Anal. Chem. 2000, 72, 3486-3491
Achievement of Near-Reversible Behavior for the [Fe(CN)6]3-/4- Redox Couple Using Cyclic Voltammetry at Glassy Carbon, Gold, and Platinum Macrodisk Electrodes in the Absence of Added Supporting Electrolyte Melissa B. Rooney, Darren C. Coomber, and Alan M. Bond*
Anal. Chem. 2000.72:3486-3491. Downloaded from pubs.acs.org by TULANE UNIV on 01/18/19. For personal use only.
Department of Chemistry, Monash University, Clayton, Victoria 3800, Australia
Voltammetric studies in the absence of added supporting electrolyte are presently dominated by the use of nearsteady-state microelectrode techniques and millimolar or lower depolarizer concentrations. However, with this methodology, large departures from conventional migration-diffusiontheoryhavebeenreportedforthe[Fe(CN)6]3-/4process at both carbon fiber and platinum microdisk electrodes. In contrast, data obtained in the present study reveal that use of the transient cyclic voltammetric technique at glassy carbon, gold, or platinum macrodisk electrodes and K4[Fe(CN)6] or K3[Fe(CN)6] concentrations of 50 mM or greater provides an approximately reversible response in the absence of added electrolyte. It is suggested that the use of very high [Fe(CN)6]3- and [Fe(CN)6]4- concentrations overcomes problems associated with a diffuse double layer and that large electrode surface areas and faster potential sweep rates minimize electrode blockage and passivating phenomena that can plague voltammetric studies at microelectrodes. The cyclic voltammetry of the [Fe(CN)6]3-/4- couple at a range of concentrations at macroelectrodes in the absence of added inert electrolyte is compared with that obtained in the presence of 1 M KCl. The enhanced influences of uncompensated resistance, migration, and natural convection arising from density gradients under transient conditions at macrodisk electrodes also are considered. It has been commonly recommended for many decades that voltammetric experiments be conducted in the presence of excess added supporting electrolyte. This presumption is based on the belief that, to obtain reliable data, the ionic strength and conductivity of the solution must be high and constant. Thus, in most studies, a high concentration of supporting electrolyte is now added routinely because it functions to (1) decrease Ohmic drop, (2) eliminate migration as a mode of mass transport, (3) eliminate changes in ionic strength due to electrolytically consumed or generated ionic species, and (4) simplify mass transport theory. However, in the last 15 years or so, the general necessity for addition of excess supporting electrolyte has been questioned, * Corresponding author: (e-mail)
[email protected]; (fax) (03) 9905 4597.
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as microelectrode studies employed under near-steady-state conditions with little or no added excess supporting electrolyte have revealed that voltammetric signals can be obtained with minimal distortion due to Ohmic drop.1-6 Such studies compliment those obtained many years earlier under polarographic conditions at the dropping mercury electrode.7 The reasons for wishing to undertake electrochemical studies in the absence of excess supporting electrolyte are numerous. Among the most important are the following: (1) to eliminate the need for, and often the waste of, expensive electrolyte, particularly for experiments in organic solvents; (2) to extend the useful voltage range in organic solvents; (3) to eliminate possible interference of supporting electrolyte in synthetic electrochemistry; (4) to raise the upper concentration limit for the analyte under study (because one need not dissolve the 10-50-fold excess of supporting electrolyte concentration); (5) to compare electrochemical data to spectroscopic data or studies of homogeneous electron transfer, as the latter