Simultaneous and Independent Potentiostatic Control of Two Indicatlor Electrodes Application to the Copper(l1) /Copper(l) /Copper in 0.5M KCI at the Rotating Ring-Disc Electrode
System
D. T, Napp, D. C. Johnson, and Stanley Bruckenstein Department of Chemist,vy, University of Minnesota, Minneapolis, Minn. 55455 Experiments with the ring-disc electrode system usually involve simultaneous and independent measurement of currents through the ring and disc electrode, while holding the potential of one of the electrodes constant and controlling the potential of the other electrode in a prescribed manner. An instrument based upon operational amplifiers is described which provides simlultaneous and independent potentiostatic control of the ring and disc electrodes. Its performance wa!; evaluated by studying the copper(ll)/copper(l)/copper system in 0.5M KCI. Several new relationships a t e given for currents observed at the ring electrode in terms of currents at the disc electrode and were verified experimentally using this new instrument.
SEVERAL APPLICATIONS of the rotating ring-disc electrode are known. For example, this electrode was used to investigate the mechanism of the electrolytic reduction of Cu(I1) in a sodium sulfate solutior (I), and evidence was found for the formation of Cu(1) as an electrode intermediate. An amalgamated gold ring-disc electrode was used to study the reduction of oxygen in 0.1N NaOH (2), and it was confirmed that the first wave in the reduction of oxygen at an amalgamated gold electrode results in the production of hydrogen peroxide. The construction of a ring-disc electrode using carbon paste has been described ( 3 ) , and the electrode was used to study the electrode process in the anodic oxidation of N,N-dimethylaniline ( 4 ) . In these er periments the intermediate oxidation states which were produced at the disc electrode were collected at the ring electrode. Effective use of the ring-disc electrode configuration requires independent potentiostatic control of each electrode with respect to the reference electrode potential. No previously reported circuit performs this function automatically. This requirement has been satisfied by the use of an analog control system construci ed with stabilized operational amplifiers. This system is described below and has been tested using the Cu(II)/Cu(I)/Cu system in 0.5M KC1 to verify various theoretical preciictions concerning steady-state currents at ring-disc electrodes. Figure 1 shows the schematic of the circuit for the instrument used with the ring-disc electrode. The operational amplifiers (Philbrick K2-W amplifiers) were stabilized by Philbrick K2-P and K2-PA chopper-stabilizing amplifiers in zero-gridcurrent configuration. 'The circuit in Figure 1 makes use of two circuits which have been described previously (5). The (1) L. M. Nekrasov and N . P. Berezina, Dokl. Akad. Nauk SSSR 142, 855 (1962). ( 2 ) A. N. Frumkin and L. :V.Nekrasov, Zbid., 129, 820 (1959). ( 3 ) Z. Galus, C. Olson, H. Y. Lee, and R. N. Adarns, ANAL. CHEM., 34,164 (1962). (4) Z. Galus and R . N. Adams, J. Am. Chem. SOC.,84,2061 (1962). (5) W. M . Schwarz and I. Shah, ANAL.CHEM., 35, 1770 (1963).
