scan rate is the important parameter and stirring rate is only of significance in the deposition step, as i t is in all variations of anodic stripping voltammetry. It is certainly a convenience to continue the stirring during stripping. The avoidance of the “rest period” decreases the chance of chemical oxidation of reduced phase by traces of oxygen and other oxidizing impurities, if no potential is applied. On the other hand, if the deposition potential is left on, the amount deposited is not proportional to id.
The peak currents of the measurements given in Figure 3 occurred at a potential of -0.620 i 0.005 volt us. AgCl(satd. KCl)/Ag. Thus E, is a useful qualitative measurement in the same sense as is Ellz. However, the provision that the approximations are acceptable must be observed. I n Table I the dependence of E, on Y is noted for the cadmium example. Calculated values of E, are from Equation 9 with E’’ taken as the polarographic half-wave potential (7), -0.595 volt us. AgCl(satd. KCl)/Ag.
The resolution of current peaks obtained with this type of electrode is rather remarkable, as shown in Figure 4. This combination of sensitivity and resolution is not matched by any of the other electrodes used with linear scan voltammetry and is perhaps better than A.C. polarography where the width of the peak is strongly dependent upon the transfer coefficient. A comparison between this electrode and the hanging mercury drop electrode is given in Figure 5 . So that the conditions would be as close as possible, the stripping curve of the mercury-film electrode was calculated from the experimental data given by Shain and Lewinson (IS); extrapolation of the curve was necessary in order to obtain a value for C E O . LITERATURE CITED
(1) DeFord, D. D., 133rd Meeting, Amer-
ican Chemical Society, San Francisco, Calif., April 1958. (2) Gardiner, K. W., Rogers, L. B., ANAL. CHEM.25, 1393 (1953). (3) Granville, W. A., Smith, P. F., Langley, W. R., “Elements of the Dif-
ferential and Integral Calculus,” Ginn & Co., New York, 1941. (4) Kemula, W.,Galus, Z., Kublik, Z., Bull. Acad. Polon. Sei., Ser. Sei. Chim. 7,732 (1959). (5) Jahnke, E., Emde, F. “Tables of Functions,” p. 6, Dover Publications, New York, 1945. (6;‘Malmstadt, H. V., Enke, C. G., Electronics for Scientists,” p. 371, W. A. Beniamin. Inc.. 1962. (7) Meites, L., “Polarographic Techniques,,’ Interscience, New York, 1955. (8) Neeb, R., 2. Anal. Chem. 180, 161 (1961). (9) Ramaley, L., Brubaker, R. L., Enke, C. G., ANAL. CHEM.35, 1088 (1963). (10) Reinmuth, W. H., Ibid., 33, 185 (1961). (11) Ibid., Columbia University, N. Y., private communication, 1965. (12) Shain, I., “Stripping Analysis,” “Treatise on Analytical Chemistry,” I. M. Kolthoff and P. J. Elving, Eds., Part I, 5’01. 4, Chap. 50, Interscience, New York, 1963. (13) Shain, I., Lewinson, J., ANAL.CHEW 33, 187 (1961). (14) Tries, W. T. de, Dalen, E. van, J. “
I
Electroanal. Chem. 8 , 366 (1964).
RECEIVEDfor review June 24, 1965. Accepted August 16, 1965. Work supported in part by the U. s. Atomic Energy Commission under Contract AT(30-1)-905.
Effect of Direction of Polarization on Current-Potential Curves at Stationary Electrodes LAURA CHUANG and PHILIP J. ELVING The University of Michigan, Ann Arbor, Mich. Reverse polarization a t a stationary electrode-i.e., variation of the potential from a value at which an electrode reaction normally occurs to one a t which it normally ceases-often allows observation of the voltammetric wave caused by electrolysis of the product of the original electrode reaction. Reverse polarization affords a simple means of identifying electrode reaction products and obtaining data for unstable species. Application of the technic to two reversible systems (ferri-ferrocyanide and nitrosobenzene-phenylhydroxylamine) and two irreversible systems (azoxybenzenehydrazobenzene and azobenzenehydrazobenzene) is described.
