for the Analysis of Antimony (III) and Bismuth (III)... 29 Temperature Effect on Transition Time................. 32 The Determination of Antimony (III) and Bismuth (III). 3h Analysis of Mixtures of Antimony (III) and Bismuth A (III .............................................. 3 Other Chronopotentiograms............................. h? IV. DISCUSSION AND CONCLUSIONS.................................. 51 LITE-AME CIT'ED.OOOOOOOIIOOOOO0.00.00.00.00000000000000000000000 SS vii TABLE I. III III. VI. VII. VIII. IX. X. LIST OF TABLES Page Summary of Calibration Results for Constant Current Supply. Manual Determination of Lead in 0.1 M Potassium Nitrate at Room TenjpemtureO0.00.00.00.0000.0.0.0...OOOOOOOIOOOOOOO... Transition Times for Various Concentrations of Lead (II) in 0.1 M PetaSSillm ChloridBOOOOOOOOOOOOOOOOOIOOOOOOOOOOO.00... 1 2 Evaluation of i‘f’ for 0.001 M Lead (II) in 0.1 M Potassium Chloride Using a Mercury-Coated Platinum ElectrOdeOOIOOOOOOOOOOOOOIOIOOIOOOOOOCIOOO0.0.0.0.000...00. Effect of Acid Concentration on the Hydrolysis of Antimony (III) and Bismth (111)0000000000000000OOOOOOOOOOOOOOOOOOOO El/ Values for Bismuth (III) and Antimony (III) in Various ACid MedmOOCOOOOCOOOOIOCOOOOOOOOOCQOOIDCCOOOOOOOOOOOOOOOIO Temperature Effect on Transition Time for 6 x 10-4 M Bismth (III).......O0.0.0.000...OCOOOCCOOOOOCOCOCCOOOOO0.. Determination of Bismuth (III) in 1 N Nitric Acid.......... Determination of Antimony (III) in 1 N Nitric Acid......... Some Mixtures of Antimony (III) and Bismuth (III).......... viii 16 20 23 28 29 31 32 36 to M: LIST OF FIGURES FIGURE 1. 10. 11. 12. 13. 1h. 15. 16. 17. 18. 19. Electrolysis Cell-:Actual Size............................. Circuit Diagram for Constant Current Supply................ . Arrangement of.Apparatus........................ ...... ..... . I-Calibration Plots for Constant Current Supply............ II-Calibration Plots for Constant Current Supply........... Method for the Determination of Transition Time............ Chronopotentiograms of Lead (II) in 0.1 M Potassium Nitrate 1 . Calibration Plot of 1' /2 vs. Concentration for Lead (II) in 0.1 N Potassium Nitrate................................. 1 Calibration Plot of i’f’/2 vs. Concentration for Lead (II) in0.1NPOtaSSiumChlorideoooooooooooooooooooooooooocooooo Effect of the Mercury Film on the Overvoltage of Hydrogen.. Effect of the Various Acids as Supporting Electrolytes for Determining Bismuth (III) and Antimony (III)............... . _4 Effect of Temperature on Transition Time for 6 x 10 M Bismuth (III) in l N Nitric ACid I O O O O O O O O O O O C 0 O O O O O O O O O O O O 0 Calibration Plots for Bismuth (III)........................ Calibration Plots for Antimony (III)....................... Calibration Plots for Various Mixtures of Antimony (III) and Bismth (III)..........ID..00....COOCOOOCOOOOCCCOOCC.CC Chronopotentiograms of Typica1.Mixtures of Antimony (III) and Bismth (111)...0...’...C...O....OOCCIOCOOOOOCOOOOIOOOC Chronopotentiogram of 0.001 M Thallium (I)................. Chronopotentiogram of 0.001 M Cadmium (II)................. Chronopotentiogram of 0.001 M Copper (II).................. ix Page 10 12 1h 17 18 19 21 22 2h 27 30 33 39 A2 145 146 ha 19 50 I. INTRODUCTION Chronopotentiometry is based on the interpretation of the variation of the potential at a working electrode as a function of time when a constant current flows between the working electrode and the auxiliary electrode both of which are immersed in an unstirred solution. The potential of the working electrode is measured against a reference electrode such as the saturated calomel electrode. Even though the fundamentals pertaining to chronopotentiometry were worked out at the beginning of the century, it wasn‘t until the early 195073 that the potentialities of applying chronopotentiometry in analytical chemistry were realized. The chronopotentiometric method of analysis offers several advan- tages that conventional polarography does not possess. No fluctuation in the measured quantity is observed. Mercury or solid electrodes can be used and the performance of both can be treated theoretically. i If a solid electrode is used, plated metals may be anodically stripped back into solution and reruns made. The sensitivity of chronopotenti- ometry is greater so that lower concentrations of reducible or oxidizable ions can be determined. No maximum suppressors need be added to the solutions to be analyzed. The work which follows was