do not require the presence of excess electrolyte. As noted above, low scan rate, near-steady-state voltammetry at microelectrodes has become the method of choice for low (no) electrolyte studies because the technique minimizes the effects of Ohmic drop.8-10 Furthermore, voltammetry obtained with microelectrodes in the absence of excess supporting electrolyte has been successfully predicted by theory, incorporating migration and Ohmic drop, for a wide range of analytes.6,11-13 However, there (1) Bond, A. M.; Fleischmann, M.; Robinson, J. J. Electroanal. Chem. 1984, 168, 299-312. (2) Bond, A. M.; Fleischmann, M.; Robinson, J. J. Electroanal. Chem. 1984, 172, 11-25. (3) Ewing A. G.; Feldman B. J.; Murray R. W. J. Phys. Chem. 1985, 89, 12631269. (4) Wightman, R. M. Science 1988, 240, 415-417. (5) Ciszkowska, M.; Stojek, Z.; Osteryoung, R. A. Anal. Chem. 1990, 62, 349354. (6) Ciszkowska, M.; Stojek, Z. J. Electroanal. Chem. 1999, 466, 129-143. (7) Heyrovsky, J.; Kuta, J. Principles of Polarography; Academic Press: New York, 1966; pp 65-72, 240-252, and 351-356. (8) Oldham, K. B. J. Electroanal. Chem. 1987, 237, 303-307. (9) Bruckenstein, S. Anal. Chem. 1987, 59, 2098-2103. (10) Bond, A. M.; Luscombe, D.; Oldham, K. B.; Zoski, C. G. J. Electroanal. Chem. 1988, 249, 1-14. (11) Amatore, C.; Fosset, B.; Bartlet, J.; Deakin, M. R.; Wightman, R. M. J. Electroanal. Chem. 1988, 256, 255-268. 10.1021/ac991464m CCC: $19.00
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also are occasions where “anomalous” results have been reported.12,14,15 One such case is the voltammetry of ferrocyanide (K4[Fe(CN)6]) and ferricyanide (K3[Fe(CN)6]) in aqueous solution. In the presence of the usual added electrolyte (e.g., 0.1 M KCl), the chemically reversible [Fe(CN)6]3-/4- couple is detected. However, two groups have independently reported almost nonexistent reduction currents for 1 mM (or lower) concentrations of K3[Fe(CN)6] in the absence of excess electrolyte.14,15 The “irregular” voltammetry of K3[Fe(CN)6] at carbon microelectrodes was attibuted to a “dynamic diffuse layer effect”,14 whereas the irreproducibility and surprisingly low currents detected for K4[Fe(CN)6] (e10 mM) and K3[Fe(CN)6] (10 mM) than usually employed when indifferent electrolyte is present. Subsequently, analyte concentrations in the usual millimolar region were investigated. It was predicted that detrimental effects resulting from a diffuse double layer, as suggested by Lee and Anson,14 should be similar at macro- versus microelectrodes, but should be avoidable by using uncommonly high concentrations of K3[Fe(CN)6] or K4[Fe(CN)6] to form a welldefined double layer. Contrary to the widespread supposition that microelectrodes are best or even mandatory for investigations without added electrolyte, our results reveal that conventional electrodes produce superior voltammetric results for the [Fe(CN)6]3-/4couple in the absence of excess supporting electrolyte. EXPERIMENTAL SECTION Reagents. Potassium ferricyanide (K3[Fe(CN)6]), potassium ferrocyanide (K4[Fe(CN)6]), and potassium chloride were Analar grade commercial products, used as received. For those experiments in which supporting electrolyte was present, KCl was added to give a 1 M concentration. All solutions were prepared with water from a MilliQ-MilliRho purification system (18 MΩ cm) and had a typical pH of 6.5. Electrochemistry. Voltammetry was performed using a Bioanalytical Systems BAS model 100A electrochemistry system (12) Cooper, J. B.; Bond, A. M.; Oldham, K. B. J. Electroanal. Chem. 1992, 331, 877-895. (13) Amatore, C.; Paulson, S. C.; White, H. S. J. Electroanal. Chem. 1997, 439, 173-182. (14) Lee, C.; Anson, F. C. J. Electroanal. Chem. 1992, 323, 381-389. (15) Beriet, C.; Pletcher, D. J. Electroanal. Chem. 1993, 361, 93-101. (16) Bond, A. M.; Feldberg, S. W. J. Phys. Chem. B 1998, 102, 9966-9974.