L
on stationary electrode voltammetry contains the frequent injunction that the indicating electrode should be polarized in the forward direction-Le., toward more negative potential if a reduction process is to be observed or toward more positive potential for an oxidation. Thus, Lord and Rogers (3) report that reverse polarization-e.g. , from positive toward ITERATURE
1506
ANALYTICAL CHEMISTRY
zero potential for an oxidation-resulted in poorly defined waves, when there were waves at all. I n connection with an extensive investigation of voltammetry at solid electrodes, studies of the effect of direction of polarization on the voltammetric behavior of reversible and irreversible redox systems at a stationary pyrolytic graphite electrode (P.G.E.), make explicit the basis for the injunction indicated and, in addition, provide a basis for the possible study of the products and reversibility of electrode reactions. I n the latter connection, the technic simulates very low frequency cyclic voltammetry. The limitations and factors involved in applying the technic of reverse polarization have been evaluated. The ferro-ferricyanide system was studied because i t is the reversible redox system, whose behavior at solid electrodes has been most extensively studied. The nitrosobenzene-phenylhydroxylamine system is reversible at both the P.G.E. and D.M.E. (1). The azobenzenehydrazobenzene and azoxybenzenehydrazobenzene systems behave irreversibly at the P.G.E. (2).
EXPERIMENTAL
Test solutions of reagent grade potassium ferricyanide and ferrocyanide were prepared in 0.5X KC1 solution. Synthesis of phenylhydroxylamine, purification of nitrosobenzene, azoxybenzene, azobenzene, and hydrazobenzene, and preparation of buffer and test solutions have been described ( 1 , 2 ) , as have the apparatus and disk-shaped P.G.E. used. Procedure. I n t h e case of reducible species, the i-E curve is recorded from a potential, a t which reduction does not occur, toward more negative potential. After t h e wave is located, the scan is stopped a t a potential about 0.1 volt more negative t h a n the peak potential, E,, the direction of polarization reversed, and a voltammogram recorded going toward less negative potential. For oxidizable species, the initial scan is from an appropriate potential toward less negative potential; the direction of polarization is reversed after the conventional oxidation wave is seen. I n order to simulate normal operating conditions, potential ranges were obtained by various combinations of initial potential, direction of polarization, and indicating electrode polarity via the settings on the Leeds &: Northrup Type E Electro-Chemograph used.
Table 1.
Run A- 1 A- 2 A- 5 A-4 A-6 A-3
E , VOLT
Figure 1. Voltammograms for forward and reverse polarization of 0.4mM nitrosobenzene in pH 1.6 buffer (50% ethanol) at a stationary pyrolytic graphite electrode Forward polarization (from 0.4 to 0 volt), reduction wave B. Reverse polarization (from 0 to 0.4 volt), composite reduction w a v e
A.
REVERSIBLE REDOX SYSTEMS
Representative data are presented in Table I. The only factor that affects t.he shape of the wave is the direction of polarization. Different combinations of instrument. setbings controlling polarizat.ion direction and applied potential affect the shape only in so far as t’hey affect the time during which oxidation and reduction may occur (cf. subsequent discussion). If a reducible species is scanned from less negat’ire potential t,oaard its E,, a normal reduction wave results (cf. Figure 1A; Table I : X-1,2,5). If t.he polarization is from potential more negative than E, toward less negat’ive potential, a composite wave results with it’s base on t’he cathodic side and its peak on the anodic side of galvanometer zero (cf. Figure 1B; Table I : d-3,4,6). The same-in the reverse sense-occurs for a n oxidizable substance (cf. Table I : B-1,2 and B-3,4). This phenomenon of apparent partial current reversal has not been systeniat’cally discussed in the lit,erature. The wave pattern under reverse polarization a t the D.iLf.E. is merely a retrace of the regular reduction wave with agreeing i d and E I l 2values. The difference in behavior at the two t.ypes of electrodes is explicable in terms of t’he different mass transport situations. On reverse polarization at a
Voltammetric Data for Ferrocyanide and Ferricyanide at a Stationary Pyrolytic Graphite Electrode on Variation in Polarization Direction
Potential sweep at P.G.E.,= volt 0 . 5 to 0
Type of wave observedb EP,2,‘volt A. l.0mM K3[Fe(CN)6]in 0.5M KC1 Reduct ion 0.230
iP,, pa.
