c1CYCLIC CHELONO, DIFPU- c2SOLVE GENERATED EQUA- 903 FORMAT (5HRR =, F10.5, SION CONTROLL, PLANE TION BEGIN AT 96 READ 8HFRACT =, F10.5) , READ IN K IN NOSIG FOR ACCURACY GO TO 920 NOSIG RR FRACT, TWO 96 IF(M- 1)300,100,102 300 PRINT905 SOLUBLE ElPECIES 100 Z=Y 905 FORRSAT (2X,5HEItROR) READ 900,K,NOSlG, RR, M=M+l 920 STOP FRACT 102 IF (Z) 98,200,99 EXD DIMENSION X (100),T (1 00) , 98 IF (Y) 71,200,73 END R(100) 99 IF (Y) 73,200,71 C GENERATION OF EQUA- 71 T(N) = T(N) + 10.0 **(-LA) LITERATURE CITED TIONS GO TO 10 (1) Alden, J. R., Chambers, J. Q., Adams, DO200N = 1,K 73 T(N) = T(K) - 10.0 **(-LA) R. N., J. Electroanal. Chem. 5, 152 T(N) = 0.0 LA=LA+I (1963). M=l 199 IF (NOSIG - LA) 300,200,71 (2) Bard, A. J., ANAL. CHEM. 33, 11 (1961). LA = 0 200 CONTIXUE (3) Churchill, R. V., “Operational Mathe- 10 DO 80 I = 1,N c3EQUATION SOLVED PRINT matics,” p. 39, McGraw-Hill, New York, SUM = 0.0 ANSWER 1958. DO 60 J = I,N DO201 J = 1,K,2 (4) Galus, Z., Lee, H. Y., Adams, R. N., = 201 R(J) = T(J)/T(J 1) J. Electroanal. Chem. 5, 17 (1963). 60 SUM SUM -- T(J) + (5) Murra,y, R. W., Reilley, C. N., Ibid., X(1) = SQRTF(SUM) PRINT 903, RR, FRACT 3, 182 (1962). 80 CONTIXUE PRINT 901 (6) Piette, L. H., Ludwig, P., Adams, Y = X(1) - 1.13 PRINT 902, [S,T(K),R(N), R. K., ANAL.CHEM. 34, 916 (1962). (7) Reinmuth, W. H., Ibid., 34, 1446 IF (1 - pu’) 81,96,300 K = 1.K1 (1962). 81 SIGN = 1.0 500 CONTIkUE (8) Testa, A. C., Reinmuth, W. H., Ibid., DO 95 L = 2,N 900 FORMAT (2110, 2F10.5) 33, 1324 (1961). = FORMAT [4(3X, lHN, 6X, SIGN -SIGN 901 RECEIVEDfor review March 27, 1963. 95 Y = Y + SIGN * RR *X(L) - 4II TAU, 5X, 5HRATIO)//] Accepted May 8, 1963. Work supported SIGN - SIaK * FRACT 902 FORMAT [4(1X, 113,2F10.6)/] by the Robert A. Welch Foundation.