equipped with a preamplifier for microelectrode measurements (Bioanalytical Systems, West Lafayette, IN). All experiments were performed in a three-electrode cell. The auxiliary electrode was a Pt wire, terminating in a circular Pt wire mesh (d ) 2 cm) which rested on the bottom of the electrochemical cell; the distance from the working electrode to this auxiliary base was ∼2 cm. Use of a conventional reference electrode system can result in the leaking of ions from the electrode into the bulk solution, a source of contamination that cannot be ignored in experiments with no deliberately added inert electrolyte. For this reason, the quasireference electrode used was a Pt wire, insulated along its length and bent in the shape of a hook, such that the bare tip was separated from the center of the working electrode surface by ∼2 mm. Working electrodes consisted of GC, Au, and Pt macroelectrodes with 3-mm diameters (Metrohm, Herisau, Switzerland), as well as microelectrodes of 12- (GC, Pt) and 10-µm (Au) diameters (Microglass Instruments, Greensborough, Victoria, Australia). Unless otherwise indicated, prior to each voltammetric experiment, working electrodes were manually polished with an aqueous slurry of 0.1- or 0.05-µm alumina particles (Bioanalytical Systems) on a Microcloth polishing cloth (Buehler, Lake Bluff, IL) and then rinsed thoroughly with water. Typically, voltammetric studies at each type of macrodisk working electrode were conducted in the same solution of K4[Fe(CN)6] or K3[Fe(CN)6] for each concentration investigated. Various scan rates were employed with each set of conditions. However, unless otherwise indicated, data are reported at a scan rate of 20 mV s-1, where IR drop is minimal under transient conditions. To minimize the interference of oxygen, the solutions were purged with high-purity nitrogen prior to working electrode placement and subsequent voltammetry. All experiments were performed with the cell at room temperature (293 K). Simulations. Simulations of the experimental data incorporating planar diffusion, uncompensated resistance, and migration were performed using the software described elsewhere.16 The diffusion coefficients for [Fe(CN)6]3-, [Fe(CN)6]4-, and K+ were obtained from the literature, and were 7.6 × 10-6, 6.3 × 10-6, and 2 × 10-5 cm2 s-1 respectively.7,15 RESULTS AND DISCUSSION Macrodisk Electrodes: K4[Fe(CN)6] Oxidation and K3[Fe(CN)6] Reduction, 50-100 mM. Given the failure of millimolar or lower concentrations of [Fe(CN)6]3- or [Fe(CN)6]4to evoke a significant and reproducible steady-state voltammetric response in earlier studies,14,15 we initially examined the response of K4[Fe(CN)6] and K3[Fe(CN)6] at higher (g50 mM) concentrations without added supporting electrolyte and then compared results when 1 M KCl was present. In stark contrast to published voltammetric data, we now find that, under our high depolarizer concentration conditions, cyclic voltammograms obtained with macroelectrodes for the oxidation of 50 mM K4Fe(CN)6 in the absence of added inert electrolyte are of similar quality to those obtained in the presence of 1 M KCl at GC, Au, and Pt electrodes (Figure 1). Even without the addition of KCl, it appears that we have a sufficiently high ionic strength to provide a well-defined double layer. Equally impressive results were obtained for the reduction of 50 mM K3Fe(CN)6 in the absence of added electrolyte at GC and Au electrodes (Figure 1). However, in the absence of Analytical Chemistry, Vol. 72, No. 15, August 1, 2000
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Figure 1. (Top) Cyclic voltammetry of 50 mM K4[Fe(CN)6] in aqueous solution at GC, Au, and Pt macroelectrodes (d ) 3 mm, v ) 20 mV s-1). (Bottom) Cyclic voltammetry of 50 mM K3[Fe(CN)6] in aqueous solution at GC, Au, and Pt macroelectrodes (d ) 3 mm, v ) 20 mV s-1). Solid line: in the absence of added inert electrolyte. Broken line: in the presence of 1 M KCl.
