Solid Electrode Voltammetry with Renewal of Electrode Surface Richard J. Lawrance and James Q. Chambers Department of Chemistry, Unioersity of Colorado, Boulder, Colo. 80302
THEPREPARATION of “clean” electrode surfaces is a n experimental technique of some controversy among those who practice solid electrode voltammetry. Various chemical, electrochemical, and mechanical means have been employed by different workers. However, even with a freshly prepared surface, adsorption of reactants, intermediates, o r products can complicate steady-state and current-time measurements a t solid electrodes. I n the case of mercury, the dropping mercury electrode presents the well known unique advantage of producing fresh electrode surfaces in the course of the current-potential (i-E) curve. We have attempted a similar approach for solid electrodes by combining the carbon paste electrodes of Adams ( I , 2) with the methodology of the dropping mercury electrode. Thus the instrument described below is capable of carrying out successive potentiostatic experiments o n freshly prepared electrodes of known area. The instrument was evaluated for use as a device for electroanalytical measurements. The approach described in this paper differs significantly from previous work. Other researchers have devised methods for renewing the solution layer in the electrode-solution interface by means of a sudden movement of the solution with respect t o the electrode or oice oersa. Descriptions of these methods and references t o previous work can be found in the papers of Cozzi et al. (3) and Roffia and Viannello (4). In the work presented in this report, not only is the solution in (1) R. N. Adams, ANAL.CHEM., 30,1576 (1958). (2) C. Olson and R. N. Adarns, Anal. C/zim.Acta, 22,582 (1960). (3) D. Cozzi, G. Raspi, and L. Nucci, J. Electroanal. Chem., 6 , 267, 275 (1963); 12, 36 (1966). (4) S. Roffia and E. Viannello, Ibid., 12, 112 (1966).
the electrode-solution interface replaced, but the electrode surface itself is renewed. Thus history effects due t o previous electrolysis are minimized. In this respect the electrode system resembles that of Eyring et al. (5, 6) who produced fresh electrode surfaces in solution by scraping a metal electrode surface. EXPERIMENTAL
The electrode arrangement consists of 24 wells, 0.500 =t 0.005 cm diameter, drilled into a Plexiglas wheel and filled with carbon paste. The wheel is mounted on a shaft which revolves at 0.35 rpm (see Figure 1). The lower portion of the wheel is immersed in solution and potentiostatic experiments are performed on successive electrodes as the wheel revolves through a quiet solution. Electrolysis occurs at a given electrode when it is at the bottom of the wheel, hence each electrode surface has been immersed in solution for approximately 15 seconds before a potential is applied to it. The limiting current resulting from this measurement is schematically represented by the dotted line in Figure 2. Since the diffusion layer containing the electrolysis products tends to be carried away with the preceding electrode, each electrode encounters a fresh solution and a current decay curve results for each electrode. In order to reduce the height of the current spike displayed on the recorder, the output of the current measuring amplifier is fed t o the recorder only during the time span tl to t2 for each electrode. (5) T. A. Anderson, R. S. Perkins, and H . Eyring, J . Am. Chem. Soc., 86, 5596 (1964).
(6) R. S.Perkins, R. C. Livingston, T. N. Andersen, and H. Eyring, J. Phys. Cliem., 69, 3329 (1965).
time
r ELECTRODE
/ CELL
T O POTENTIOSTAT
2%
Figure 1. Pictorial representation of revolving wheel electrode
134
ANALYTICAL CHEMISTRY
Figure 2. Schematic representation of current-time behavior
T
MICRO SWITCH
Figure 3. Block diagram of timing circuit
A t all other times the voltage corresponding to the current at ry is held o n a capacitor and fed to the recorder until time rl in the lifetime of the next electrode. Thus a current-time curve is obtained resembling the solid line in Figure 2. The time t2 is determined b j a solid state circuit of conventional design, which senses tht: current rise for each electrode and initiates a RC timing .letwork. A block diagram of this circuit is given in Figure 3. All the results reported in this paper were obtained with a time tz of 4.71 seconds (=t2,3Z at the 95 confidence level). Separate measurements with a n uncalibrated delayed time base of a Textronix Model 585 oscilloscope indicated that tz was constant to better than 1 %. The time tl is determined by a microswitch which is activated by one of 24 cams mounted o n a wheel mechanically coupled to the electrode wheel. This time is adjusted by careful positioning of the cam wheel by means of a set screw. The time t1is not constant from electrode to electrode; this causes the variation of the height of the current spikes in the i-E curves. The transistor circuit of Figure 3 should be used to set a second timing circuit to determine t1 more accurately and consequently imprcwe the appearance of the resulting i-E curves. Two methods were u x d for the graphical measurement of the limiting current. The first consisted of averaging the values of the current (at f2) for 24 electrodes on the plateau of the wave. This gave a value whose average deviation was less than 2 %. The second method consisted of drawing a straight line on the plateau parallel to the charging current and measuring the limiting current between this line and the charging current. Since the latter method resulted in values within the limits of error of the instrument and was considerably simpler, it was used throughout this study. Electrode surface preparation was similar to procedures in the literature ( I , 2, 7) except that excessive care was not taken (7) C. A. Chambers and J. Q. Chambers, J. Am. Chem. SOC.,38, 2922 (1966).