High Speed Controlled Potential

ALLEN J. BARD Department of Chemisfry, The University of Texas, Austin, Texas

b An cell for rapid con- reference to calibration curves, etc.-the and depends upon such experimental trolled potential coulometric determi- long electrolysis times usually required conditions as electrode shape, cell nations, employing a large electrode sometimes discourage potential users of geometry, and turbulence of flow. It is area-to-solution volume ratio and using this technique. The aim of this study probably better to write simply ultrasonic and nitrogen stirring, was was to consider the factors governing the designed. This cell allowed deter- speed of an electrolysis, and to design a minations to be performed with total cell capable of performing a controlled where m is a mass transfer constant. electrolysis times of less than 100 potential coulometric analysis in a short There is frequently no direct propor- seconds. The apparcitus was tested time. tionality between p and A; the dimen- by determining silver (I) and iodide by For a single electrode reaction carried sions of the electrode, rather than the electrodeposition of silver and silver out at potentials at which the rate of the area, are more important (3). Com- iodide, respectively. From 2.5 to reaction is limited by the rate of mass pletion of electrolysis is generally taken 25 pmoles of each was determined transfer of the electroactive species to at the time when the current has de- with an average error of 2 to 0.270. the electrode, the current decays ac- cayed to 0.1% of its initial value, that is The application of t7is cell to the cording to the equation (7) study of mechanisms of electrode t = 6.9/p (4) reactions was also considered. it = ;&-P’ (1) To decrease the electrolysis time, p must where it is the current at time t, 6 is the be made as large as possible. In this initial current, and p is a function of the study an electrolysis cell was designed ONTROLLED potential coulometry electrode dimensions, solution volume, with a large electrode area-to-solution C has been useful both as an ana- cell geometry, and rate of mass transfer. volume ratio, which employed ultra- lytical technique and for the investiga- For a simple Nernst diffusion layer sonic and nitrogen stirring. With this tion of mechanisms of electrode re- model of convection, p is given by the cell 811 “effective p” of about 0.1 actions. The time required to perform expression second-’ was obtained, so that elec- a controlled potential determination is trolysis times were only slightly longer p = DA/6V (2) usually hetween 20 miniites and 2 hours, than one minute. dqiriidiiig upou tlir c.qxrirnonta1 ap- wlitve z) is the diffusion coeflicient of I l:Lr:i t I IS f?mjIloyed. hltl~ough ContIolled the electroactive species, d is the elec- EXPERIMENTAL potential coulometric analysis has the trode area, V is the total solution vol- Apparatus. Kith a suitable po- advantage of being an rabsolute method ume, and 6 is the thickness of the diffu- tentiostat and coulometer, the design -Le., allowing the direct determination sion layer. The actual dependence of p of the electrolysis cell usually governs of the quantity of a substance without upon these variables is very complex, the electrolysis time. After experi-

VOL 35, NO. 9, AUGUST 1963 1125 other contact of the transducer was pecially in the early stages of elec- connected by means of a small metal trolysis, that electromechanical po- piece, 3, leading around the Teflon tentiostats and coulometers cannot be gasket and connecting to the brass wall used. In this investigation an electronic of the piece. This, in turn, connected , based on one commercially to the outside (ground) terminal of the available from Brinkmann Instruments, BNC connector. An inlet, 1, was used Great Neck, L. I., N. Y.,as a Wenking to direct a stream of compressed air or Potentiostat, with an output of other coolant against the transducer about 25 volts at 250 ma. and a to cool it. The glass cell was held response time in the order of micro- against the transducer by the upper seconds, was employed. The coulometer threaded brass piece, which screwed was based on a -to-frequency onto the second brass piece and pressed converter and a digital counter (2). the lip of the cell down against the trans- A block diagram of the apparatus is ducer gasket. shown in Figure 3. A Sargent Model Another Teflon gasket between the SR recorder, measuring the voltage drop upper part of the cell lip and the upper over a standard resistor, Kas used to machined brass piece relieved the strain obtain current-time curves. Since the on the glass lip. This rather complex recorder could not follow the rapid mounting arrangement assured that current changes during the first few the transducer and cell would be seconds of electrolysis, the ball-and- rigidly held in place and could be disk integrator which is connected to easily disassembled for cleaning or for this recorder is not suitable for determin- making cell or transducer modifications. ing the total number of coulombs in- il simpler cell and transducer mounting volved in the electrolysis. A Model could probably be designed. 35 ultrasonic generator (McKenna Lab- The cell cover, which contained a tube oratories, Santa Monica, Calif.) , op- bil- closed at the bottom with a fine porosity erating at 1 Mc. per second at Figure l. Schematic diagram of elec- sintered glass disk, 9, for the auxiliary power levels up to 35 watts, was used electrode, 10, fitted to the cell body with a one-inch barium titanate trans- trolysis cell by means of a standard taper joint. ducer in these experiments. Additional standard taper joint con- Procedure. The electrolyses mere nections in the cell cover permitted performed by adding about 7 ml. of menting with several types of cells, the introduction of a reference elec- supporting electrolyte solution to the we found the one shown schematically trode, 7, nitrogen, 8, and sample cell, deaerating, and prc-electrolyzing in Figure 1 to be most useful. Details aliquots. With this arrangement, and the solution at the control potential of the cell construction are shown in because a stirrer did not have to be Figure 2 and are described below. positioned, the cell could be assembled The solution volume was about 7.5 very rapidly. I mi. when the cell was filled to slightly The requirements for the potentiostat above the bottom of the auxiliary and coulometer are somewhat more electrode chamber. A platinum gauze stringent for high speed electrolg& electrode, 6, wound in a tight spiral than for the usual slower methods. so as to fill the sample solution com- The current changes so rapidly, es- pletely, was used as a . With this electrode design, the usual types of stirring by rotating the working electrode or by employing a propeller or magnetic stirrer could not be used. Ultrasonic stirring eliminated the need for moving parts within the cell, and by making the base of the cell a barium titanate transducer driven by an ultra- sonic generator, effective stirring could be accomplished. A rapid flow of nitrogen through the solution also 6 aided in increasing the mass transfer rate. The base of the cell consisted of three 10 pieces of machined brass. The bottom piece screwed into the second piece aI~~iKii 9 and held the ultrasonic transducer, 5, I !I1 'Ill! pressed between two Teflon gaskets, against the upper lip of the second piwe. -7 The upper Teflon gasket was shaped so that when the glass cell portion was 4- -6 finally attached, solution would not come in contact lvith the brass. 2 Electrical connection to the trans- ducer was made with a BNC connector. The middle terminal, 2, was connected to one contact of the transducer; the

