Alternating Current Voltammetry with Controlled AI te rnati ng PotentiaI SIR: Electronic control of the alternating potential of an electrode is herein demonstrated to be a valuable improvement for alternating current voltammetric studies in practical systems. Alternating current voltammetry (or ax. polarography) has been limited in its range of applications by the requirement for low cell and circuit resisantces. The alternating voltage across the electrode solution interface differs from the applied value when current flows, and the variation is not linear as peaks are recorded. A number of ways are used to reduce resistances (4). Point-bypoint manual adjustment of the alternating potential gives improved concentration us. peak-height relationships ( I ) , but is not applicable to automatic recording. The phase angle of the current must be corrected for resistance effects by extensive calculations before the data are suitable for the evaluation of kinetic parameters (1, 2). Instruments that electronically control the alternating potential have been reported, though the advantages with respect to high resistance systems have not been emphasized (6, 7). An instrument patterned after the
circuit devised by Booman (3)is in use in this laboratory for controlled potential scan voltammetry and other direct current techniques. Its use has been extended t o alternating current voltammetry by applying the alternating voltage of desired wave form, frequency, and magnitude across a &ohm resistor in series with the ground connection of the Philbrick (6) Model K2-P stabilizing plug-in unit of the potential-controlling amplifier. Both the d.c. scan and the superimposed a x . are simultaneously controlled. EXPERIMENTAL AND RESULTS
Scans were first recorded for the electrolysis a t a stationary pIatinum wire electrode of a 10+M solution of potassium ferricyanide in 1N KCl using a conventional a x . instrument. The d.c. scan rate mas 1 volt per minute, and the superimposed a x . was a 5-mv. (r.m.s.), 84-C.P.S. sine wave. The alternating current was amplified, rectified, and recorded on the Y-axis of a Mosley Model 3s X-Y recorder. The d.c. potential of the electrode was plotted on the X-axis using a saturated calomel reference electrode and a
Leeds & Xorthrup Model i 6 6 4 pH meter. The external circuit resistance was 15 ohms, and the cell resistance for the a x . electrode system was 18 ohms for I S KC1 background electrolyte. Traces were recorded without added external resistance, and nith 50 and 100 ohms added. The experiment was repeated using the potential-controlling device. The alternating current from the current amplifier of the unit was detected, rectified, and recorded as before. External resistances 17 ith values up to 1 megohm were added in series with the auxiliary working electrode. Lissajous figures mere continuously observed, and photographs were made for phase angle comparisons. Figure 1,8, illustrates the result of small variations of resistance when using noncontrolled potentials. In addition, the phase angle of the current was changed by the added resistance. Figure 1,B, shows that peaks recorded with the potential controlling device are not influenced by resistances of A
C
B
D
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llA.
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I
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E vs. S.C.E. Figure 1. A.c. voltammetric scans for reduction of 10-3M K3Fe(CN)o in 1 N KCI background electrolyte using conventional instrument
Figure 2. Charging-current patterns for a platinum wire electrode in 1N KCI with an applied square wave, 84 C.P.S. and 10 mv. (peak to peak)
A.
A.
Using noncontrolled potentials Resistance added 1. None 2. 50 ohms 3. 100 ohms E. Using electronic control of ax. and d.c. potentials with no resistance, 5000 ohms, and 10,000 ohms added
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ANALYTICAL CHEMISTRY
E. C.
D.