to prepare very smooth surfaces. The 24 electrode surfaces on the wheel could be prepared easily in less than 5 minutes. The ratio of grams of graphite (Union Carbide grade 38 graphite powder) to grams of mineral oil in the carbon paste was 1.92. All chemicals were of reagent grade and used without further purification. Double distilled water was used to prepare all solutions. All studies were carried out at room temperature (25' =t2" C) and in the presence of air. A Moseley Model 135A X-Y recorder was used to display the i-E o r i-time curves. The three-electrode circuits employed have been described (8). RESULTS AND DISCUSSION
Three test systems were chosen for the evaluation of the instrument. The oxidation of 3,3'-dimethoxybenzidine in sulfuric acid affords a n example of a diffusion controlled process (9) and the Fe(III)jFe(II) couple in sulfuric acid exhibits moderately large overpotentials and obeys the equations of charge-transfer controlled kinetics (10) at carbon paste electrodes. The oxidation of 5-aminoindole in acidic solution (pH 1.87) was chosen as a n example of a n oxidation process complicated by surface phenomena (11). A typical i-E curve for the oxidation of the benzidine compound is shown in Figure 4. A sigmoid curve with a well defined limiting current ( i l z m )was obtained at all concentration - i)/i] levels. Logarithmic analysis of the wave form {ln[(iL%m L'S. E) gives the expected RT/nF slope. The limiting current was shown t o be proportional to the concentration of the (8) J. R. Alden, J. Q. Chambers, and R. N. Adams, J . E/ectroanal. Cliem., 5 , 152 (1963). (9) J. F. Zimmermann, Ph.D. Thesis, University of Kansas, 1964. (10) Z. Galus and R. N. Adams, J . Phys. Clzem., 87, 866 (1963). (11) C. A. Chambers and J. Q. Chambers, unpublished data, 1966. VOL. 39, NO. 1 , JANUARY 1967
135
900
800
MV’ WS. S.C.E. 700 600
500
400
-50
-100 G
3 I2 W K
-150 0
-‘I
4 0 . 2
500
Figure 4.
-200
I
550 MV VS. S.C.E.
6 3
I
’
I
Oxidation of 5.00 X 10-3M 3,3’-dimethoxybenzidine in 1M HzS04a t revolving wheel electrode The solid line has a slope of 30 mv per decade. The sweep rate was 1 mvjsec
Table I. Oxidation of 3,3’-Dimethoxybenzidine at the Revolving Wheel Electrode Concentration, irim/C mM ba/mM) AV. 5.00 26.0 26.1 & 0 . 1 26.0 26.2 1 .OO 25.7 26.1 f 0 . 2 26.3 26.2 0.50 25.7 26.3 i 0.5 26.4 26.8 0.20 26.1 26.3 f 0.4 26.9 26.0 0.10 25.7 26.1 f 0 . 5 26.1 26.7 25.2 26.7 26.5
slowly through the solution during the potentiostatic measurement. This constant is easily determined for a particular system by measuring the diffusion coefficient in a quiet solution at a stationary carbon paste electrode. For the conditions employed in this study, the result was iltm =
k
(12) P. Delahay, “New Instrumental Methods in Electrochemistry,” p. 51, Interscience, New York, 1954. 136
ANALYTICAL CHEMISTRY
E F ADC ~
4% =
1.14
The terms in Equation 1 have their conventional notation; t is given by the time tn,4.71 sec. Current-potential curves obtained with this electrode can be shown t o obey the equations of charge-transfer controlled processes. A n i-E curve and the corresponding kinetic analysis obtained a t this electrode in a solution of ca. 5 m M Fe(I1) and 5mM Fe(II1) in 1 M sulfuric acid is shown in Figure 5. Since the overpotentials involved were on the order of 200 mv or greater, heterogeneous rate constants were evaluated from the equation for a totally irreversible electrode process, Equation 2 (13). The diffusion coefficients
_i -electroactive species over a 50-fold concentration range (see Table I). In the simplest approximation the current is given by the Cottrell equation (12) multiplied by a constant which takes into account the fact that the electrode is actually moving
k
8112
Xexp hzerfc X
id
X
=
k2/tiD
used in the calculations, obtained from potentiostatic measurements o n separate ferric and ferrous solutions, were 5.56 (13) P. Delahay, “New Instrumental Methods in Electrochemistry,” p. 76, Interscience, New York, 1954.
