2310
Anal. Chem. 1902, 54, 2310-2314
plexes are stable in aqueous solution. LITERATURE CITED (1) Blaedel, W. J.; Dinwiddie, D. E . Anal. Chem. 1974, 4 6 , 873. (2) Westall, J. C.; Morel, M. M. F.; Hume, D. N. Anal. Chem. 1979, 5 1 , 1792. (3) Gilbault, G. G.; Meisel, T. Anal. Chem. 1969, 41, 1100. (4) Smith, G. F.; McCurdy, W. H., Jr. Anal. Chem. 1954, 26, 371. (5) Flaschka, H. A. “EDTA Titrations”;Pergamon Press: New York, 1959. (6) Manahan, S.E . !norg. Chem. 1966, 5 , 482. (7) Anderegg, G. Critical Survey of Stablllty Constants of EDTA Complexes”; Pergamon Press: Oxford, 1977.
(8) James, R. 5.;Wllllams, R. J. P. J . Chem. SOC. 1961, 2007. (9) Larsen, E . E . Anal. Chem. 1974, 46, 1131. (IO) Kwlk, W. L.; Ang, K. P. J . Chem. SOC.,Dalton Trans. 1961, 452.
RECEIVED for review June 1, 1982. Accepted July 23, 1982. This work was supported under the U.S.-Latin American Cooperative Science Program, by the Consejo Nacional de Ciencia y Technologia in Mexico (Grant NSF/PT/79/0111) and by the National Science Foundation (Grant INT.8000529) in the U.S.A.
Electrochemical Pretreatment of Glassy Carbon Electrodes Royce C. Engstrom Department of Chemistry, University of South Dakota, Vermillion, South Dakota 57069
The effects of preanodlzatlon and precathodlzatlon on the electrochemlcal oxldatlon of hydroqulnone, ferrocyanide, and hydrazine were studied at glassy carbon electrodes. Pretreatment was characterized with respect to the sequence of voltages applied to the electrode, the amplltude of applied voltage, and the duration of pretreatment. For all three electroactlve species, preanodization at a voltage greater than 1.5 V vs. SCE was requlred to activate a freshly pollshed electrode. Ferrocyanide and hydrazlne also required precathodlzatlon to remove an inhibitory layer formed durlng preanodizatlon. I n all cases, pretreatment resulted in a substantlal Improvement In the half-wave potential of the voltammetric wave and In the reproduclblllty of the wave. The results are Interpreted in terms of three dlstlnct electrode surface condltlons.
Reports involving the use of carbon electrodes often describe pretreatment procedures that were found necessary to observe reproducible and well-defined electrochemical behavior. Taylor and Humffray noted that dipping a freshly polished glassy carbon electrode in chromic acid produced a significant increase in the rate constant for the iron(II1)-iron(I1) system ( I ) . Blaedel and Jenkins found that glassy carbon electrodes required “preconditioning” and “pretreatment” (2, 3). Pretreatment, performed immediately before an experiment, consisted of applying 1.35 V and -1.35 V for 2 min each. Preconditioning, performed at any time in the history of the electrode, consisted of cycling the electrode repeatedly between those two voltages. After such treatment, the half-wave potential for the oxidation of dihydronicotinamide adenine dinucleotide (NADH) became less anodic by approximately 0.3 V. Similar pretreatment was used by Moiroux and Elving in their study of the same system (4). Exposure of pyrolitic carbon to radio frequency plasma in an oxygen atmosphere also resulted in a substantial improvement in the oxidation potential for NADH (5). Pyrolytic carbon film and glassy carbon electrodes showed improved catalytic ability toward the ferricyanide-ferrocyanide system following application of an 8-V (peak-to-peak) signal at a frequency of 70 Hz (6). The basal plane of pyrolytic graphite was shown to have advantages as an electrochemical detector for high-performance liquid chromatography, provided the electrode was preanodized in citrate-acetate buffer (7). Recently, electrochemical pre0003-2700/82/0354-23 10$01.2510