0.228 0,227 0.200(0.214) 0.197 (0.206) 0.192 (0.203)
0 to 0 . 5
5.96 5.76 5.90 5.78 ( 3 . 7 6 ) 5 . 6 8 (3.84) 6.32 (4.40)
ComDosite redn. Composite redn. - 0 . 5 to 0 . 5 Composite redn. B. l.0mM K4[Fe(CN)e]in 0.5M KC1 B-1 0 to 0 . 5 Oxidation 0,200 6.00 B-2 Oxidation 0.200 6.00 B-3 0 . 5 to 0 Composite oxidn. 0.232 (0.208) 5.92 (2.74) B-4 Composite oxidn. 0.227 (0.210) 5.80 (3.48) c. 0.5mM K3[Fe(Cx)s]and 0.5mM K4[Fe(C?;)s] in 0.5M KC1 c-4 Oto0.5 Composite redn. 0.199 (0,207) 5.20 (4.56) Composite redn. c-3 0,197 (0.207) 5.40 ( 4 , 6 4 ) c-1 0 . 5 to 0 Composite oxidn. 0.230 (0.220) 5 . 6 4 (4.92) c-2 Composite oxidn. 0.226 (0,220) 5 , 4 4 (4,76) Geometric area of pyrolytic graphite disk electrode: 12.57 mm.2 “Composite oxidn.” refers to a wave, which would be expected to be an anodic wave, but is actually composed of both anodic and cathodic current portions. “Composite redn.” similarly refers to an expected cathodic wave, which is composed of both cathodic and anodic components. Figures in parentheses refer only t o the anodic portion of a composite oxidation wave and vice versa for a composite reduction wave.
and oxidizable species of a reversible couple, the cathodic current of the composite wave should be due to reduction of both the reducible species and the accumulated electrode reaction product of the oxidizable species. iit the D.M.E., the fall of the mercury drop aids in dis-
stationary electrode, the original electrode reaction product, which has not had sufficient time to diffuse away completely, can be electrolyzed when the necessary potential is reached, causing a composite wave. Consequently, in a solution containing both the reducible
I
-0.5 I
0
I
I
-0.2
I
I
-0.4 E , VOLT
1
-0.6
I
I
-Q8
Figure 2. Voltammograms of 0.2mM azoxybenzene in p H 6.6 buffer ethanol) at a stationary pyrolytic graphite electrode A. Forward polarization (from -0.2 to - 1.1 volt) E. Reverse polarization (from -0.9 t o 0 volt) C.
(50%
Reverse polarization (from -0.6 to 0 volt) V b l . 37,
NO. 12,
NOVEMBER 1965
1507
Table II. Comparison of Half-Peak Potentials on Forward and Reverse Polarization for Azoxybenzene, Azobenzene, and Hydrazobenzene at a Stationary Pyrolytic Graphite Electrode E,,2, volt, for volt, for E.12, volt, for
PH 1.6 3.3 4.2 5.1 5.4 6.6 7 9 8.6 Y,U
11.4 12.5
aioxybenzene Forward Reverse
azobenzene Forward Reverse
-0.298 -0,451
-0.030 -0,124
-0.028 -0,141 -0,211
-0.018 -0.115 -0.156
-0.602
-0.147
-0.696 -0,765
-0.223 -0.280
-0.419 -0.502
-0,239 -0.290
-0,780 -0,893 -0.910
-0.270 -0.350 -0.384
-0.603 -0.718
-0,270 -0.373
persion of the electrode reaction product and the new drop forms in a solution containing essentially only the original species. I n reverse polarization the length of time, during which a potential is applied a t which a faradaic process occurs, influences the amount of electrode product that can be observed and, consequently, the reproducibility of the composite wave height. I n the reduction of ferricyanide ( E , = 0.23 volt), Run A-3 (Table I) started at -0.5 volt and A-4 at 0 volt. Since the scan rate was 0.2 volt/minute, the P.G.E. was exposed to a reducing potential for 2.5 minutes less in A-4 than in A-3 and the amount of ferrocyanide accumulated was less (cf. lower anodic current of A-4 as compared t o A-3). When a solid electrode is rotated, the wave has the form of a plateau rather than a peak and reverse polarization retraces the wave similar to behavior at the D.M.E. I n reverse polarization of nitrosobenzene a t p H 1.6 a t potentials more positive than Ep12,a small anodic wave (ca. 0.5 pa.) appeared, which is of minor significance (main wave : 11.O pa.) and which may be due to oxidation of reaction product adsorbed on the electrode or trapped in imperfections of the electrode surface or in the graphiteglass interface.
hydrazobenzene Forward Reverse No wave found -0.108
-0.140
-0,187 -0.177 -0.290 -0,293 -0.314 -0.340 -0.385
-0.318 -0.406 -0.480 -0.530 -0.560 -0.704 -0.762
reduction. I n a separate experiment, hydrazobenzene gave at p H 6.6 an oxidation wave of Epi2 = -0.23 volt. EPi2values from forward and reverse polarizations for azoxybenzene, azobenzene, and hydrazobenzene over the p H range are summarized in Table I1 and Figure 3. E,i2 values for reverse polarization of both azoxybenzene and azobenzene fall on the same straightline plot of Ep12for the forward polarization of hydrazobenzene, indicating that both are electrolytically rcduced to hydrazobenzene, whose presence is detected on reverse polarization. Hydrazobenzene, on the other hand, is oxidized only to azobenzene.