undertaken to extend analytical appli- cations by the use of a platinum electrode for the chronopotentiometric determination of dilute solutions of arsenic (IIIL,antimony (III) and bismuth (III) separately, or in mixtures. To carry out this work, an electrolytic cell had to be designed and constructed and a constant current source had to be assembled and calibrated. II. HISTORICAL AND THEORETICAL BACKGROUND Credit has been given to weber for initiating the studies on chronopotentiometry in 1879 (33) when he conceived a method for the determination of the diffusion coefficient of zinc sulfate. It was Sand, however, who developed the theoretical aspects of the method and derived the first working equation relating the various factors involved (26). In his work, Sand studied the variation of ionic concentrations at electrodes in a solution of copper sulfate and sulfuric acid during electrolysis.’ He devoted special attention to the liberation of hydrogen during electrolysis. He concluded that only after the copper ion concentration had gone to zero at the surface of the electrode, would hydrogen be evolved. The duration of electrolysis for the depletion of the copper ions at the-cathode (transition time) was determined experimentally and the following equation was derived to verify this mathematically. 71/2 5 171/2 EDI/2 C N in which, 2 1 - current density in amperes/cm. ’T’ = transition time in seconds. N = number of electrons involved in the over-all electrode process. F = the Faraday, 96,h9h coulombs. 2 D = diffusion coefficient in cm /second. C = concentration in moles/ml. Sand proposed several conditions which must be met in order that the above equation apply. The solution must be confined in a cylindrical vessel bounded by the electrodes. No convection currents must be allowed to develop. The diffusion of the salt in solution should proceed according to Fick's law'and its transport values be constant. He stated that this formula can be made the basis of an approximate method for determining diffusion coefficients. Cottrell (5) and Karaoglanoff (17) applied the method to the verification of Fick's law of diffusion and to the determination of diffusion coefficients. Karaoglanoff carried out his work on the electrolytic reduction and oxidation of iron alum, ferric sulfate, ferric chloride, ferrous ammonium sulfate, and ferrous sulfate. He verified Sand's formula to the extent that in electrolysis at constant current, potential at an electrode varies as a function of time and that oxidation or reduction processes taking place are diffusion controlled. Rosebrugh and Miller (25) developed a complete mathematical ‘ treatment of changes in concentration which occur at the electrode during electrolysis. Particular attention was given to the changes which occur within the first fraction of_a second after the current begins to flow; Studies in the field up to the middle of this century pertained mainly to reactions occurring on the surface of the working electrode. Hickling, Taylor and Spice (12, 13, 1h, 15, 16) studied the anodic behavior of copper, silver, nickel, gold or platinum electrodes in acid, neutral or alkaline media. They identified the various stages in the polarization of these metals by employing a cathode ray oscillograph. Hakkad and Emara (30, 31, 32) also studied surface film formation on cOpper, nickel or platinum electrodes. They employed a direct potentiometric method using a special cell and constant current unit. They confirmed their findings by the oscillographic methods employed by Hickling, _e_t 31. Other work on the anodic passivation of gold has been carried out by Butler and Armstrong (2) and by Shutt and walton (28). Stackelberg‘gt'gl, (29) in 1953 made application of the method of voltammetry at constant current for the determination of diffusion coefficients of 'Ag(I), Tl(I), Cd(II), Pb(II), Zn(II), Zn(0H)4”, 103‘, Fe(CN)65 and Fe(CN)e= in concentrations of 0.001 M in aqueous solutions of sodium chloride, potassium chloride and potassium nitrate up to 3 M in concentration. Delahay and Berzins (l, 7) in the same year presented rigorous mathematical analysis for three types of electrode processes in electrolysis at constant current in unstirred solutions. These processes were a cathodic process followed by re-oxidatian resulting from reversal of the current, reduction of a two-component system and stepwise reduction of a single substance. Rigorous interpretations of potential- time