treatment of pyrolytic carbon fiber electrodes was found to cause a substantial decrease in the peak potentials for the oxidation of dihydroxyphenylacetic acid and ascorbic acid (8). A 7 0 - H ~signal with an amplitude of a t least 2.6 V yielded well-resolved peaks for the otherwise overlapping oxidations of the two compounds. While the above pretreatment procedures were arrived at largely by trial and error, most of the workers suggested that pretreatment involves the oxidation-reduction of functional groups on the carbon surface. Chemical evidence presented in the 1950s suggested the existence of quinone, phenol, lactone, chromene, and carbonium groups on graphite (9, 10). Polarography (11,12) and controlled-potential coulometry (13) of carbon suspensions resulted in the observation of a welldefined electrochemical process on the carbon surface, which was attributed to the reduction of a quinone group. Spectroscopic examinations using internal reflectance infrared spectrometry indicated the presence of carbonyl groups on the surface of graphite, with the possibility of a cyclic anhydride (14-16). The work of Miller et al. resulting in a chiral electrode involved the introduction of titratable acid groups onto graphite through heating in air ( 1 7 ) ,the groups being concentrated on the edge surface of the graphite microcrystallites (18). Other work involving chemical modification of electrodes apparently involved chemical bonding through either quinone or carboxyl functionalities (19-21). Currentpotential curves taken of graphite electrodes yielded three anodic and three cathodic peaks (22). The peaks were attributed to the oxidation-reduction of quinone-hydroquinone groups in differing environments on the electrode surface. Similar work with glassy carbon showed two anodic and cathodic processes, one of which was thought to involve a surface functional group and the other the chemical adsorption of oxygen (23). The differential capacitance of glassy carbon and the edge orientation of pyrolytic carbon showed potential dependences suggestive of the presence of surface functional groups (24). The work reported here concerns glassy carbon, a material reported to contain both trigonally and tetrahedrally bonded carbon in an arrangement of tangled aromatic ribbons with varying degrees of cross-linking (25,26). The glassy carbon surface has some of the characteristics of edge-oriented pyrolytic carbon, based on capacity measurements (24). The present work was undertaken as a result of observations made during studies of electrode fouling, where it was noted that 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
after polishing a glassy carbon electrode to remove adsorbed material, voltammograms were often poorly defined and nonreproducible. Pretreatment procedures were sought that would cause the electrode to efficiently and reproducibly catalyze electrochemical reactions and that would be relatively convenient to porform. Three electroactive species were chosen for study, hydroquinone (27,28),ferrocyanide (29,301, and hydrazine (31, 32). Electrochemical pretreatment was systematically evaluated with respect to the sequence of voltage applied to the electrode, magnitude of the applied voltage, and the duration of pretreatment. In addition, experiments were performed to obtain information concerning the nature of the processes involved during pretreatment.