Figure 2. Detailed diagram of electrolysis cell

1. Compressed air inlet for coolingt ransducer. 2,3. Electrical connection I to transducer. 4. Teflon gaskets. 5. Barium titanate ultrasonic trans- ducer. 6. Platinum gauze electrode. 7. . 8. Nitrogen inlet. 9. chamber. 10. Auxiliary electrode. The cell body was borosilicate glass. The transducer mounting was brass 6 cm.

1126 rn 1 TIME, scc. Figure 4. Effect of varying conditions upon log current vs. time behavior for electrodeposition of iodide as Agl on a silver anode. Solution contained 12.5 pmoles iodide in Figure 3. Block diagram of controlled potential coulometric about 6.5 ml. of 1M KN03. Control potential was -0.20 analysis apparatus volt vs. saturated mercurous sulfate electrode until the background current reached 1. Gauze electrode, ultrasonic and nitrogen stirring a low, constant value. The counter 2. Gauze electrode, only nitrogen stirring was set for a preset time, the elec- 3. Gauze electrode, no stirring trolysis started, and the sample in- 4. Electrode of 16-gauge silver wire coils, ultrasonic and nitrogen stirring troduced. At the completion of the electrolysis, the cour ter was reset and the correction due to the background and apparently is involved with the these studies, may prove useful for more current was determined. When sample electrolysis of material in the immediate efficient mass transfer, especially in volumes were kept small (500 pl. or vicinity of the electrode (within the larger cells. less), up to four determinations could diffusion layer). After about 6 to 8 Nitrogen stirring had the disad- be performed before the cell had to seconds the slope changes to that charac- vantage of causing droplets of solution be cleaned and refilled with supporting teristic of the rate of masstransfer. In un- to collect on the walls of the upper por- electrolyte solution. stirred solutions this slope is about 0.01 tions of the cell, leading to lorn results. second-’, while in a solution with both Flushing down the cell walls with a small RESULlS ultrasonic and nitrogen agitation, it is amount of the electrolysis solution with- Determination of Silver. Silver about 0.05 second-’. Curve 3 in Figure drawn during the determination in a was determined by plating onto the 4 indicates that the current decays to dropping pipet decreased errors due to platinum electrode from 1M KKOs 1 and 0.1% of its initial value in about this effect. at a potential of -0.40 volt us. a 44 and 68 seconds, respectively, in- Application to Electrode Mechanism saturated mercurous sulfate electrode. dicating an over-all effective-p of about Studies. When secondary chemical Results of the analysis of 2.5 to 25 0.1 second-l. Since the initial slope is reactions occur along with the elec- pmoles of silver are given in Table I. usually not larger than 0.5 secondp1 trolysis reaction, the number of The total electrolysis time, including the (corresponding to a p of 1.2 second-’), coulombs of consumed may time required for satr.ple addition, was the electrolysis time would probably be more or less than the amount ex- 100 seconds. be no shorter than about 6 seconds with Determination of Iodide. Iodide this cell, even with the best possible was determined by formation of AgI conditions. upon anodic polarization of a silver- The effect of ultrasonics on electrode Table 1. Determination of Silver plated platinum electrode by the reactions has been studied extensively Supporting electrolyte, 1M K?;03. Solu- method of Lingane and Small (8), from (9, 11). Ultrasonics primarily increase tion volume, above 7 ml. Platinum elec- trode at -0,40 volt us. saturated a supporting e1ectrol:rte of 1Ji KNO, the rate of mass transfer to the electrode, mercurous