Anal. Chem. 1904,56,1741-1744
I
-11
4 I"
I 'I,
!
I
Figure 1. Anodic stripping voltammograms of solutions, 0.01 N in sulfuric acid, containing different rhenium concentrations: (0).no rha nium present; (1) 5.0 X lo-* g of Re/mL; (2) 1.0 X g of Re/mL; g of Re/mL; (4)2.0 X g of Re/mL; deposition time, (3) 1.5 X 2 min; sweep rate, 1.2 V mln-'; measuring mode, DP modulation amplitude, 40 mV. solution consisting of 20 mL of 0.01 N H2S04. The air is expelled by introduction of pure nitrogen gas for 10 min. The next working step consists in enrichment of traces of rhenium in the amalgam layer. A voltage of -0.8 V is applied to the electrode rotating at 1500 rpm as long as enrichment continues. Then the voltage is applied to the electrode at rest for an additional 20 s. This working step is terminated by electrode polarization in the direction of the anode. The potential on the HgAuRDE is varied from -0.8 V to 0.0 V. A speed of 1.2 V min-' is recommended for the potential scan. If the differential pulse technique
1741
is used, a modulation amplitude of 40 mV has proved to be useful. All the voltammograms have been recorded with a current sensitivity of 1.5 X lo4 A mm-'. The method of standard addition can be used for the quantitative evaluation of the recorded signals. The latter method can be repeated several times without replacement of the active electrode surface. To remove the amalgam layer with a view to regenerating the electrode, it will suffice to apply a voltage of +1.0 V to the electrode for about 1 min duration. A platinum electrode or also an Ag/AgCl electrode has been successfully used as auxiliary electrode. The perrhenate solution used has been prepared by dissolution of a known amount of rhenium powder (Specpure), using a mixture of sulfuric acid and hydrogen peroxide, while heating, fuming of the major portion of the acid, and dilution of the cooled concentrate with deionized water until an appropriate volume is obtained. Registry No. Re, 7440-15-5; Au, 7440-57-5; Au amalgam, 12607-42-0; perrhenate, 14333-24-5.
LITERATURE CITED Heyrovsky, J. Nature (London) 1935, 135, 870-871. Llngane, James J. J . Am. Chem. SOC. 1942, 64, 1001-1007. Ismagulova, A. 0. Zavod. Lab. 1901, 4 6 , 1088-1090; Chem. Abstr. 1981, 94, 57562. Agasyan, P. K.; Nikolaeva, E. R.; Chavdarova, R. Vestn. Mosk. Unlv., Ser. 2 : Khlm. 1970, 33, 336-339; 8 9 , Chem. Abstr. 1970, 8 9 , 190374. Muldagalleva, I . Kh.; Rozhdestvenskaya, Z. B.; Songina, 0. A. Sov. Electrochem. (Engl. Transl.) 1973, 9 , 762-764; Chem. Abstr. 1973, 79, 99769. Belenko, 1. A.; Speranskaya, E. F. Usp. folyarogr. Nakoplenlem, Mater. Vses Konf . "Amal'gamnaya folyarogr Nakopleniem €e Primen. Nauchn. Issled.", 1973 16-18; Chem. Abstf. 1975, 8 2 , 023600.
.
RECEIVED for review November
.
3, 1983. Accepted April 2,
1984.