EXPERIMENTAL SEC1’ION Electrochemistry. 1111 electrochemical experiments were performed with a laboratory-built rotating-disk-electrode apparatus, used in connection with a PAR Model 174 polarographic analyzer (Princeton Applied Research Corp., Princeton, NJ). The working electrodes were short lengths of 3 mm diameter glassy carbon rods (Tokai Mfg., Tokyo, Japan) epoxied into a Plexiglass housing (Epotek 320, Epoxy Technology, Inc., Billerica, MA). A saturated calomel electrode (SCE) was used as a reference electrode, and all voltages in this paper refer to the SCE. A platinum wire served as the auxiliary electrode. Reagents. Solutions were prepared with water purified by distillation, charcoal adsorption, ion exchange, and microfiltration on a commercially available system (Barnstead NANO-pure, Boston, MA). Reagent grade chemicals were used without further purification. Hydrmine sulfate was obtained from Sigma Chemical Co. (St. Louis, MO) and used as received. The supporting electrolyte, unless stated otherwise, was 0.10 F potassium nitrate and 0.010 F potassium hydrogen phosphate, adjusted to pH 7.0 with dilute nitric acid. The electroactive species was present at 1.0 mM unless stated otherwise. Procedure. The glassy carbon electrodes were polished using 4/0 emery paper, followed by 0.3 fim alumina slurry and 0.05 wm alumina slurry on wool polishing cloth. One minute was spent on each polishing medium. The electrodes were rinsed copiously with water between each medium and after the final polishing step. An electrode was then placed in the cell and immersed in the cell contents, which bad been deaerated for 15 min. Unless stated otherwise, d voltammogramswere taken with the electrode rotating at 500 rpm, and at a voltage scan rate of 10 mV/s. The scans were ordinarily taken from an initial potential of -0.2 Y to a final potentnal of 1.0 V. Electrochemical pretreatment procedures were performed by setting the applied voltage to the desired value with the “initial potential” control of the polarographic analyzer for the desired time interval. After pretreatment, the voltage was set to -0.2 V, and the current allowed t o decay for about a minute before initiating the scan. After the scan, the voltage was returned to -0.2 V until another scan was performed or until the electrode was removed for repolishing. RESULTS AND DISCUSSION Effect of Preanodization and Precathodization. While pretreatment procedures involving cycling the electrode between positive and negative potentials have proven effective in some cases (2-4), it was desired to examine the effects of the positive and negative voltages separately. A freshly polished electrode was placed in the cell containing thie electroactive species and an anodic scan obtained. Then a voltage of +1.5 V was applied for 5 min (preanodization) and the scan repeated. After the electrode was repolished, a voltage of -1.5 V was applied (precathodization) and the scan repeated. The results are summarized in Figure 1. For hydroquinone (Figure IA), preanodization of the electrode (curve 2) resulted in a substantial decrease in the half-wave potential as compared to that found a t a freshly polished electrode (curve 1)or a precathodized electrode (curve 3). Cycling of the potential +1.5 V and -1.5 V repeatedly (curve 4) resulted in no significant improvement over that produced by preanodization alone. The half-wave potential
2311
1-
50w 0
!J
1.0
L
0.4
-0.2
Applied Potential, volts Figure 1. Effects of electrochemical pretreatment: (A) hydroquinone at 0.10 mM, (8)ferrocyanide at 1.0 mM, (c) hydrazine at 1.0 mM;cwve 1 taken on freshly polished electrode, curve 2 after preanodization at 1.5 V, curve 3 after precathodization at -1.5 V, curve 4 varies, see text.
shown for curves 2 and 4 is near that observed at the dropping mercury electrode and is near the formal electrode potential for the quinone-hydroquinone system a t pH 7. The oxidation of ferrocyanide (Figure 1B) responded quite differently t o pretreatment than did that of hydroquinone. Preanodization of the electrode a t 1.5 V virtually eliminated the ferrocyanide wave (curve 2). Precathodization alone did nothing t o improve the wave (curve 3), however, preanodization followed by precathodization (curve 4) resulted in half-wave potential significantly less anodic than that found a t a freshly polished electrode (curve 1). The oxidation of hydrazine (Figure IC) showed the most dramatic response to electrochemical pretreatment. At a freshly polished electrode, a wave was not observed at all (curve 1);however, the current was somewhat above background. Preanodization alone produced a substantial increase in current (curve 2), but still a well-defined wave was not observed. Precathodization alone produced no improvement a t all (curve 3); however, preanodization followed by precathodization resulted in a well-defined wave with a measurable half-wave potential (curve 4). The results of this experiment point out that the effectiveness of electrochemical pretreatment is dependent upon the electrochemical species undergoing electrolysis. The process that activates the electrode with respect to hydroquinone actually deactivates the electrode with respect to ferrocyanide. For all of the electroactive species, it made no significant difference if the pretreatment was performed with the electroactive species present during pretreatment or if it was added after pretreatment. Voltage Dependence of Pretreatment. T o further characterize the effect of preanodization on electrode performance, we varied the voltage used during preanodization. The experiment was performed by first polishing the electrode and then preanodizing for 5 min a t various voltages. In the case of hydroquinone, scans were recorded directly following preanodization, since the results presented above indicated precathodization was unnecessary. For ferrocyanide and hydrazine, the preanodization step was followed by precathodization a t -1.2 V for 1min and then the anodic scan was recorded. The half-wave potential was recorded for each voltammogram and plotted against preanodization voltage. The results are shown in Figure 2.