sulfate electrode. Electrolysis at a potential of -0.20 volt us. a sat- although alteration of the structure of time, 100 seconds urated mercurous sulfate electrode. Re- metals deposited under the influence of KO. of sults of the analysis of 2.5 to 25 pmoles ultrasonics has also been reported. Too deter- Silver Silver Av. of iodide are shown it1 Table 11. The high levels of ultrasonic power caused mina- taken, found, dev., cell has also been used successfully for a the solution to heat up very quickly and tions pmoles pmoles pmole number of other electrolyses, including eventually boil. Ultrasonic radiation 4 2.501 2.447 0.035 3 12.50 12.42 0.04 the electro-oxidation of hydrazine and is also capable of causing sonochemical 4 25.01 24.94 0.28 iodide, and the electrolysis of diphenyl- reactions (5, IO), such as the dissocia- picryl hydrazyl in an acetonitrile tion of water into hydrogen and hydroxyl medium. Details of this work will be radicals, which might react with the Table II. Determination of Iodide presented elsewhere. electroactive substance or cause an in- Supporting electrolyte, 1M KN03. Solu- Current-Time Behavior. Figure 4 crease in the background current. Most tion volume, about 7 ml. Silver-plated illustrates the effect of variation of of the electrolyses performed here were platinum electrode at -0.20 volt us. the electrode area and stirring con- conducted at ultrasonic power levels of saturated mercurous sulfate electrode. ditions during the determination of below 30 watts. In kinetic studies Electrolysis time, 100 seconds iodide ion upon log it us. t behavior. where a constant temperature is de- KO. of deter- Silver Silver Av. Similar behavior was found during the sirable electrolyses can be carried out mina- taken, found, dev., electroplating of silver. The curves with only nitrogen stirring at the ex- tions pmoles pmoles pmole are characterized generally by two pense of some increase in p. Higher 4 2.500 2.463 0.101 slopes. The larger, initial slope of about levels of ultrasonic power, or radiation 3 12.50 12.43 0.03 0.1 to 0.3 second-’ appears on curves of a frequency different than the 1 2 25.00 25.04 0.14 in both stirred and unstirred solutions megacycle per second employed in

VOL. 35, NO. 9, AUGUST 1963 1127 ference of the chemical reaction de- (or 0.01 peq.) assuming a voltage change Table 111. Effect of Side Reactions on creases as the rate of the electrolysis of 0.5 volt and a double layer capacity of Coulometric Oxidation of Hydrazine in (governed by p) increases. In the high 20 pf. per sq. cm. The cell described Usual and High Speed Cell speed cell the nospp values were closer to here was useful only for electrolyses with Supporting eIectrolyte, 0.05M HISOl 4, even at high concentrations of hydra- solid . Constructing a cell Hydrazine zine. Actually, because the potentio- for use with mercury cathodes is appar- concentration, stat employed in this study had a limited ently more dificult and is currently be- mM no.,, current output, the cell was not op- ing investigated. Usual cell;5 erating under limiting current conditions p estimated at about second-' during the total duration of the elec- ACKNOWLEDGMENT 0.1 4.02 trolysis, and the effective p was smaller The author is grateful to John Dooley 1 .o 3.80 than that obtained under optimal condi- for aid in the construction of the poten- 10.0 3.27 tions. With a potentiostat of higher tiostat and cell, and to Harvey Kappler High speed cell; output capacity a p of about 0.1 second-'. for experimental work on the hydrazine p = 1.2 X lo-" second-' should be obtainable (p is practically reaction. 3.3 3.83 independent of the concentration of the 16.0 3.80 electroactive substance), and an nospp LITERATURE CITED