curves were developed for cases for which only approximate treatments are available in polarography and transitory voltammetry. Gierst and Juliard (10) carried out a kinetic study of electrode processes occurring by a non-steady state electrolysis under constant current from a study of the relation between the transition time and the constant current. They concluded that the product of i’rg/Z is linearly proportional to D1/2 and to the concentration only if the electrolysis is exclusively controlled by the diffusion of the depolarizer. In l95h, Delahay and Mattax (9) verified by experiment that the accuracy of evaluating transition time is primarily determined by three factors. These factors are the degree to which convective transfer interferes with the diffusion process, the precision with which transi- tion time can be determined and finally, the precision on the current density measurement. Delahay and Mamantov (8) in a review of the theoretical principles of chronopotentiometry presented relationships concerning transition time and potential-time curves for reversible, irreversible, consecutive, stepwise and kinetic processes. For the consecutive process, when two or more ions are present in solution which are reduced at different potentials, the concentration of the ion first reduced may be calculated from its transition time by means of the original Sand equation. However, for a second ion, since the total electrolysis current now divides between two diffusion currents, the state of concentration polarization with respect to the second ion will be reached less rapidly and the transition time is correspondingly enhanced. The following general equation applies to diffusion-controlled chronopotentiometric systems where more than one reducible ion is present. (n+1'2+1’n>1/2 -('r.+r.+""1'n-11/2 = 171/2 NnFDnl/zcn 21 The potential of the working electrode can be calculated from the Nernst equation, and by studying the reaction cathodically can be shown to vary toward more negative values during electrolysis. RT m4,“ 014*” During electrolysis, the concentration of metal ion decreases at the electrode surface because of the consumption of this ion, while at the same time the concentration of metal increases. .As a result, the po- tential becomes more cathodic as the electrolysis proceeds. Initially, no metal is present so the potential should theoretically be positive infinity. At the end of the electrolysis, the concentration of metal ions should be zero and the potential should theoretically be negative infinity. It is also shown mathematically (8) that the value of E T7;, the potential at one-fourth of the transition time, is equal to the polarographic half-wave potential. Reilley, Everett and Johns (23) discussed the feasibility of the method for analytical purposes in the cathodic reduction and anodic oxidation of simple ions at platinum and mercury pool electrodes. Several supporting electrolytes were examined and practical apparatus and technique described. Nicholson and Karchmer (21) made application of the method to the determination of lead ion in nitric acid solution. Their results confirm the analytical usefulness of the method. Reilley and Scribner (2h) described a new method of end-point detection in a volumetric method which they called chronopotentiometric titrations. The relationship between chronopotentiometry and potenio- metric titrimetry, the possible types of titration curves and the factors which govern the sensitivity of the method were discussed. The method has the advantages of the amperometric method but is applicable to the titration of small volumes or concentrations and in situations where stirring is undesirable. Nikelly and Cooke (22) analyzed for very dilute solutions of metals by anodically removing them under controlled conditions. Recently, Laitinen and Ferguson (19) found that salts dissolved in high temperature molten systems may be studied by chronopotentiometry. Using platinum electrodes, measurements made on the chlorides of Bismuth (III), cadmium (II), silver (I) and copper (I) in a lithium chloride-potassium chloride eutectic mixture at I450o showed the theoretical relationships among applied current density, concentration and transition time. The accuracy of the method was i 2.6%. For the most complete work on theory and experimental evaluation of chronopotentiometry, Chapter 8 in the book by Delahay (6) is an excellent review. III. EXPERIMENTAL Chemicals