+
2312
ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982 io
Q, OE
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Cathodization Potential, volts Figure 4. Restoration of ferrocyanide wave by precathodization.
20
Prea n od i z a t i on Pot entia I, voi t s Figure 2. Dependence of half-wave potential on preanodization voltage: (U) hydroquinone, ( 0 )ferrocyanide, (A)hydrazine.
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Inhibition of ferrocyanide wave following preanodization
All three species showed similar dependence on preanodization voltage. Preanodization had to be performed at a voltage of approximately 1.5 V or higher in order to achieve a decrease in the half-wave potential. The half-wave potential became less dependent on the preanodization voltage at higher voltages. In the case of hydrazine, further impravement in half-wave potential may have resulted from preanodization voltages greater than 2.0 V; however, higher voltages were not investigated because the current would have exceeded the range of the polarographic analyger. The voltage necessary to activate the electrode is likely to be dependent on the supporting electrolyte; however, it is interesting to note that the anodic voltage used in the pretreatment procedure of Blaedel and Jenkins (3) is similar to that f o m d necessary here, when the difference in reference electrodes is accounted for. In the case of ferrocyanide, the voltage dependence of pretreatment was examined more closely. The effect of preanodization alone (without subsequent precathodization) was to inhibit the oxidation of ferrocyanide, as already mentioned. The voltage dependence of that inhibition is shown in Figure 3. The amplitude of the ferrocyanide wave decreases abruptly when the preanodization voltage reaches a value greater than 1.5 V. The ferrocyanide wave can be restored through application of cathodic voltages, as shown in Figure 4. Application of -0.5 V restores the ferrocyanide wave to its original height. It is important to note that when preanodization alone is performed, the voltage required to inhibit the ferrocyanide wave is the same as the voltage re-
Figure 5. Dependence of haif-wave potential on preanodization duration: (A) hydrazine, (B) hydroquinone, (C) ferrocyanide: ( 0 )preanodized at 1.5 V, (A)preanodlzed at 1.75 V.
quired to activate the electrode toward ferrocyanide when precathodization follows preaodization. Time Degendence of Anodization. The dependence of the half-wave potential on the time spent at preanodization is shown in Figure 5. While there are significant differences in the time necessary to activate the electrode toward the three species, similar behavior is shown in all cases. After approximately 8 min, further improvement in the half-wave potential was minimal. As might be expected, a higher preanodization voltage causes activation in a shorter period of time, as shown for hydrazine. Preanodization at 1.75 V caused activation toward hydrazine in about 3 min. The slightly lower half-wave potential resulting from the higher preanodization voltage is consistent with the data of Figure 2. The duration of precathodization for ferrocyanide and hydrazine pretreatment was not critical. For example, the half-wave potential of ferrocyanide remained within a 5-mV range using cathodization times ranging from 10 s to 5 min. Reproducibility o f Voltammetry at Pretreated Electrodes. From an analytical standpoint, pretreatment procedures were sought that would provide a reproducibly active surface a f t q electrode polishing. The pretreatment procedures selected as "optimum" were 1.5 V for 10 min for hydroquinone, 1.5 V for 5 min followed by -1.0 V for 1 min for ferrocyanide, and 1.75 V for 5 min followed by -1.2 V for 10 s for hydrazine. It should be repeated that the precathodization time was not critical for either ferrocyanide or hydrazine. The results are
ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982 2313 Table I. Effect of Pretreatment on Reproducibility before after pretreatment pretreatment
-
species hydroquinone ferrocyanide hydrazine
E,,,,V 0.127 0.312
S,V
E,,,,V
0.005 0.053
0.000
>1
0.250 0.299
S,V 0.002 0.001
0.015
A
Table 11. Rotation-Rate Dependence of Cathodic Peak rotation during peak peak cathodic potential, amplitude, V mA anodization scan -0.82 0.33 ON ONa -0.82 0.33 OFF ON -0.61 0.29 ON OFF -0.59 0.27 OFF OFF a ON represents a rotation rate of 500 rpm. f
q-
,
, J ,
0 0
0.8
1.2
1.6
Preanodization Potential, volts Flgure 7. Charge passed during preanodization at various voltages. Preanodization duration was 10 min.