Figure 2. (A) Effect of scan rate (v) on the cyclic voltammetry of 100 mM K3[Fe(CN)6] in the absence of added inert electrolyte in aqueous solution at a GC macroelectrode (d ) 3 mm). Increasing current corresponds to v ) 20, 50, 100, 200, 500, and 1000 mV s-1. Insets: (B) effect of instrumental IRu compensation on the cyclic voltammetry of 100 mM K3[Fe(CN)6] in aqueous solution at a GC macroelectrode (d ) 3 mm, v ) 20 mV s-1). Broken line: uncompensated IRu. Solid line: nominally 100% IRu compensated cyclic voltammogram. (C) As for (B), but v ) 1000 mV s-1.
added electrolyte, a significantly decreased peak current, more than expected for migration alone, is observed for the reductive component of the process at the Pt electrode. The difference in the average peak potentials (Ep, av) obtained in the presence and absence of KCl is predominantly attributed to the medium-dependent potential of the Pt quasi-reference electrode, which was used to avoid adventitious addition of electrolyte that occurs via leakage from conventional reference electrodes (Figure 1, Pt). The presence of additional uncompensated resistance (Ru) also contributes to the significant enhancement of the peak-to-peak separation (∆Ep) observed in the absence of added electrolyte. Data obtained at faster scan rates exacerbates 3488 Analytical Chemistry, Vol. 72, No. 15, August 1, 2000
Figure 3. Comparison of experimental and simulated voltammograms for (A) 50 mM K4[Fe(CN)6] and (B) 50 mM K3[Fe(CN)6] in the absence of added electrolyte in aqueous solution (GC electrode, d ) 3 mm, v ) 20 mV s-1). Solid line: experimental data. Broken line: simulation.
the effect of uncompensated resistance (as shown for 100 mM K3Fe(CN)6, Figure 2A). While the influence of uncompensated Ohmic drop (IRu) can be minimized via application of instrumental compensation (Figure 2B,C), we feel the contribution of IRu drop is better handled by inclusion of this term into the theory (see ref 16 and Figure 3). In the case of K4[Fe(CN)6], the enhanced oxidation current results from the significant participation of migration of [Fe(CN)6]4toward the working electrode that occurs with oxidation of a negatively charged species in the absence of excess inert electrolyte (Figure 1). Conversely, when the negatively charged [Fe(CN)6]3- is being reduced, it will migrate away from the working electrode. Thus, in the case of K3[Fe(CN)6] reduction at GC and Au electrodes, migration is the predominant reason for the decrease in peak current relative to that obtained in the presence of 1M KCl (Figure 1). Nonetheless, the response is clearly dependent on the electrode material, and surface passivation apparently remains a problem at Pt electrodes. Indeed, the irreversible adsorption and decomposition of ferricyanide on Pt
surfaces has been noted and related to the formation of Prussian Blue deposits.17 However, such deposits were not observed in our experiments with [Fe(CN)6]3-/4- at Pt electrodes, presumably due to the shorter duration of our experiments and the lower rate of electrolysis at stationary electrodes versus rotating disk electrodes at high rotation rates (employed in ref 17). The voltammetric results obtained in the absence of added electrolyte were simulated according to predictive theory, which incorporates both the effects of Ru and migration (Figure 3). Close inspection of the voltammetry of 50 mM K4Fe(CN)6, particularly at very positive potentials where the current should be dominated by mass transport, reveals the presence of larger experimental current than that predicted by theory (Figure 3A). We attribute this enhanced current to two phenomena. First, the simulated voltammogram assumes a simple planar electrode geometry; hence, the semi-infinite linear diffusion assumed does not take into account the radial component that contributes to the steadystate current detected at unshielded electrodes.18 A more significant contribution under slow scan rate conditions appears to be the effect of convection resulting from electrochemically generated fluid density gradients within the depletion layer. As K4[Fe(CN)6] solutions have a slightly higher density than K3[Fe(CN)6] solutions, oxidation or reduction of either species results in the flux of K+ ions and a subsequent difference in density of the diffusion layer versus the bulk solution.18-20 The resulting natural convective flow, which is dependent on electrode