3 3 ohms cell and circuit resistance with no potential control 3 3 ohms cell and circuit resistance with 100 ohms added in series 600 ohms cell resistance using potential control Same as C with 3 0 0 0 ohms added
high ohmic resistance systems could not be studied effectively. CONCLUSIONS
The most important practical advantage of electronically controlled a x . voltammetry is the application t o high resistance systems, where conventional a x . voltammetry is inapplicable. Phase selective detection of the current, as reported by Smith and Reinrnuth (7), extends the utility even more. LilERATURE CITED
(1) Bauer, H. H., Elving, P. J., ANAL. CHEM. 30. 334 (1958). (2)-Bauer, H. H., Elving, P. J., Australian J.Chem. 12,335 (1959). (3) . . Booman, G. L. ASAL. CHEM. 29, 213 (1957). (4) Breyer, B., Gutmann, F., Hacobian, S.. Australian J. Sci. Research Ser. A 4,‘597 (1951). (5) Kelly, M. T., Fisher, D. J., Jones, H. C., Maddox, W. L., Stelzner, R. JV., Laboratory Instruments Session, I.S.A. Conference, Sen- York, Sept. 26, 1960. (6) Philbrick, George A., Researches, Inc., Boston, Mass., “GAP/R Electronic Analog Computers,” ‘‘L4pph.tions Manual for Philbrick Octal Plug-In Computing Amplifiers,” and catalog data sheets. ( 7 ) Smith, D. E., Reinmuth, W. H., ANAL.CHEM.32, 1892 (1960). DONALD E. WALKER RALPHS. ADAMS Department of Chemistry University of Kansas Lawrence, Kan. JOHN R. ALDEN School of Electrical Engineering University of Kansas Lawrence, Kan. RECEIVED for review October 21, 1960. Accepted December 7 , 1960. I - - -
0.9
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E
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S. C. E.
Figure 3. A.c. voltammetric scan of 2 X 1 O-3M KI in 75% acetone-25% water with 0.25M HzS04 background electrolyte using controlled potential (4) 4600 ohms cell resistance
10,000 ohms or less, and no difference in the phase angle of the current was noted. Resistances up to 100,000 ohms did not change the peak height, but the total a x . current increased during part of the alternating cycle, which indicates some loss of control. Resistances above 500,000 ohms caused overloading of the amplifiers, and resulted in clipping of the sine-wave current. The charging of the double layer capacitance for the electrode system in I N KC1 with an applied square wave was studied as an indication of the ability of the instrument to control the
ax. potential. The square wave response is shown in Figure 2. Potential control results in rapid charging of the double layer, and better separation of capacitive and faradaic currents than in noncontrolled systems. A solution of 2 i< 10-3AVKI and 0.25M H2S04in a mixed solvent composed of 7 5 7 , acetone-25% water was oxidized using potential control and the scanning conditions previously described. The cell resistance as determined by an a x . bridge was 4600 ohms. The resulting scan is shown in Figure 3. Without potential control, such
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Determination of Cesium as Permanganate SIR: The solubility of cesium permanganate in water a t temperatures a few degrees above 0” C. is lower than that of any other metal permanganate. At 1O C. a saturated solution has only a faint violet tint, and the solubility product of the salt a t this temperature is 1.5 X This indicates the possibility of practically quantitative preripitation, a t least in a sufficiently concentrated cesium solution a t a low temperature in the presence of a sufficiently high concentration of permanganate. EXPERIMENTAL
Evaporate the neutral solution, containing not less than 5 mg. nor more than 50 mg. of cesium, to dryness in a 50-ml. beaker or Erlenmeyer flask. Dis-
solve the residue in 1 ml. of water and half immerse the vessel in a bath of salted cracked ice. Add dropwise from a buret 6 ml. of 0.1X ammonium permanganate while swirling the liquid in the vessel. Allow the vessel to stand in the bath a t least 10 minutes. Prior to beginning the precipitation, half immerse in the same bath a micro wash bottle containing about 25 ml. of propionic acid. At the same time place a weighed filter crucible in a small container near some dry ice so that the entire crucible is cooled by carbon dioxide vapor. At the end of the cooling period of a t least 10 minutes fit the cold crucible to a suction apparatus, and transfer the precipitate and solution rapidly to the crucible with the aid of successive I-ml. portions of the propionic acid. Wash the precipitate with a few additional 1-ml. portions of pro-
pionic acid and allow the suction to continue until the precipitate appears to be dry. Remove the crucible, touch the bottom of it to a piece of filter paper to remove any adhering propionic acid, and place it in a drying oven maintained a t 110’ to 120’ C. Dry for about an hour, cool, and weigh. Multiply the weight of the precipitate by 0.5270 to obtain the weight of cesium or by 0.5593 to obtain the weight of cesium as oxide. Instead of drying and weighing the cesium permanganate, some time may be saved by dissolving the precipitate and titrating the solution. To do this, first dry the crucible containing the precipitate briefly, place it in a funnel leading to a 250-ml. beaker, and dissolve the precipitate by adding in successive small portions a solution prepared by adding 2 ml. of concentrated sulfuric acid to 25.00 ml. of VOL. 33, NO. 2, FEBRUARY 1961
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