00
-0 8 Applied Potential, volts
-04
-I 2
Flgure 6. Cathodic: scans following preanodization at 1.75 V. The current axis zero has been displaced for each scan. The duration of preanodization is indicated for each scan. Scan rate was 50 mV/%
summarized in Table I, where it can be seen that a significant decrease in the half-wave potential and the standard deviation of that half-wave potential occurred for all1 three electroactive species. The standard deviation for the hydrazine half-wave potential before pretreatment is not shown because the voltammetric waves did not reach a plateau (see Figure 1). Behavior of the Glassy Carbon Electrode during Pretreatment. T o gain insight into the processes that occur during pretreatment, we performed some experiments in supporting electrolyte alone, without the addition of the electroactive species. In the first experiment, cathodic scans were recorded following preanodization. It was found that after preanodization, cathodic scans yielded a peak such as those shown in Figure 6. The amplitude and peak voltage of the cathodic peak were quite reproducible as long as the voltage and duration of preanodization remained constant. For example, three identical preanodizations produced cathodic peaks whose mean amplitude had ii relative standard deviation of 1.6%, and whose peak potential varied by 40 mV. The cathodic peak was not the result of a reaction involving the supporting electrolyte, since similar behavior was observed in 0.1 F sodium fluoride. The cathodic peak was considered therefore to originate from a reaction involving either water or the electrode itself. The cathodic peak did not appedr until the preanodization voltage reached 1.5 V, and the peak amplitude increased as preanodization voltage increased. The anodic voltage required to generate the cathodic peak corresponds to the voltage required to activate the electrodes toward the electroactive species or to inhibit the oxidation of ferrocyanide following preanodization alone. The data shown in Figure 6 show that as the time of preanodization at +1.75 V increasesi the cathodic peak amplitude increases. A plot of the charge density under the cathodic peak as a function of preanodization time shows a linear relationship, with a slops C/(cm2 min) anodization time. The linearity of 3.7 X indicated that, for preanodization times af up to 15 min, the electrode surface did not become “saturated” with the species responsible for the cathodic peak.
That the cathodic peak is due to a surface bound species is shown by the rotation rate dependence of the peak as summarized in Table 11. During the cathodic scan, the amplitude, peak potential, and shape of the peak were independent of rotation rate. When the rotation wm switched off during preanodization, the resultant cathodic peak occurred with a lower peak potential, and the peak amplitude was somewhat diminished. The production of the surface bound species therefore does depend on the electrode rotation rate. In Figure 6, the peak potential shifts to more negative values as the length of preanodization time is increased. This does not mean that the voltage required to cause the process becomes increasingly more cathodic, however. If the voltammograms of Figure 6 are superimposed, it can be seen that the cathodic process begins at essentially the same voltage regardless of the duration of preanodization. Rowever, longer preanodization times cause a greater accumulation of the surface bound product on the electrode. Consequently, more time is required to electrolyze the surface bound product, and the current peaks at a more cathodic voltage. The charge passed during preanodization was measured by determining the area under the current-time profile for a period of 10 min at various preanodization voltages. As shown in Figure 7, the charge increased rapidly at a preanodization voltage of 1.5 V or higher. From Figure 7, the charge passed C. The charge passed in the resulting a t 1.75 V is 5.6 X cathodic peak (Figure 6, 10 min trace) is 2.5 X low3C. The cathodic peak accounts for only 4.5% of the total charge passed during preanodization. The remaining charge passed during preanodization apparently goes into surface processes not reduced in the cathodic peak or into the production of solution species, or both. The behavior observed here is in some ways similar to that observed at pyrolytic graphite electrodes by previous workers (33) who found that anodization at 1.5 V for 1.5 min in 1 M H2S04produced a subsequent cathodic peak at 0.3 V. The amplitude of that peak increased with increasing anodization potential as in this study. The nature of the “film” giving rise to the cathodic peak was not determined, however, it was reported that the film did not prevent the reduction of Ce(1V).