orientation, increases mass transfer of the electroactive analyte to the electrode surface, thereby increasing the current detected at the electrode surface. This convection effect has been observed for ferricyanide reduction and ferrocyanide oxidation with steady-state voltammetry at microelectrodes in the presence of high concentrations of excess electrolyte.20 Recent studies on polyoxomolybdate electrochemistry in the presence and absence of added supporting electrolyte, employing transient voltammetry at macrodisk electrodes, reveal that gravity-induced natural convection effects are enhanced in the absence of added electrolyte.21 The enhanced steady-state current observed for [K4Fe(CN)6] in our studies is dependent on electrode orientation, as predicted for natural convective effects,20 and also is minimized by addition of 1 M KCl and the use of high scan rates. Convective effects caused by density gradients at the electrode surface remain evident in the case of reduction of 50 mM K3[Fe(CN)6] in the absence of added electrolyte (Figure 3B) when slow scan rate conditions of 20 mV s-1 are employed. In this case, migration away from the working electrode results in a decrease in the current that opposes the increase caused by convective effects. Nonetheless, the effect of convection is revealed by the disagreement of experimental and simulated results in the very negative potential region (beyond -400 mV) and in the resulting offset of the oxidative current on the reverse scan. Investigation of voltammetry at different electrode orientations reveals the expected dependence of steady-state current on electrode orienta(17) Kawiak, J.; Kulesza, P. J.; Galus, Z. J. Electroanal. Chem. 1987, 226, 305314. (18) Laitinen, H. A.; Kolthoff, I. M. J. Am. Chem. Soc. 1939, 61, 3344-3349. (19) Lee, W. W.; White, H. S.; Ward, M. D. Anal. Chem. 1993, 65, 3232-3236. (20) Gao, X.; Lee, J.; White, H. S. Anal. Chem. 1995, 67, 1541-1545. (21) Bond, A. M.; Coomber, D. C.; Feldberg, S. W.; Oldham, K. B.; Vu, T. Unpublished work, Monash University, 1998-2000.
Figure 4. (Top) Cyclic voltammetry of 0.5 mM K4[Fe(CN)6] in aqueous solution at GC, Au, and Pt macroelectrodes (d ) 3 mm, v ) 20 mV s-1). (Bottom) Cyclic voltammetry of 0.5 mM K3[Fe(CN)6] in aqueous solution at GC, Au, and Pt macroelectrodes (d ) 3 mm, v ) 20 mV s-1). Solid line: in the absence of added inert electrolyte. Broken line: in the presence of 1M KCl.
tion. Under fast scan conditions (g100 mV s-1) the effect of natural convection is minimal, although this advantage is offset by enhanced contributions from IRu drop. Macrodisk Electrodes: 5 and 0.5 mM K4[Fe(CN)6]. In principle, the shapes of cyclic voltammograms of highly charged species should be independent of concentration of [Fe(CN)6)]4with respect to the relative influence of IRu drop.16 This is because, on decreasing the concentration, the effects of increased resistance and decreased current magnitudes should cancel each other with respect to the IRu term. Results obtained with 5 mM solutions of ferrocyanide (not shown) have shapes similar to 50 mM voltammograms with both oxidative and reductive processes well-defined, although again at Pt electrodes, the currents are significantly smaller than those predicted on the basis of migration-diffusion theory and those obtained in the presence of 1 M KCl. However, as the concentration of K4[Fe(CN)6] is lowered to 0.5 mM, far more irregular voltammetry is observed at all electrode materials (Figure 4). For GC and Au, the oxidative component is clearly broadened and the current is decreased relative to that obtained when KCl supporting electrolyte is present. More obviously, the reductive component is virtually eliminated. In fact, a small reductive wave was only identified on the first scan; successive potential scans revealed no detectable faradaic current for the reductive process. The nonidealities observed in the absence of added electrolyte when low [Fe(CN)6]4- concentrations are utilized have been attributed to a number of factors. Surface adsorption and blockage by the reduction product, ferricyanide, or a reaction product was proposed by Beriet and Pletcher15 to explain the complex steadystate voltammetric behavior observed for low-millimolar concentrations of K4[Fe(CN)6] at a Pt microdisk electrode. However, one might expect such a blockage to have an equivalent effect on the oxidative current of a subsequent cycle in our cyclic voltammetric experiments. In fact, the effect of repetitive potential cycling on the oxidative current is minimal, in contrast to the near extinction Analytical Chemistry, Vol. 72, No. 15, August 1, 2000