and Solutions Reagent grade chemicals were used to prepare individual 0.1 M stock solutions of lead nitrate in distilled water, antimony trichloride in h N hydrochloric acid and bismuth nitrate in h N hydrochloric acid. These stock solutions were analyzed by precipitating lead as lead chromate (3) and bismuth as the oxy-chloride (3h). Antimony was determined by a permanganate titration (3h). A 0.1 M arsenic stock solution was prepared by dissolving primary standard grade arsenious oxide in 1 N potassium hydroxide, neutralizing the base, then making the final solution 1 N in hydrochloric acid. Primary standard grade silver nitrate-was used in the current supply calibration. Also pre- pared were individual 0.00l M solutions of reagent grade thallous nitrate, cadmium chloride, cupric nitrate and zinc nitrate. One normal nitric acid, 0.1 N potassium chloride or 0.1 N potassium nitrate was used as the supporting electrolyte throughout the work, although other common supporting electrolytes were tried for comparison. Matheson oil- pumped nitrogen was bubbled through each solution prior to electrolysis to remove any oxygen present. Apparatus Cell The electrolysis cell shown in Figure 1 was designed for simplicity and convenience in its use. Actual construction was carried out.by Mr. Gene Hood in the laboratory glassblowing shop. The cell consists of an 10 —_<\ / An date Cathol‘nlfe Gemini-talent Compartment 4*; ' ; FIGURE I. ELECTROLYSIS CELL. ACTUAL SIZE ll anode compartment and a cathode compartment separated by a sintered glass disk to prevent free mixing of anolyte and catholyte. Stopcocks at the bottom of both compartments were attached for easy draining. The cathode compartment is surrounded by a jacket through which constant temperature water can be circulated. The cell was supported by means of a heavy duty clamp bolted rigidly to a heavy steel stand. Several mats were then placed under the steel stand. Vibrations to the cell were thus kept to a minimum. Electrodes various sized platinum electrodes were prepared by heat-soldering 0.018 mm. thick platinum foils onto platinum wires sealed into the end of six mm. diameter soft glass tubes. .Approximately one cm. from the sealed end inside the glass tube, the platinum wire was silver soldered to a copper wire of similar thickness which extended out of the end of the glass tube and to the current supply connection. Constant Current Supply A simple constant current unit shown schematically in Figure 2 was constructed to supply any current from approximately h-2500 micro- amperes. The circuit is arranged so that the current can be directed through the cell or by-passed around it. The current can also be made to flow in either direction. CURRENT SUPPLY CIRCUIT DIAGRAM CODE (For Figure 2) R1. 1000 ohms, 2-watt resistora R2. hOOO ohms, 2-watt resistor8 R3. 32 megohms, l-watt resistor8 R4. 16 megohms, l-watt resistora R5. 8 megohms, l-watt resistora R6. h megohms, 1-watt resistora R7. 2 megohms, l-watt resistorb R8. 1 megohms, l-watt resistor: R9. 500,000 ohms, 1-watt resistor R10. 200,000 ohms, l-watt resistor R11. 100,000 ohms, l-watt resistor R12. 50,000 ohms, 2-watt resistor R13. 20,000 ohms, lO-turn Helipot R14. 20,000 ohms, 2-watt resistora T1. Thordarson T-22R32 Transformer, 350-0-350 V, 110 ma., SV.-2A.; 6.3 V. CT-3A; 6.3V. CT-3A. 31,82. SPST switch 83. SPDT 3-position switch S4. DPDT 2-position switch 35. 11-position switch 01,02. 8 mfd., electrolytic condenser, h50 volts. O'U‘O‘ aWire‘W’ound Type bCarbon type .>.._n_a3m hzmmmao ._.z<._.mzoo mo“. EdmoSo 5.3050 .N 959.... 12 :I It A I I1 1595 we: A; 80 :UU J. 'IJ)>>>>II.«~ «D HUI .0 L" rm 15112 In 12,]le n.» J m e GEO e w w mam * v I . titsilrlzsikli x e .e .e 13 Calibration of Glassware All volumetric flasks, pipeta and burets were calibrated by standard techniques. Corrections were then applied when necessary. The dilute solutions used in this work were prepared from the analyzed stock solutions using this calibrated equipment. Arrangement of the Apparatus The arrangement of the apparatus is shown in Figure 3. A saturated calomel electrode, a working electrode, and glass tubes for nitrogen inlet and outlet are uniformly arranged into the catholyte compartment and supported by means of a rubber stopper. The two-way nitrogen system may be adjusted for initial sweeping of the solution, or alternately for passing nitrogen above the solution during electrolysis. A fritted glass disk at the bottom of the