CONCLUSIONS The observations presented above suggest that three different surface conditions exist on the glassy carbon electrode during the course of the pretreatment. The first condition, prevalent on a freshly polished electrode, is a relatively inactive
2314
ANALYTICAL CHEMISTRY, VOL. 54, NO. 13, NOVEMBER 1982
condition. The electrochemical reaction of all three electroactive species studied here is poorly catalyzed on this surface. The reason for the low activity may result from the lack of functional groups on the surface; however, there are other possible explanations. These possibilites include the presence of impurities on the surface that hinder the oxidation reactions. Impurities may come from the polishing step, where alumina could be deposited on the electrode or carryover of the electrode housing material could occur. Panzer and Elving reported that no carryover of epoxy or housing material occurred during their polishing procedures (34). The second type of surface results after anodization, where anodization causes activation of the surface as well as formation of a material that is later reduced in the well-defined cathodic process. The deposited material is likely an oxide layer of some sort, since anodization is performed a t voltages where oxygen evolution is probably taking place (23). The exact nature of this layer is uncertain; however, the charge contained in the cathodic stripping peak indicates a multilayer structure. For example, the charge density under the peak recorded after 10 min of preanodization (Figure 6) corresponds to a density of 11electroactive groups per A2, if one assumes 2 equiv per group. Such a density is unlikely for groups attached directly to the surface of the electrode. The oxide layer has a strong inhibitory effect on the oxidation of ferrocyanide and hydrazine but does not affect the oxidation of hydroquinone. Such behavior has been well-documented in the case of oxide layers on platinum electrodes (35) and may be linked to the adsorption of the electroactive species (36). The activation of the electrode surface in this second condition must involve either the oxidation of the surface to produce functionalities capable of catalyzing the electrochemical reaction or the oxidative removal of impurities from the surface. The activation process may occur directly by electrochemical oxidation or indirectly by oxygen produced during anodization. The coincidence of the voltage needed to activate the electrode and the voltage needed to cause the appearance of the cathodic stripping peak suggest that the two processes are related. The third surface condition is that of the activated electrode after the oxide layer has been cathodically reduced. This surface appears active toward all of the electroactive species studied here and would be a surface similar to that obtained in those pretreatment procedures that involve cycling the electrode between positive and negative voltages (2-4). The surface states proposed here are compatible in some respects to those suggested by Laser and Ariel in their study on the anodic behavior of glassy carbon (23). They suggested that a freshly polished electrode underwent anodic oxidation
to create a redox couple that was subsequently oxidizable and reducible, but not removable under ordinary electrochemical conditions. This may correspond to the activation of the electrode reported here. In addition, Laser and Ariel reported a cathodic process at a voltage around -0.6 V vs. SCE. This may correspond to the cathodic peak observed here (Figure 6), but because of the limited cathodic range in their study, Laser and Ariel did not observe the peak nature of the process.