3
600.
600
MV VS.'SC.f: 400 200
0
-200
MV
us. S.C.E.
-I
1
30
20 35
-
40
t
D
3
B
40-
k
z
Y W
60 3 0
45
30
50
too 0 0,
I20 0:
t 0-
'2 I-
)
E a
t i
' 2 02
Figure 6. Current-potential curve for the oxidation of 5.3 X 10-3M 5-aminoindole in a solution of pH 1.87 Curve A was obtained on previously used electrodes and curve B was obtained with freshly prepared surfaces. The sweep rate was 2 mvjsec
0
02
0
Figure 5. Current-potential curve for 4.69 X 10-3M Fe(II1) and 5.22 X 10-3M Fe(I1) in 1M H2S04 The sweep rate was 2 mv/sec
X 10-6 and 6.71 X cm*/sec for Fe(II1) and Fe(II), respectively. The concentration of Fe(I1) was determined immediately after the i - E curve of Figure 5 was recorded by titration with standard Ce(IV) solution using the revolving wheel electrode as a n indicator electrode ( E a p p = < 0.45V us. SCE). The value of log ( k J , the logarithm of the heterogeneous rate constant at the Eo' potential, was determined to be -5.15 0.08 ( c m p x ) from three separate measurements. This value is smaller than that previously reported (IO), but agrees well with the v a h e obtained from the separation of the totally irreversible anodic and cathodic peak voltammograms of a cyclic voltammogram recorded o n the same solution (log k , = -4.9) (14). These two examples demonstrate that this electrode can be used t o obtain analyti1:ally and kinetically significant data. However, the most significant feature of this instrument is that the electrode surface can be renewed in the course of a i-E curve. This is of particular importance for organic electrode reactions for $whichreactants, intermediates, and/or products may be adsorbed o n the electrode surface [or extracted into the mineral oil phase of the carbon paste (15)]. The oxidation of organic compounds a t a carbon paste electrode during the first sweep of a cyclic voltammogram will sometimes cause the electrode t o become inactive during sub-
*
sequent cycles. The oxidation of 5-aminoindole in buffered acidic solutions falls into this category. The oxidation wave for 5-aminoindole (pH 1.87) at the revolving wheel electrode is shown by B in Figure 6. The electrode surfaces used t o obtain this wave were renewed manually while the i-E curve was being recorded. A regular i-E curve was obtained which did not exhibit a limiting current. The form and height of this wave were reproducible. Although the mechanism of this oxidation is not understood, it is of interest t o compare this curve with the i-E curve A of Figure 6 which was obtained with a set of electrodes which had previously been used for the oxidation of 5-aminoindole. Clearly previous oxidation has caused the rate of oxidation of the indole t o decrease (perhaps related to the adsorption or extraction of a n oxidation product). These effects are sometimes subtle and can be overlooked in single sweep or rotating disk voltammograms. This electrode permits a systematic study of these phenomena at new electrode surfaces. CONCLUSIONS This instrument is capable of making electroanalytically significant measurements. It has potential use in the study of organic electrode reactions which involve adsorption of reactants or products o n the electrode surface. While the electrode requires technical development before approaching a solid electrode analogy of the dropping mercury electrode, it does hold promise in this direction. ACKNOWLEDGMENT We acknowledge the patient technical assistance of John Cowan, who designed the timing circuit.
(14) R. S. Nicholson and I. Shah, ANAL.CHEM., 36,706 (1964). (15) C . A. Chambers, Masters Thesis, University of Kansas, 1965.
RECEIVED for review July 18, 1966. Accepted November 10, 1966 VOL. 39, NO. 1 , JANUARY 1967
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