At p H 1.6, hydrazobenzene normally does not show an oxidation wave because it rearranges to an electrolytically inactive species during the time required for solution preparation and conditioning. However, it can be generated at the stationary P.G.E. in a p H 1.6 solution of azobenzene and its E,jz measured by reverse polarization; the valie thus found falls on the straightline plot of hydrazobenzene Ep12 us. PH ( 2 ) . Ordinary light isomerizes trans-azobenzene to cis; in presence of the cis isomer, E,/z of the trans isomer is shifted in basic solution about 20 mv. more negative ( 2 ) . Epj2values of azobenzene generated in hydrazobenzene solution of p H 7.9, 8.6, and 9.0 are about 30 mv. less negative than those of azobenzene reduction waves in solutions of corresponding pH, indicating that pure trans-azobenzene is generated a t the electrode surface or within the diffusion layer. DISCUSSION
A simple technic of reverse polarization at stationary electrodes, which involves recording a voltammogram from a potential, at which an electrode process occurs, toward a potential, a t which this process ceases, frequently allows observation of the voltammetric wave of the product formed in the original electrode reaction process. The technic
IRREVERSIBLE REDOX SYSTEMS
I n a typical experiment, the azoxybenzene reduction wave was obtained at p H 6.6 with a stationary P.G.E. (Epi2 = -0.718 volt; Figure 2 A ) . The solution was then scanned from -0.9 to 0.0 volt; after an indication of a reduction wave, a n oxidation wave appeared (E,,z = -0.236 volt; Figure 2B). The electrode was then resurfaced and the solution scanned from -0.6 volt, where essentially no reduction occurs, to 0 volt; no anodic wave appeared (Figure 2C). These results clearly indicate that the anodic wave (Figure 2B) is due50 oxidation of the product formed in the azoxybenzene 1508
ANALYTICAL CHEMISTRY
PH
Figure 3. Comparison over the pH range of E p p values of azoxybenzene, azobenzene, and hydrazobenzene for forward and reverse polarization a t a stationary pyrolytic graphite electrode
-- Azoxybenzene 0
0 A
(I), azobenzene (111, and hydrazobenzene (Ill) on forward polarization Hydrazobenzene on reverse polarization Azobenzene on reverse polarization Azoxybenzene on reverse polarization
can be utilized to elucidate the nature of an electrolysis product-e.g., identification of trans-azobenzene as the sole oxidation product of hydrazobenzene-and to evaluate, a t least qualitatively, the reversibility of the redox couple. Although the accumulation of electrolytic product could be due either to adsorption on the electrode or to slow dispersion and subsequent rediffusion back to the electrode, the wave shapes obtained with reverse polarization a t the D.M.E. and rotating P.G.E. indicate that the second situation prevails in the case of the compounds involved in the present study. EPiZobtained by reverse polarization may be less accurate than EpiZobtained by normal polarization-e.g., if the concentration of the electrode product on reverse polarization is low, the wave may have a drawn-out appearance which would make determination of EPil difficult. Where the reduction and oxidation potentials of irreversible redox couples are far apart-e.g., azoxybenzene (EplZ = -0.9 volt) and hydrazobenzene (Epi2 = -0.4 volt) a t p H 12.5, an improved wave shape can be obtained by briefly applying a potential where reduction occurs-e.g., - 1 . 2 volt for 2 minutes, before starting the scan from -0.5 volt toward less
negative potential to locate the oxidation wave of the electrode reaction product. Without prior knowledge of where the latter electrolyzes, the wave can be located by trial runs. Because the rate of diffusion of the product away from the electrode is generally not known, the magnitude of the peak current of the waves found on reverse polarization cannot be precisely interpreted, although fairly good estimations can be made on the basis of comparative runs. For example, the composite wave of a ferri-ferrocyanide mixture (0.5 m V each) (Table I : C-1,2) has an average cathodic current component of 4.8 pa. The average cathodic current due to 0.5mM ferricyanide is 2.93 pa. (A-1,2,5); that due to reverse polarization of 0.5mX ferrocyanide is 1.56 l a . (B-3,4); the sum, 4.5, is considered close to the value for the mixture, because the continuous transition from anodic to cathodic current in a composite wave does not permit adequate residual current corrections. Similarly, the sum of the expected anodic currents due to ferrocyanide and reverse polarization of ferricyanide is 4.9 pa. (3.00 plus 1.90); the observed anodic current of the composite reduction wave of the mixture is 4.6.