Resistance of Nonaqueous Solvent Systems Contalnlng Tetraalkylammonium Salts. Evaluation of Heterogeneous Electron Transfer Rate Constants for the Ferrocene/Ferrocenium Couple K. M. Kadish,* J. Q. Ding, and Tadeusz Malinski' Department of Chemistry, University of Houston, Houston, Texas 77004 Prior to 1965 most cyclic voltammetric studies were carried out in aqueous media. This is not the case today where many types of nonconducting organic solvents are commonly used (1). Even with the addition of supporting electrolytes the resistance of these solutions may be quite high. For example, the specific resistance of aqueous 1.0 M to 0.1 M KCl solutions is in the range of 9.9-85.6 il cm (2). In contrast, specific resistance in nonaqueous media may range between 20 il cm and 40 000 il cm, and corrections thus must be made for IR loss. Several instrumental approaches to reduce the magnitude of IR loss have been described (3-9). Two of the most commonly used methods involve the utilization of a three-electrode system and positive feedback. This latter electronic compensation in the potentiostat is proportional to the cell current and thereby approxmately cancels the IR drop. In addition to positive feedback, other quite complicated instrumental methods can be used. These include the storage of measured ohmic potential drop in an analog sample-and-holding memory which is returned as a correction to the potentiostat input (5)) 'Present address: Department of Chemistry, Oakland University, Rochester, MI 48063. 0003-2700/84/0356-1741$01.50/0
or a use of the potential difference between two probes located at different distances from the working electrode to provide a correction signal to the potentiostat (7). Noninstrumental methods of reducing IR loss include the use of a Luggin capillary (I), as well as use of a Pt wire quasi-reference electrode (10). In addition, a low concentration of reactant can be utilized to reduce the total current. Even under optimum conditions, however, some uncompensated resistance remains (9,II) This does not cause problems in aqueous media but may become significant in nonaqueous solvents where absolute values of uncompensated resistance are much higher. The effect of uncompensated resistance is to shift the cathodic and anodic peak potentials of a cyclic voltammogram such that larger peak separations, Epa-Ew,are obtained with increase in scan rate (3). A similar increase in peak separation is also observed for the case of slow, or quasi-reversible, electron transfer kinetics and it is in this increase in peak separation with scan rate which is used to evaluate the magnitude of the heterogeneous rate constant, k" (12). In fact, with a given electrode configuration, and a single concentration of reactant, it is impossible to differentiate a slow electron transfer reaction from one which is rapid but is measured 0 1984 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 9, AUGUST 1984
Table I. Specific Resistance ( a cm) and Heterogeneous Electron Transfer Rate Constants for Oxidation of Ferrocene in Different Organic Solvents Containing 0.1 M TBAP at 22 "C ko,b,c
solvent (abbreviation)
P , S ~cm
cm/s
acetonitrile (CH,CN) acetone (Me,CO) nitromethane (CH,NO,) methanol (MeOH) dimethylformamide (DMF) dimethylacetamide (DMA) methylformamide (MF) butyronitrile (PrCN) dimethyl sulfoxide (DMSO) formamide (F) benzonitrile (PhCN) pyridine (Py) nitrobenzene (PhNO,) methylene chloride (CH,Cl,) dichloroethane (EtCl,) ethanol (EtOH) propanol (PrOH) tetrahydrofuran (THF)
132 f 2 (128)" 181 f 4 203 * 3 223 f 3 250 t 5 250 5 258 f 5 264 t 4 365 t 5 450 t 7 477 f 8 569 t 6 640 t 10 725 f 5 861 f 8 1800 i 5 0 1960 f 50 2670 f 50
0.09 0.08 0.07 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.05 0.05 0.04
a Reference 9. Values represent an average of six measurements obtained at the lowest concentration of Fc M ) over a potential range of sweep between 0.05 and 8.0 V/s. Diffusion coefficients ranged between 1.0 and 2.4 X cm2/sdepending upon solvent. Rate constants are represented to fO.01.