bubbler increases the dis- persion of the nitrogen. The length of the saturated calomel electrode was extended thirty-two mm. by molding.an epon-type plastic resin onto the top. This allows the tip to dip several cm. into the solution. The anode was supported at the level of the solution bridge by means of a rubber stopper. All glass tubing was of five or six mm. diameter and extended at least one inch above the rubber stopper. While the current passes between the two electrodes, the voltage change between the working electrode and the saturated calomel electrode is observed on a model 9600 Beckman Zeromatic pH meter and recorded automatically as a function of time by a Honeywell-Brown Electronik recorder, model 1h .mahdm4aa4 no h2m2u02...n_n_:m hzummao hz1 . I. . '0 .. ... .- . . s - . . .. .... .. . - . . . . . ... ...- . I .. . .. . . . . I .- . .- I - r . . -. . .. .. .... . . . .- . . . . u. . . . -.. . ... .. . . . . -. . . .. I LI 1 o . .. .n . . v . . . . .. . . . . . 1.. A .. . . . i . . .. . . - . .. .. .n ..a I 3* \ka noo+u II . . .- -. . c . . . . . . . . ... . ... . . . .. -.l.- -.-..- . -... . . . .i. . . . . . .. - . . . ...x a .. ..u. . . . . v . . ... . ... . . . ..A . I T. . .1 u ... ... u . Y A. . '. . t . » I. .- . -0. . 0 a o .. t I . .o I a v I o ... ... .. ... .... .v. .. ...-.. ....... ...- . 21....“ N X 6’. . . v -H ....0- I sorta .. is... I 1 .01 U - ....v . . 8m . -VIIAquva . . ... . . c. ' III- .... .0. -- . .... .. e o ...-I .. . ..-: II. .- .... ... ... I4 .vsv lo. .. .... ... ‘ ' .. . «I.. v. . .1 v ... In. ' ' ... .. .... .. . -. . .. I . ..it to O O . . . . . . . . . -I . . . . . . -. .... .I. . . - . . ,. .. . . I? ... .j ;_ - - So. .00 I D iii-.-.. 00h . ' ' . IL.. ..I ..v. .. .. .. . . III. 0 I ..I v...Iol..... on... .50 to. . .Iolii... .... .. -.Itl in... .VIVIL .4irvo.. .. . . . . . - .. I+|¢i6 ..I.TI04-IA0... v66. nIbI Iv OTTO... . ..l ...L III ....A VQ.OI vIcQV .....1 .0 - . . .IIOIV elo‘alflolbno.... I6.11|c|¢vvxv$vo.. v Avficc. a... V..+.1 o A . - . _ I- , , . U .- -I, I - I n II+o.IIIv¢ v.v§c Isl. '. r 4i. 1. 0410.61 I»! tot-LI... Vole. . .b‘ .A 34-Oct 904 oIbJeivI. .IIII L-olllvt. (I.-. .. .l .ILVIIIIf .IJIA- ve4..17+ I..AT9-o.. 11 II .. .. I... 1 Ibcoilol..I+l Li...ItI.Y¢rO.OI Io . . :qu 1~1¢4I4 . v4 v. o oltlb. I . L I « oi. Av YO. .91 #0 I91 Ioi r- .. 1 T94- . .LI ATIQI . iii...- OO“ . - xii: . :05... x i .-.-31x.-...I-i.-i:-oh . 8 tit - I I ..r T 9!. I -. . ., . Emfhfuta..- «Mas-Jaw ”H.4Nx .UIIW ii a N 1 p .1wa 4» A tsiu dIHLIvIIf I -..-.I. (4. - r w 6 . IT: . 1 151...th . .11le +1 v . v o $1 I .190. To 6... I H 4¢I4 q .I - . . -l§ .... I > 4» A «I #I riwol VIIIIO IOIOIII . . , II... . .1, Y All . 1r TAT; - v.9» . Io? rI he a .. I . . L . . M Li. . «L . J I 'I ... «» * “t 'i It. I Iii i t—ov >—J I I—«L J . by... "7‘ g . . ...; .. L I H ._ _, - TL... ..-. - Vial. IN. - ”uh- fi um . ”I ...-W ...”th rt». e n.1,; wt... nth-fie»? . . a d .: _::_:.-3IT..:1.. -..-.-e.1.I-.HII.,I.._»L . rut-4 P. HH... 0:1 :H ...1 19 surface area of the platinum electrode used was both geometrically and experimentally measured for comparison. The voltage changes were followed manually on the Beckman Zeromatic pH meter while time was followed with a stopwatch. The corresponding potentials and times were then plotted manually. All transition times were measured graphically according to the suggestion of Delahay (8). In Figure 6, segment BE is constructed so that AB and FE are one-fourth the distance of AC and FD, respectively. OP, which then represents the transition time at E1/;, is drawn parallel to AC and DF. All measurements were made using a pair of dividers. O c/ Tune "1 fl 1 P Volt- >07 Figure 6. Method for the Determination of Transition Time. 20 Table II shows the results of manually determining transition times at room temperature for 0.0005 to 0.0015 M lead (II). These chrono- potentiograms are shown in Figure 7. In Figure 8, the relationship between 1' 1/2 and concentration is shown. The dotted line shows the calculated 1'1/2 values assuming the diffusion coefficient of lead to be 0.98 x 10’5cm2 per second (18) and the area of the electrode to be 2.10 cm2 as determined by geometric measurement. The solid line 1 represents the experimentally evaluated 1' /2 values. TABLE II MANUAL DETERMINATION OF LEAD IN 0.1 M POTASSIUM NITRATE AT ROOM TEMPERATURE Pb(II) conc. Current Selector T 11/2 A, cm.2 i 1' 1/2 (moles/liter) Position (Sec.) (Calc.) c 0.0005 6-300 6.0 2.h6 1.83 1.00 0.0008 6-300 19.0 h.h8 2.08 1.12 0.0010 6‘300 32.0 5.66 2.10 1.13 0.0015 6-300 89.0 9.h3 2.32 1.25 Average Electrode Area = 2.08 cmz. Measured Electrode Area = 2.10 cmz. Chronopotentiograms obtained after the above described preliminary work were automatically recorded by the Brown Electronik recorder. Table III lists the values of’i/l'l/2 for 0.01 to 0.00005 M lead (II) in 0.1 M potassium chloride while Figure 9 shows these results graphically. O The temperature of the cell was maintained at 25.00 i 0.05 C. ,o 21 6-300' Ina I .t a h .m it .a .m m r e ..t e D Manual m 8 two 9| P U s in N e r r U -C t. .