o Data of Karp and Meites (6). even closer to 4.00 should result (1). (1) Bard, A. J., Mayell, J. S., J. Php. Chem. 66, 2173 (1962). (2).. Bard, A. J., Solon,., E., ANAL. CHEM. CONCLUSIONS 34, 1181 (1962). (3) Delahay, P., "New 1nstt;ymental pected if only the electrode reaction Short electrolysis times yield ad- Methods in , Chap. occurs (1, 4). The variation of the vantages other than merely decreasing 9, Interscience, New York, 1954. amount of electricity consumed during the analysis time. High speed con- (4) Geske, D. H., Bard, A. J., J. Phys. an electrolysis has been used to explain trolled potential coulometry diminishes Chem. 63, 1057 (1959). the mechanism of electrode reactions (5) Haissinsky, M., Klein, R., Rivayrand, the effect of chemical side reactions dur- P., J. Chim. Phys. 59, 611 (1962). and to estimate the rate constants of ing analytical determinations and per- (6) Karp, S., Meites, L., J. Am. Chem. the secondary chemical reactions. For mits the application of thia technique to SOC.84. 906 (1962). example, in a study of the controlled the study of electrode reactions where (7) Lingahe, J.' J., Anal. Chim. Acta 2, .591--- (1948),- - -- ,. potential coulometric oxidation of side reactions are occurring at faster (8) Lingane, J. J., Small, L. A., ANAL. hydrazine, Karp and Meites (6) found rates. The accuracy of the coulometric CHEM.21, 1119 (1949). that because a chemical reaction con- determination should be increased, be- (9) Littlewood, K.. J. Row. Insl. Chem. sumed some electroactive material, the cause the background current is in- . 86,78 (1962). ' apparent number of coulombs per creased less than the current due to the (IO) Weissler, A., J. Acoust. SOC.Am. 25. 651 11953). of hydrazine oxidized (nospD)was less electroactive species. Transfer of solu- (11) Yeage< E., Hovorka, F., Ibid., 25, than the 4.00 expected on the basis of tion into and out of the auxiliary elec- 441 (1953). the electrode reaction alone (Table 111). trode chamber is also less for a short for review February 8, 1963. electrolysis time. On the other hand, RECEIVED NIHI+ 4 NI 5H+ 4e Accepted May 17, 1963. Division of + + using large effective electrode areas in- Analytical Chemistry, 142nd Meeting Note that noSepdecreases with increasing creases the error due to charging of the ACS, Atlantic City, N. J., September, 1962. Work supported by The Universit

Po Ia ro g r a p hy of Se Ie ni u m(IV)

GARY D. CHRISTIAN and EDWARD C. KNOBLOCK Division of Biochemistry, Walter Reed Army Institute of Research, Walter Reed Army Medical Center, Washington 12, D. C., and WILLIAM C. PURDY Department of Chemistry, Universify of Maryland, College Park, Md.

b As many as three polarographic lated elementary selenium which re- authors attributed the waves to the reduction waves are found for sele- sists further reduction. The polaro- stepwise reduction to the +Z elemental, nium(1V) depending upon the pH of graphic characteristics of selenium(lV) and -2 oxidation states, respectively. the medium. The limiting current of in a number of electrolytes are de- In very dilute solution, the first two all waves is diffusion controlled but scribed. waves merged. They reported a single only the second wave is reversible. wave for the reduction of selenite ion The first wave exhibits a four-electron to the element in ammoiiiacal solution. change while the second wave cor- HERE is considerable confusion in Lingane and Xedrach (13) found that responds to a two-electron change. Tthe literature concerning the poIar- selenide ion gave a dissolution wave In basic medium, the third wave is due ographic behavior of selenium. The at the dropping mercury electrode to a six-electron change. However, in first of selenium was (DME) similar to that of sulfide ion. acid medium, coulometric reduction at reported by Schwaer and Suchy (17) These authors also studied the po- potentials corresponding to the third who described three waves for selenium larography of selenite and selenate wave results in the formation of coagu- (IV) in IN hydrochloric acid. These ions (14). They attributed the selenite

1 128 ANALYTICAL CHEMISTRY