under conditions of high uncompensated resistance. For this reason our own laboratory has attempted to compare new measurements of heterogeneous electron transfer rate constants with another redox couple whose value of ko is much larger than that for the compound of interest under the same experimental conditions. In principle, this comparison enables one to more easily evaluate the significance of a measured rate constant as measured by cyclic voltammetry in nonaqueous media. For example, if the heterogeneous rate constant of the second couple is much faster than that for the compound of interest, one may confidently assume that the calculated rate constant for this compound is not an apparent value limited by uncompensated IR loss. This internal comparison of two rate constants has already proven to be quite useful for cyclic voltammetric measurements obtained in methylene chloride (13)and acetone (14) where the ferrocene/ferrocenium couple (Fc+/Fc) has been selected as the couple for internal comparison. The ferrocene/ferrocenium couple (Fc+/Fc) has frequently been used as a standard for correction of liquid junction potentials (15)and thus is a logical choice for internal comparison
with other heterogeneous electron transfer rate constants in nonaqueous solvent systems. Several values of ko have been presented for the Fc+/Fc couple in nonaqueous media (16-18) but there are no systematic data on the rate constants of this couple as measured by cyclic voltammetry. Thus, one aim of this paper is to provide such data. In addition, we have also presented specific resistances of 18 nonaqueous electrochemical solvents containing varying concentrations and types of supporting electrolytes. These data complement the type of information obtained by the kinetic measurements and should be especially useful for selection of a solvent-supporting electrolyte system in thinlayer spectroelectrochemistry where large area electrodes are often employed and uncompensated resistances are substantial. Finally, these specific resistances may be used to calculate uncompensated solution resistances for a given working and reference electrode configuration (9). This information should be of great help in selecting a given nonaqueous solvent for kinetic and thermodynamic measurements of novel redox couples. EXPERIMENTAL SECTION Tetraethylammonium perchlorate (TEAP), tetrapropylammonium perchlorate (TPAP),and tetrabutylammonium perchlorate (TBAP) were obtained from Eastman Kodak. Electrolytes were recrystallized from ethyl acetate and then dried under vacuum at 60 "C. Tetramethylammonium perchlorate (TMAP) was purchased from Alfa and treated in a similar fashion. Eighteen different nonaqueous, aprotic solvents were used in this study. These solvents are listed in Table I which also gives the commonly used abbreviation. All solvents were distilled over appropriate drying agents as described in the literature and in most cases were stored over 4-A molecular sieves before use. The resistance of each solution was measured both by means of an impedance instrument whose design has been described elsewhere (20)and with a Yellow Springs Co. Model 31 resistance bridge. No differences in specific resistance, p , were observable as a function of the measurement technique and values of p measured with a bridge and with the ac impedance method were interchangeable within experimental error. The presented ko values were all obtained from cyclic voltammetric measurements by using the technique of Nicholson (12). Cyclic voltammetric measurements were obtained with a three-electrode system using an IBM EC/225 voltammetric analyzer, coupled with a Houston Instruments Model 2000 X-Y recorder or a Tektronix 5111 storage oscilloscope. The working electrode was a Pt button and a Pt wire served as the counterelectrode. The surface of the working electrode was treated with concentrated (1:l) nitric acid, rinsed with water and solvent, and polished on Whatman qualitative grade filter paper before use. A commercial saturated calomel electrode (IBM) was used as the reference electrode which was separated from the bulk of the solution by a fritted glass bridge. For these studies a Luggin capillary was employed, and positive feedback (which was built into the potentiostat) was utilized. The
Table 11. Cyclic Voltammetric Data for Different Concentrations of Ferrocene in CH,CN and EtCl, Containing 0.1 M TBAP M ferrocene l o w 3M ferrocene scan rate, AE ko,b AE solvent VIS m6' $a cm/s m6' $" cm/s CH,CN 0.05 63 7 0.085 63 7 0.085
EtC1,
0.10 0.50 1.00 5.00
65 69 75 92
5 2.6 1.7 0.74
0.086 0.100 0.092 0.089
65 70 75 94
5 2.4 1.7 0.72
0.086 0.092 0.092 0.087
0.05 0.10 0.50
63 66 72 77 89
7 4 2 1.4 0.82
0.065 0.052 0.059 0.058 0.058
63 66 73 78 91
7 4 1.9 1.4 0.80
0.065 0.052 0.056 0.058 0.057
1.oo
3.00
From ref 1 2 .