The reverse polarization technic, which is in essence a slow form of cyclic voltammetry accomplished with a n ordinary polarograph, has been used to study two reversible and two irreversible redox couples; in every case, the product of the electrode process was successfully identified. I n the case of an unstable compound (hydrazobenzene a t p H 1.6), where ordinary voltammetry is too slow to record the behavior of the compound before it disappears, the compound can be studied by reverse polarization of the more stable member of the couple. ACKNOWLEDGMENT
The authors thank Dr. Ilana Fried for stimulating discussion. LITERATURE CITED
(1) Chuang, L., Fried, I., Elving, P. J., ANAL.CHEM.36, 2426 (1964).
(2) Xbid., 37, 1528 (1965). (3) Lord, S. S., Rogers, L. B., Ibid., 26, 284 (1954).
RECEIVEDfor review March 8, 1965. Accepted August 27, 1965. Work supported in part by the U. s. Atomic Energy Commission and the Horace H. Rackham School of Graduate Studies of The University of Michigan.
Polarography of Selected Cations in Fused Sodium Nitrate-Potassium Nitrate Eutectic at 250" C. H. S. SWOFFORD, JR., and CHARLES L. HOLlFlELD Departmenf o f Chemistry, University o f Minnesota, Minneapolis, Minn.
b
The polarographic
reductions of
TI(I), Cd(ll), and Pb(l1) are examined in the fused salt mixture. Reversible behavior is observed for both TI(I) and Cd(ll) which have half-wave potentials of -0.828 volt and -0.665 volt vs. the Ag-Ag(I) reference electrode, respectively. Inconsistent with previously reported work in a lower melting nitrate eutectic, the reduction of Pb(ll) appears to proceed irreversibly with a nonconcentration-dependent prewave preceding a diffusion controlled second wave a t higher conPossible excentrations of Pb(ll). planations for this phenomenon are discussed in light of the experimental data.
C
URRENT-VOLTAGE STUDIES in
fused salt media have generally been carried out with the use of solid indicat(22). Polaring microe!ectrodes graphy-Le., voltammetry using a dropping mercury electrode (DME)-has
55455
not been employed by a large number of investigators. Two factors are doubtless responsible for its restricted use a t elevated temperatures; an obvious health hazard and a more serious limitation imposed by the necessity of working in media melting a t temperatures sufficiently low so that the volatilization of mercury from the surface of the drop does not interfere with the diffusion and/or electron transfer process. I n fact, Delimarskii and Markov (3) have stated that the D M E is not useful a t temperatures above 215' C.; that this statement is incorrect is evidenced by the work of Swofford and Laitinen (15) and is further challenged by the present tTork. Kachtrieb and Steinberg were the first workers to make use of the D M E in fused salt mixtures (9, IO). Their initial study established its use in the ternary eutectic h'H4N03-LiN03-NH4C1 a t 125" C. However, the instability of this mixture led these investigators to a second study employing a ternary mixture composed of 30 mole yoLiN03,
17 mole % NaN03, and 53 mole % KN03 a t 160" C. as a solvent; these authors found the Ilkovic equation to be applicable and reported halfwave potentials for the reversible reductions of Ni(II), Pb(II), Cd(II), and Zn(I1). Colichman (2) has employed the D M E in molten ammonium formate a t 125' C. and has observed reduction waves for a wide variety of compounds. Stability constants for the chloro complexes of Pb(II), Cd(II), and Ni (11) have been evaluated by Christie and Osteryoung (1) using a D N E in fused LiKOs-KaXO3 a t 180' C. Previous to the present work, and the work of Swofford and Laitinen ( I @ , Inman and Bockris (6) are apparently the only workers who have attempted to use metallic mercury for voltammetric investigations a t temperatures in excess of 200' C. These investigators used a hanging mercury drop to determine chronopotentiometrically the association constants for chloro complexes of cadmium in molten KNO3-XaNOs eutectic a t 260" C. VOL. 37, NO. 12, NOVEMBER 1965
1509