- g . t v o O , O o i 0.00:5 M Pb " «uneven IN O.|M POTASSIUM NITRATE. FIGURET. CHRONOPOTENTIOGRAMS OF LEADII .mhdmk; «saws—.0.“ 2.6 2. =o «AL. “.0 hon—n. zo_.r 2 N8 25 Use of a MercuryePlated Platinum Electrode The relatively low overvoltage (11) for hydrogen discharge at a platinum electrode limits the working range of this type electrode in chronopotentiometry, especially in acid solution. The working range, however, can be extended as much as 0.3-0.h volts in the negative direction by employing a mercury-coated platinum electrode. This shift is brought about by the much higher overvoltage for hydrogen discharge on mercury than platinum. Although the shift with a mercury-coated electrode is not as pronounced as with pure mercury, it is sufficient to obtain good Chronopotentiograms for many additional ions. Many attempts in the past were made to prepare suitable and repro- ducible mercury surfaces on clean platinum. Reilley and Scribner (2h) obtained a mercury-copper surface on a platinum foil electrode for use in chronopotentiometry by first plating a thin film of copper onto the p1atinum.surface and then immersing in mercury. The method by which Marple and Rogers (20) electrolytically plated mercury onto a platinum micro-electrode for use in polarography (h) was employed in this work with considerable success. The method was rapid and gave reproducible mercury films on the platinum surface. Various plating times were tried with four minutes being the time chosen. The following procedure was used for preparing this mercury film on the platinum surface. 'The electrode to be mercury-plated was first cleaned by immersing in hot concentrated nitric acid, rinsed with distilled water, allowed to soak at least two minutes in saturated potassium chloride and again rinsed with distilled water. It was then supported in approximately 26 twenty-five ml. of saturated mercuric nitrate at a distance of two cm. from a small coiled platinum wire which served as the anode. The current from two 1.5 volt dry cells, connected in series, was allowed to flow for four minutes. The electrode was then removed, rinsed with distilled water and stored in saturated potassium chloride until ready for use. It must not be allowed to stand in open air for more than a few minutes and any water clinging to it, when ready for use, must be shaken off. Evidence for the importance of the mercury film is shown in Figure 10. Part B of Figure 10 shows that a mercury film deposited on the platinum electrode in four minutes is sufficient to shift the hydrogen discharge so that a well-defined chronopotentiogram for antimony (III) in 1 N nitric acid medium is obtained. Part A of Figure 10 shows that the four minute mercury deposit is not sufficient for zinc (II) in 0.1 N potassium chloride. An additional two minute plating time was necessary . To check performance and working area of a new mercury coated platinum electrode, a series of runs was made using several current selector settings for 0.001 M lead (II) in 0.1 M potassium chloride. ifl/z C was obtained. The mean A mean deviation of i l.5h% for calculated electrode area was 2.77 cm? as compared to the geometrical measurement of 2.69 cmz. These results can be seen in Table IV. 2'! o o l 4-ma'n. Plate tine ‘ .m P. 3 H I v r o tional ;..;A“l b FIGURE IO. EFFECT OF THE MERCURY FILM ON THE OVERVOLTAGE OF HYDROGEN. 28 . C OOHN\HDN\H h 2 U 0004” «\0L. am a ~50 000<¢ m0.0 04.0 00.0 0.04 m0.0 4m.0 00.0 n.0m 04.- 004-0 00.0 00.0 00.0 0.00 00.0 00.0 40.0 0.40 04.- 000-0 00.0 04.0 40.4 0.00 00.0 04.0 00.4 0.00 04.- 0-0 00.0 00.0 00.0 0.00 00.0 00.0 00.0 0.00 04.- 004-0 40.0 04.0 00.0 0.00 40.0 04.0 00.0 0.00 04.0 000-0 00.0 44.0 00.4 0.00 40.0 04.0 m0.4 4.m0 04.- 0-0 Adwwmwv «\HH H «\HW A “0‘9 Awmwwwv 0D”. 