Calculated by using diffusion coefficients of 2.4 x
l o - * cm2/sin CH,CN
and 1.4
X
cmz/sin EtCl,.
ANALYTICAL CHEMISTRY, VOL. 56, NO. 9, AUGUST 1984
IR loss was compensated based on standard procedural methods, i.e., looking for the onset of potential oscillation (which is only an approximate procedure). Residual uncompensated IR loss was evaluated in a systematic way by looking at the cyclic voltammetric behavior as a function of the ferrocene concentration. This method of evaluating uncompensated resistance has been used in past cyclic voltammetric studies (13,14) and, under the present experimental conditions, gave values of uncompensated resistance ranging between 60 0 (acetonitrile)and 350 0 (dichloromethane).
RESULTS AND DISCUSSION Values of specific resistance, p, for 18 different 0.1 M TBAP solutions are presented in Table I. Values of p range between 132 D cm in acetonitrile and 2670 D cm in tetrahydrofuran. The value in acetonitrile is within experimental error of the 128 D cm reported by Whitson et al. (9) under the same experimental conditions, while the remainder of the values parallel earlier less numerous data in 0.5 M TBAP (21). At this concentration of supporting electrolyte, the lowest value of p was reported in acetonitrile (39 D cm) and the highest in THF (583 D cm). Also included in Table I are the heterogeneous electron transfer rate constants for the Fc+/Fc couple in each solvent system. These values represent an average of six k o measurements obtained over ranges of potential sweep between 0.05 and 8.0 V/s. Typical data are shown in Table 11. As seen in this table, self-consistent rate constants are obtained at low concentrations and low scan rates indicating the absence of measurable contributions due to uncompensated resistance. The ko values presented in Table I are about 2-3 times larger than values calculated by cyclic voltammetry in CH3CN (0.044 cm/s), MezSO (0.038 cm/s), DMF (0.033 cm/s), and EtOH (0.016 cm/s) (26). Our value in CH3CN, 0.1 M TBAP (0.09 cm/s) is smaller than that reported by Sharp et al. (17)in CH,CN, 0.1 M TEAP (0.22 cm/s), but is within experimental error for measurements by cyclic voltammetry between two different laboratories using two different types of supporting electrolytes. No other values are available in the literature for comparison with other solvents. In addition to possible changes in k o with the type of supporting electrolyte, the observed rate constant for a given redox couple in a given solvent is also known to vary with the electrode material, the condition of the electrode (29), and the technique used for the measurement. Of importance in this present study is that all of the rate constants in Table I were obtained by the same technique, at an identical electrode and bridge, and with the same type and concentration of supporting electrolyte. Thus, the k o values presented in this table show a good trend in kinetic changes of the Fc+/Fc couple with changes in the nonaqueous solvent environment. The changes in rate constants with changes in solvent are not surprising because the solution-supporting electrolyte system is known to influence mass-transfer phenomena and may also change the characteristics of the electron transfer itself, Le., the half-wave potential, the electron transfer rate constant, and the charge transfer coefficient, a. For the specific Fc+/Fc couple, the rate constants are found to vary inversely with the specific resistance of the solution. While one might assume that this decrease is due to resistance effects, this is not the case. A detailed theoretical discussion of this effect will be the subject of a separate paper (22). Values of specific resistance in different solvent-TBAP combinations are presented in Table 111. In solvents containing TBAP, the most commonly used supporting electrolyte, values of p ranged from 44.4 D cm and 38500 0 cm depending upon the solvent and concentration of supporting electrolyte. A t 0.1 M TBAP, p ranged between 132 0 cm (CH,CN) and 2670 D cm (THF). In all solvents the values of p decreased with increasing concentration of supporting electrolyte. For example, in DMF