circuitry for the potentiostatic control of indicating electrode 1, I-1, consists of the reference voltage follower, F-1, the control amplifier, A-1, whose output is connected to the counter electrode, C.E., and the current follower, CF. The current follower maintains the indicating electrode at virtual ground potential by means of its current feedback loop. The control amplifier maintains the difference in the potential between electrode I-1, El-l, and the reference electrode potential, E a r / ,equal to the signal potential, EsLp,when R1is equal to
R,'. EI-I - ERef
= Esig
(1)
I n practice we determine the potential of I-1 by measuring the output of F-1 ; R1and R1'were matched only to f1 %. The circuitry required for potentiostatic control of the second indicator electrode, 1-2, consists of the reference voltage follower, F-1, the inverter, I , the control amplifier, A-2, and the voltage follower, F-2. The control amplifier functions by means of its current loop to maintain the value of the potential difference between electrode 1-2, and the potential of the reference electrode, E R ~equal ~ , to minus the signal potential, E'S40,
EI-2
- ERef
=
-E'.qrg
(2)
when (R3
i,PR4)+ (R5
(3)
and
Rp,
+ p'Rq'
=
R7
(4)
p is the fraction of R4 in the feedback loop of I , while p' is the fraction of R4'in series with Rs. Hence the potential of electrodes I-1 and 1-2 may be controlled independently of each other. Because the current loop for control amplifier A-2 includes the indicating electrode I - 1 , it appears at first glance that the independent control of the current through indicating electrode I-1 has been destroyed. However, amplifier A-1 operates to maintain the equality of Equation 1 and thus the counter electrode, C.E., serves to complete the current loop for both indicating electrodes-i.e., the current through C.E. is the algebraic sum of the currents through 1-1 and 1-2. In order to obtain the condition given in Equation 3, Etsfewas set to zero, and R 4 adjusted until scanning Est, f 1.0 V produced less than f O . l mV change in the potential of I-2 with respect to the reference. The condition given by Equation 4 was obtained by setting Esrgat zero volt and adjusting Rd' until the potential between and the output of F-2 differed by less than *O,l mV from zero volt as varied by k l . 0 V. The above potentials were measured with a potentiometer. When these balancing procedures were VOL. 39, NO. 4, APRIL 1967
481
I
R R7
I
Figure 1. Schematic diagram of instrument Platinum counter electrode isolated from bulk solution by fritted glass = Saturated calomel reference electrode = Control amplifiers for electrodes I-1 and 1-2, respectively = Recorder RD,Ra, RE,R7 = Resistors, 100 K, 1 % = Potentiometers, 30-turn, 5K = Resistor, 97.6 K, 1 % = Feedback resistor for the current follower, 10 K to 2 Meg, 1% = Current-measuring resistor for electrode I-2,10 to 1 K, 1% =
used, the deviation from Equations 1 and 2 was less than kO.1 mV for all values of Esfgand in the range * l . O V. As has been pointed out by a reviewer, “there is an interaction between the two electrode control signals, in the uncompensated ZR drop. Thus, the uncompensated ZR drop between Ref and 1-1 will cause an equivalent loss of potential control in 1-2. Although the adjustable controls were designed to minimize this (effect), exact balance is a function of the currents flowing at the two electrodes, and the balance point will shift as the currents change. Although this error will be small in most cases, fairly large currents are encountered with convection electrodes, and the uncompensated IR drop must be kept at the lowest possible value. . . . in addition. , the potential of I-1, as measured at F-1, includes the error resulting from the uncompensated ZR drop . . .” In the work we report below, no problems arising from any uncompensated IR drop have been observed. In performing certain other experiments, we have noted effects which can be attributed to uncompensated ZR drop. The effects are particularly noticeable if the Luggin capilllary tip is not positioned at a point close to and normal to the disc electrode surface. An x-y recorder may be used to record current-potential curves. The potential of I-1 is determined with respect to the reference potential by placing the x-axis input of the recorder across the reference follower, F-1, and ground. The potential of the electrode 1-2 is determined by driving the x-axis of the recorder with the signal voltage E’sfo. The signal potentials EBig and Erst, may be generated by conventional ramp generators. The current through I-1 is determined by placing the y-axis input of the recorder across the output of the current follower, CF, and ground. The current through 1-2 is determined using a recorder with potentiometric input across the output of
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482
ANALYTICAL CHEMISTRY
potential follower F-2 and the output of control amplifier A-2. The potential difference measured will be equal to the product of the current in 1-2 and the resistor R9. Simultaneous recording of the currents through 1-1and 1-2 should be possible with an x-y-y ’ recorder. EXPERIMENTAL
The above described instrument was tested with a rotating platinum ring-disc electrode (RPRDE) in 10-3M CuCk 0.5M KC1 solution. A rotation speed of 2500 rpm was used. The current-potential curves were recorded on a Houston Instruments HR-97 T-2 x-y recorder, All potentials were measured with respect to the SCE. The potential of the disc electrode (Z-1, Figure 1) was scanned at a rate of 750 mV/ minute through a potential range of f0.4 to - 0.6 V, and the disc and ring currents were recorded in successive scans. 0.5” C. The electrochemical cell was thermostated at 25 The dimensions of the RPRDE are as follows: disc electrode radius (R,) = 0.3874 cm; ring electrode inside radius (Rz)= 0.3949 cm; ring electrode outside radius (R3) = 0.4054 cm. The various ring electrode currents observed in this experiment depend upon the ring electrode potential and the disc electrode potential E+ The notation iR(E‘&,, E8