0 «NHL. A 0...va 7m. w\mmmbv ”wwwwmmm m 50m H 55.0 0.00.0050 E mnoaomqm Eejm OMB-adolwgomm: 4. .0sz: mQHmoamo 0 Emmfiom a do 2H 3: Dad 2 450.0 8h m\ ..P H .08 ZOHB4DA¢>M .0 >H mama Determination of a Proper Supporting Electrolyte for the Analysis of Antimony (III) and Bismuth (III) Solutions of 0.001 M antimony (III) and 0.001 M bismuth (III) each, were prepared in acid solution for the prevention of hydrolysis. Table V shows the effect of varying the acid concentration for several acids. TABLE V Effect of Acid Concentration on the Hydrolysis of Antimony (III) and Bismuth (III) Acid HNO3 HCl H2804 Normality Bi I I Sb III Bi III b III Bi III Sb III 1.5 Soluble Soluble Soluble Soluble Soluble Soluble 1.0 Soluble Soluble Soluble Soluble Hydrolyzes Soluble 0.5 Hydrolyzes Soluble Soluble Soluble -—-———- Soluble O 2 Slight Slight Slight Slight ' Hydrolysis Hydrolysis Hydrolysis Hydrolysis For comparison, solutions of antimony (III) and of bismuth (III) were made up in 1 N nitric acid, in 1 N hydrochloric acid, in 0.5 N hydro- chloric acid and in 1.5 N sulfuric acid. Figure 11 shows the chrono- potentiograms obtained with the various solutions and different current positions. 'With 1 N nitric acid the least shift and greatest spread in the El/4 values of antimony (III) and bismuth (III) are observed. For these reasons, 1 N nitric acid was chosen as the supporting electrolyte for subsequent work on antimony (III) and bismuth (III). A summary of these results appears in Table VI. 3 . _: >202....z< oz< _: IPDZmE @z_z_s.mw._.wo mo... mmk>40mkow4w z_._.mon.n5m m< mead m:o.m<> m1... ....0 ...ommmw m » _o>.. . 33> .. 00”. 0.... ‘ ‘ .. .- 00“. .2 Nana-u. I .00 . QI- 61. 9 .... O TABLE VI El/ VALUES FOR BISMUTH (III) AND ANTIMONY (III) 4 IN VARIOUS ACID MEDIA a Current E Ion Determined Curve Supporting Selector 1/4 ’T’ (0. 001 M Conc. ) Number Electrolyte Position (vs S .C.E.) (Sec.) * Bi (III) 1 1 N HNOB 7-0 -.050 37.6 Bi (III) 2 1 N HN03 7-500 -.O5O 13.6 Bi (III) 5 1 N H01 7-0 -.100 53.8 Bi (III) 6 1 N H01 7-500 -.125 16.7 Bi (III) 9 0.5 N H01 7-0 -.050 51.h Bi (III) 10 0.5 N H01 7-500 -.085 1h.8 Bi (III) 13 1.5 N H2304 7-0 -.015 30.1 Bi (111) lb 1.5 N H2304 7-500 -.ou5 15.6 Sb (111) 3 1 N 111103 7-0 -.180 h5.u Sb (III) h 1 N HN03 7-500 -.170 13.0 Sb (III) 7 1 N H01 7-0 -.160 32.8 Sb (III) 8 1 N H01 7-500 -.170 22.2 Sb (111) 11 0.5 N HCl 7-0 -.120 hh.5 Sb (III) 12 0.5 N H01 7-500 -.180 21.3 Sb (III) 15 1.5 N H2504. 7-0 -.1h0 29.5 Sb (III) 16 1.5 N H2304 7-500 -.180 1b.8 aThe curve number refers to the numbered curves in Figure 11. 32 Temperature Effect on Transition Time Table VII shows the effect of temperature on transition time for 6 x 10‘4 molar bismuth (III) in 1 N nitric acid. This is also demon- strated graphically in Figure 12. By increasing the temperature from 20.00 to 110.00 C., the transition time varied by a factor of 2.2. TABLE VII _4 TEMPERATURE EFFECT ON TRANSITION TIME FOR 6 x 10 M BISMUTH (III) T— Temp. 1' 1/2 Ev. i 1'1/2 a °c. (Sec.) II 4(vs. s.c.E,) "'?T"" 20.0 21.5 8.68 -.Oh 1.79 20.0 21.5 n.6u -.ou 1.79 22.0 25.1 5.0u -.05 1.95 22.0 26.0 5.09 -.05 1.97 2u.0 27.1 5.23 -.05 2.02 2u.0 28.3 5.32 -.05 2.06 25.0 27.2 5.20 -.05 2.01 25.0 28.6 5.30 -.05 2.07 26.0 27.2 5.20 -.05 2.01 26.0 30.7 5.53 -.05 2.1h 28.0 30.5 5.52 -.05 2.13 28.0 32.8 5.72 -.05 2.21 30.0 32.6 5.70 -.0h 2.21 30.0 35.h 5.9u' -.ou 2.30 35.0 no.7 6.36 -.oh 2.86 35.0 10.7 6.36 -.oh 2.h6 10.0 06.6 6.82 -.0h 2.6h h0.0 16.6 6.82 -.0h 2.6h aCurrent selector position was 69500 for these results. 33 0.0.. 0.5.2 2. 2. 2.25205 .2 .033 0.0“. 0.2: 20.20242.- 20 020200....sz “.0 Swat .0. 0200.... 1.. 38 The Determination of Antimony (III) and Bismuth (III) Individual solutions containing only one of the metallic ions ranging in concentration from 3 x 10-5 to l x 10“3 moles per liter of bismuth (III), and 6 x 10-4 to 2 x 10-3 moles per liter of antimony (III) in l N nitric acid were prepared from the analyzed stock solutions and chronopotentiograms recorded. A mercury-coated platinum electrode was employed, although it is not necessary for the determination of bismuth. The following procedure was developed for best results from the preliminary work on lead. The cell is rinsed several times with‘ portions of the sample solution and then both compartments are filled to approximately one cm. above the solution bridge. Constant tempera- ture water at 25.00 i 0.050 C. is circulated through the cell jacket. Air is removed from the solution by bubbling with nitrogen for at least ten minutes. The platinum working electrode which had been soaking