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8
3
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v3m
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r:
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r l m o o o . r l m o o r l v 3 o o o r l m o o r l m o o o r i m o o r l m o 00
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1743
1744
Anal. Chem. 1984, 56, 1744-1747
varied between 1360 Q cm at 0.01 M TBAP and 107 Q cm at 1.0 M TBAP. There were no significant differences between the values of p in this study utilizing ten nonaqeous solvents and those presented in two more limited studies which included three of our ten investigated solvents (9,21). Values from these earlier studies are presented in parentheses in Table 111. Data for solvents containing TPAP, TEAP, and TMAP are less numerous than those for TBAP due to the limited solubility of these salts in some solvents. However, as seen from Table I11 similar trends in p exist as a function of supporting electrolyte concentration. In a given solvent p is almost identical for TPAP, TEAP, and TMAP, and is slightly smaller than that for TBAP. In addition, the smallest value of p is again observed in CH&N and the largest (where a comparison is possible) is found in CHzClzor EtClZ. Almost all electrochemical studies in nonaqueous media utilize tetraalkylammonium salts as supporting electrolytes, the most commonly being TBAP, TPAP, TEAP, and TMAP. The usual concentration of this supporting electrolyte which has been employed in cyclic voltammetry is 0.1 M, although some studies utilize 0.2 M. In contrast, thin-layer spectroelectrochemistry (which does not use built-in IR compensation) requires much higher concentrations of supporting electrolyte, and concentrations as high as 0.50 M or 1.0 M are often utilized. As seen in Table I11 the need to use highly concentrated solutions of tetraalkylammonium salts is largely justified on the basis of the decreased specific resistance. In an earlier study, Whitson et al. (9) calculated a solution resistance of 230 Q using their specific cell configuration and a specific resistance of 128 Q cm. Using the same configuration in THF and our value of 2670 Q cm (at 0.1 M TBAP) would lead to a solution resistance of almost 5000 Q. This is an extremely high value, and under these experimental and solution conditions, great care must be taken in correcting for IR loss. This value of resistance depends, of course, upon the specific cell configuration. However, for a given electrode configuration, solution resistances can be easily calculated. Finally, we should stress that k” values presented in this paper should not be assumed to be absolute standard rate constants which would be identical by all electrochemical techniques. They are, however, self-consistent measurements obtained by cyclic voltammetry at a Pt electrode using all known precautions to eliminate uncompensated IR loss. We believe that the use and evaluation of these data when looking p
at other heterogeneous electron transfer rate constants should help to eliminate problems associated with the reporting of erroneous resistance-limited rate constants, as well as the erroneous evaluation of peak current shifts as a function of scan rate using cyclic voltammetry and the well-known and often quoted Nicholson-Shain diagnostic criteria (23). Registry No. TBAP, 1923-70-2;TPAP, 15780-02-6; TEAP, 2567-83-1;TMAP, 2537-36-2;DMF, 68-12-2;DMA, 127-19-5;MF, 123-39-7;F, 75-12-7; Py, 110-86-1;THF, 109-99-9;Fc, 102-54-5; Fc’, 12125-80-3; CH3CN, 75-05-8; Me2C0, 67-64-1; CH3NO2, 75-52-5;MeOH, 67-56-1;PrCN, 109-74-0;Me2S0,67-68-5;PhCN, 100-47-0; PhN02, 98-95-3; CH2C12, 75-09-2; EtCl,, 1300-21-6; EtOH, 64-17-5; PrOH, 71-23-8.