LITERATURE CITED Taylor, R. J.; Humffray, A. A. J . Electroanal. Chem. 1973, 42,347. Blaedel, W. J.; Jenkins, R. A. Anal. Chem. 1974, 46, 1952. Blaedei, W. J.; Jenkins, R. A. Anal. Chem. 1975, 47, 1337. Moiroux, J.; Elving, P. J. Anal. Chem. 1978, 50, 1056. Chi-Sing, D.; Kuwana, T. Anal. Chem. 1978, 20, 1315. Blaedel, W. J.; Mabbott, G. A. Anal. Chem. 1978, 50,933. Wightman, R. M.; Paik, E. C.; Borman, S . ; Dayton, M. A. Anal. Chem. 1978, 50, 1410. Gonon, F. G.; Fombarlet, C. M.; Buda, M. J.; Pujol, J. F. Anal. Chem. 1981, 53, 1386. Garten, V. A.; Weiss, D. E. Aust. J . Chem. 1955, 8. 68. Garten. V. A.; Weiss, D. E.; Wiiiis, J. B. Aust. J . Chem. 1957, 70, 295, 309. Hallum, J. V.; Drushei, H. V. J . Phys. Chem. 1958, 62, 110. Jones, I.F.; Kaye, R. C. J . Electroanal. Chem. 1969, 20, 213. Drushel, H. V.; Hailum, J. V. J . f h y s . Chem. 1958, 62, 1502. Mattson, J. S.;Mark, H. B., Jr.; Weber, W. J., Jr. Anal. Chem. 1969, 41,355. Mattson, J. S.;Mark, H. B., Jr. J . Colloid Interface Sci. 1969, 31, 131. Mattson, J. S.;Lee, L.; Mark, H. B., Jr.; Weber, W. J., Jr. J . Colloid Interface Scl. 1970, 33, 284. Watkins, 9. F.; Bekling, J. R.; Karin, E.; Miller, L. L. J . A m . Chem. SOC. 1975, 97,3549. Firth, B. E.; Miller, L. L.; Mltani, M.; Rogers, T.; Lennox, J.; Murray, R. J . A m . Chem. SOC. 1978, 98,8271. Elliott, C. M.; Murray, R. W. Anal. Chem. 1976, 48, 1247. Evans, J. F.; Kuwana, T. Anal. Chem. 1977, 49, 1635 Evans, J. F.; Kuwana, T. J . Necfroanal. Chem. 1977, 80,409. Blurton, K. F. Hectrochlm. Acta 1973, 18,869. Laser, D.; Ariel, M. J . Electroanal. Chem. 1974, 52, 291. Randin, J. P.; Yeager, E. J . Necfroanal. Chem. 1975, 58,313 Noda, T.; Inagaki. M.; Yamada, S. J . Non-Cryst. Solids 1969, 7, 285. Jenkins, G. M.; Kawamura, K. Nature (London) 1971, 231, 175. Gaylor, V. F.; Elving, P. J.; Conrad, A. L. Anal. Chem. 1953, 25, 1078. Lindquist, J. J . Electroanal. Chem. 1968, 78,204. Blaedel, W. J.; Schieffer, G. W. J . Nectroanal. Chem. 1977, 80, 259. Blaedel, W.J.; Engstrom, R. C. Anal. Chem. 1978, 50,476. Bard, A. J. Anal. Chem. 1963, 35, 1602. Karp, S.;Meites, L. J . A m . Chem. Soc. 1962, 84,906. Mamantov, G.; Freeman, D. 9.; Miller, F. J.; Zittel, H. E. J . Elecfroanal. Chem. 1965, 9 ,305. Panzer, R. E.; Elvlng, P. J. J . Nectrochem. SOC. 1972, 119, 864. Gilman, S. Electroanal. Chem. 1967, 2. Anson, F. C.; Schultz, F. A. Anal. Chem. 1963, 35, 1114.
RECEIVED for review June 11,1982. Accepted August 12,1982. This work was supported in part by a Northwest Area Foundation Grant of the Research Corporation and the South Dakota Research Institute.