in saturated potassium chloride during this bubbling period is then quickly inserted and the volume above the solution purged for a few minutes with nitrogen. Great care must be taken not to touch the electrode when inserting it to prevent deactivation. The water bath is then turned off during the electrolysis and recording to reduce vibrations. The recorder and pH meter are connected and adjusted. These instruments should be allowed a fifteen minute warmdup period prior to electrolysis. The current supply switch is turned to the forward position and the chronopotentiogram recorded. Calibration curves obtained for bismuth and antimony are listed in Tables VIII and IX and are shown graphically in Figures 13 and 1h, 35 1/ - -2 respectively. The deviation of the mean for the constant l’rc -5 obtained for bismuth (III) was i 1.70% over the range 3 x 10 to ...3 _4 _4 1 x 10 moles per liter. However, from 1 x 10 to 9 x 10 moles -5 -5 per liter, this deviation was i 1.17% and from 3 x 10 to 9 x 10 1/ i1' moles per liter was i 2.81%. The deviation of for antimony could not be obtained because the constant decreased as the concentration decreased. Even though this calibration plot for antimony is not linear over the entire range studied, it can still be employed for the de- termination of antimony (III). This abnormal behavior has been encountered by others (8, 10) and is usually explained by assuming that some chemical reaction occurs before the actual electron transfer process. These kinetic effects which can be detected by chrono- potentiometry do not necessarily appear in polarography. TABLE VIII DETERMINATION OF BISMUTH (III) IN 1 N NITRIC ACID ‘Current E . 1/ Cone . Selector 1/4 T 1/2 1 1' 2 (Moles/Liter) Position (vs. S.C.E.) (Sec.) T -—_0_—— 0.001 7-0 -.07 88.8 6.68 2.03 0.001 7-0 -.07 53.2 7.28 2.23 0.001 6-500 -.07 87.8 9.38 2.17 0.001 6~500 -.07 98.5 9 .70 2 .25 0.0009 7-0 -.ou 38.3 5.85 2.00 0.0009 700 -.Oh 36.6 6.08 2.06 0.0009 6-500 -.08 63.7 7.97 2.05 0.0009 6-500 -.08 68.3 8.02 2.07 0.0008 7-0 -.08 28.3 5.31 2.08 0.0008 7-0 -.08 28.3 5.31 2.08 0.0008 6-500 -.oh 58.8 7.80 2.18 0.0008 6-500 -.08 89.6 7.08 2.08 0.0007 7-0 -.08 22.5 8.78 2.07 0.0007 7-0 -.08 23.0 8.78 2.09 0.0007 6-500 -.08 39.8 6.26 2.07 0.0007 6-500 -.08 39.8 6.26 2.07 0.0006 7-0 -.08 15.3 3.90 1.99 0.0006 7-0 -.08 16.1 8.01 2.05 0.0006 6-500 -.08 27.2 5.22 2.02 0.0006 6-500 -.08 27.7 5.26 2.03 0.0005 7-0 -.08 11.2 3.38 2.05 0.0005 7-0 -.08 11.3 3.36 2.06 0.0005 6-500 -.08 18.9 8.38 2.01 0.0005 6-500 -.08 19 .8 8.88 2 .06 0.0008 7-0 -.08 6.85 2.61 2.00 0.0008 7-0 -.08 6.89 2.58 1.95 0.0008 6-500 -.08 12.7 3 .56 2.06 Continued TABLE VIII - Continued 37 Current Conc . Selector E1/4 T 1/ 1 1’ 1/2 (Moles/Liter) Position (vs . S .C .E .) (Sec .) T 0 0.0008 6-500 - 08 13.0 3.60 2.09 0.0002 6-0 - 08 7.32 ' 2.70 2.08 0.0002 6-0 - 08 7.08 2.68 2.06 0.0002 6-500 - 08 2.60 1.61 1.87 0.0002 6-500 - 08 2.60 1.61 1.87 0.0001 8-0 - 03 27.8 5.28 1.96 0.0001 8-0 -.03 29.5 5.83 2.01 0.0001 3-500 -.01 89.8 7.05 2.03 0.0001 3-500 -.01 50.3 7.08 2.08 0.00009 8-0 0.0 26.0 5.10 2.09 0.00009 8-0 0.0 26.5 5.18 2.11 0.00009 3-500 0.0 80.2 6.38 2.03 0.00009 3-500 0.0 80.8 6.38 2.08 0.00008 8-0 0.0 18.9 8.38 2.01 0.00008. 8-0 0.0 19.8 8.88 2.05 0.00008 3-500 0.0 38.2 5.88 2.10 0.00008 3-500 0.0 38.2 5.88 2.10 0.00006 8-0 0.0 10.6 3.25 2.01 0.00006 8-0 0.0 11.1 3.83 2.11 0.00006 3-500 0.0 17.7 8.20 2.01 0.00006 3-500. 0.0 18.8 8.28 2.05 0.00005 8-0 0.0 8.03 2.83 2.09 0.00005 8-0 0.0 7.55 2.78 2.03 0.00005 3-500 0.0 18.1 3.75 2.16 0.00005 3-500 0.0 18.1 3.75 2.16 0.00008 8-0 0.0 8.72 2.17 2.01 0.00008 8-0 0.0 8.72 2.17 2.01 0.00008 3-500 0.0 8.50 2.91 2.10 0.00008 3-500 0.0 8.85 2.97 2.15 Continued TABLE VIII - Continued 38 Current Cone. Selector EV.1 T 1/2 i 71/2 (Moles/Liter) Position (vs. S.C.E.) (Sec.) T 0 0.00003 8-0 0.0 2.60 1.61 1.99 0.00003 8-0 0.0 2.60 1.61 1.99 0.00003 3-500 0.0 5.08 2.25 2.16 0.00003 3-500 0.0 5.83 2.33 2.28 Average = 2.06 1 71/2 Average deviation = 0.035 Percent deviation from average = i 1.70 39 I I 1 o I x selou - lg ”“93 I I O o.. . 9-H I. 6 VIII; . g cgfl-gl cotao --.? It 9 1| 0|. P. u co..t.«0¢ .3 a t...” 4.20.2200 I . ...—1.54290 mo...— mho-E zo_._.202_._.z< mo“. who]: zo_._.v 06000666 6000x02 .0061 . l + H. fl l + .H0 cm 6 i 0% ~\0 B an}... H~\0 ~\0 L.V.._ N\0..—. «L. N\0 0.... 0L. \0m 00”.?»de . ___I._.32m_m oz< :_>zoz_._.z< no 5 mmmahxi mDOE<> mon— mPOJm zo_.—.205_._.z< oz< ... 15.3290 ...0 M mum:...x.2 4490;... ...O m5