LITERATURE CITED (1) Sawyer, D. T.; Roberts, J. L., Jr. “Experimental Electrochemistry for Chemists”; Wiley: New York, 1974. (2) Dobos, D. “Electrochemical Data”; Elsevler: Amsterdam, 1975. (3) Brown, E. R.; McCord, T. G.; Smith, D. E.; DeFord, D. Anal. Chem. 1966, 38, 1119, and references therein. (4) Garreau, D.; Saveant, J. M. J. Electroanal. Chem. 1972, 35, 309. (5) Bezman, R. Anal. Chem. 1972, 4 4 , 1781. (6) Whltson, P. E.; VandenBorn, H. W.; Evans, D. H. Anal. Chem. 1973, 45, 1298. (7) Sedletskii, R. V.; Limin, B. E. Electrokhimiya 1972, 8,22. (8) Thomas, W. E.; Schaap, W. 8. Anal. Chem. 1969, 4 1 , 136. (9) Whltson, P. E.; VanderBorn, H. W.; Evans, D. H. Anal. Chem. 1973, 45, 1298. (10) Belew, W. L.; Fisher, D. J.; Kelley, M. T. Chem. Instrum. 1970, , 297. (11) Garreau, D.; Saveant. J. M. J. Necroanal. Chem. 1978, 86,63. (12) Nicholson, R. S. Anal. Chem. 1965, 37, 1351. (13) Kadish, K. M.; Su, C. H. J. Am. Chem. SOC. 1983, 105, 177. (14) Zhu, T.; Su. C. H.;Lemke, B. K.; Wilson, L. J.; Kadish, K. M. Inorg. Chem. 1963, 22,2527. (15) Bauer, D.; Breant, M. I n ”Electroanalytical Chemistry”; Bard, A. J., Ed.; Marcel Dekker: New York, 1975; Vol. 8. (16) Diggle, J. W.; Parker, A. J. Nectrochim. Acta 1973, 18,975. (17) Sharp, M.; Peterson, M.;Edstrom, K. J . Nectroanal. Chem. 1980, 109, 271. (18) Armstrong, N. R.; Qulnn, K.; Vanderborgh, N. E. J. Nectrochem. SOC. 1976, 123,646. (19) Daum, P. H.; Enke, C. G. Anal. Chem. 1969, 4 1 , 653. (20) Cai, S. M.; Mallnskl, T.; Lln, X. 0.; Ding, J. Q . ; Kadlsh, K. M. Anal. Chem. 1983, 55, 161. (21) House, H. 0.; Feng, E.; Peet, N. P. J. Org. Chem. 1971, 36,2371. (22) Malinski, T.; Ding, J. Q.; Kadish, K. M., manuscript in preparation. (23) Nicholson, R. S.; Shain, I. Anal. Chem. 1964, 3 6 , 706.
RECEIVED for review July 25, 1983. Resubmitted March 16, 1984. Accepted March 26,1984. We are grateful for financial support of this work from the National Institutes of Health (Grant GM 25172), the National Science Foundation (Grant CHE-821557),and Robert A. Welch Foundation (Grant E680).
Enhanced Voltammetric Response by Electrochemical Pretreatment of Carbon Paste Electrodes K. Ravichandran and Richard P. Baldwin* Department of Chemistry, University of Louisville, Louisville, Kentucky 40292 Despite the fact that electrodes constructed from carbon substrates exhibit low background currents over a wide range of potentials and are useful for numerous electrochemical applications, electron transfer rates observed for redox processes at these surfaces are often slower than those at metal electrode surfaces. As a result, carbon electrodes often exhibit substantial overpotentials which cause the related oxidations and reductions to take place at potentials significantly in excess of their thermodynamic potentials. In order to increase the electron transfer rates, various chemical (1-3), thermal ( 4 ) ,and electrochemical (5-12) surface treatment procedures for carbon electrodes have been developed which have been 0003-2700/84/0356-1744$01.50/0
shown to produce improved electrode response compared to that of the native electrode material. Of all these modification methods, the electrochemical pretreatment reported by Engstrom (6, 7)for glassy carbon is one of the simplest to carry out experimentally. In addition, this particular modification strategy has been shown to produce a marked enhancement in the sensitivity and selectivity of liquid chromatography/ electrochemicaldetection (LCEC) for several electrochemically irreversible oxidations which exhibit high overvoltages at untreated glassy carbon (12,13). The systems examined so far have included hydrazines, hydroquinone, ascorbic acid, and nicotinamide adenine dinucleotide (NADH). In nearly 0 1984 American Chemical Society