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of the reductive current on the initial scan. Thus, attributing all effects at low concentration without added electrolyte to electrode passivation is unrealistic on the basis of data obtained by the transient voltammetry. A significant contribution to the irregular electrochemistry observed at low ferrocyanide concentrations is most likely a diffuse double-layer effect, as suggested by Lee and Anson.14 The inhibited reduction current may very well occur at potentials negative of the point of zero charge, which give rise to repulsion of the negatively charged ferricyanide anion at the electrode surface in this potential region. In the presence of added supporting electrolyte, screening provided by the modified compact double layer should avoid this effect. It follows that the welldefined voltammetry of high K4[Fe(CN)6] concentrations in the absence of KCl may also occur because of the increased availability of charge carriers and the resulting attainment of a relatively well-defined compact double layer. Clearly, the electrochemistry of 0.5 mM K4[Fe(CN)6] in the absence of electrolyte at the Pt macroelectrode is qualitatively different from that at GC and Au electrodes. On Pt, both the oxidative and reductive waves are broad, so that the value of ∆Ep is significantly greater than that observed on GC and Au. Thus, features associated with both reduced electron-transfer kinetics (in a diffuse double layer) and electrode blockage are indicated. However, while exhibiting some agreement with the steady-state results obtained by Beriet and Pletcher at the Pt microdisk electrode,15 our transient voltammetric responses obtained at the Pt macrodisk are still better defined than is the case under steadystate conditions at a Pt microdisk electrode. Macrodisk Electrodes: 5 and 0.5 mM K3Fe(CN)6]. As with K4[Fe(CN)6], the cyclic voltammograms obtained for K3[Fe(CN)6] in the absence of added electrolyte reveal anomalous behavior at lower concentrations. Data for 5 mM K3[Fe(CN)6] are qualitatively similar to that obtained for 5 mM K4[Fe(CN)6], but significantly different behavior is found at 0.5 mM for the two oxidation states (as shown in Figure 4). The results at the GC macroelectrode are in contrast to Lee and Anson’s results at carbon microelectrodes, where essentially no reduction wave was detected at low concentrations of K3[Fe(CN)6].14 Although a welldefined redox couple was obtained under transient conditions at GC electrodes, the response is significantly lower than that predicted on the basis of diffusion and migration. In addition, the GC response broadened and decreased with successive scans, an effect more noticeable with the Fe(II) reduction than the Fe(III) oxidation. Similar results were observed with the Au macrodisk electrode, although the voltammetric peaks are broadened and decreased far more severely than at GC. Thus, at GC and Au macroelectrodes, both diffuse double-layer effects and surface passivation are implicated in the nonideal voltammetry observed for low concentrations of K3[Fe(CN)6] in the absence of added supporting electrolyte. Consistent with results obtained for K4[Fe(CN)6] oxidation, the results for 0.5 mM K3[Fe(CN)6] reduction at the Pt electrode are most detrimentally affected by the absence of excess supporting electrolyte. On Pt electrodes, the voltammetry resulting from the initial potential scan (performed immediately after polishing) is not highly reproducible, and small reductive and oxidative peaks detected on the initial scan disappear on a second subsequent 3490 Analytical Chemistry, Vol. 72, No. 15, August 1, 2000
Figure 5. Steady-state voltammetry (v ) 20 mV s-1) of K3[Fe(CN)6] in aqueous solution at a carbon fiber microelectrode (d ) 12 µm). Solid line: in the absence of added inert electrolyte. Broken line: in the presence of 1M KCl.
scan. In contrast, at all three electrode surfaces, well-defined voltammograms were observed when 1 M KCl was added. Our voltammetric results at Pt macrodisk electrodes parallel those of Beriet and Pletcher, where no reduction wave was observed at a Pt microelectrode for K3[Fe(CN)6] concentrations of
