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Microelectrodes as probes in low electrolyte solutions: the reduction of quinone in aqueous sulfuric acid solution
Journal of Electroanalytical Chemistry, 374 (1994) 173-177
173
Microelectrodes as probes in low electrolyte solutions: the reduction of quinone in aqueous sulfuric acid solution Rebecca T. Robertson
and Bradford D. Pendley
l
Department of Chemistry, Rhodes College, 2000 North Parkway, Memphis, TN 38112 (USA)
(Received 10 September 1993; in revised form 21 December 1993)
There has been considerable interest in the use of steady-state voltammetry at microelectrodes to study electrochemical reactions in solutions to which little electrolyte has been added [l-11]. Investigations in such low electrolyte media are possible because the “ohmic” potential drop is much smaller at microelectrodes than at conventionally sized electrodes, leading to sigmoidally shaped voltammograms with minimal distortion. Several researchers have presented models that predict the steady-state voltammetric response of a microelectrode in solutions with small amounts of added electrolyte [12-161. In particular, Oldham predicted the voltammetric behavior for the oxidation of ferrocene and the reduction of quinone in aqueous sulfuric acid solution [13]. While several other researchers have subsequently tested the predictions for ferrocene [17-191, no report of the behavior of the quinone system has appeared, even though this system is significantly different from the ferrocene system in that sulfuric acid serves the dual role of electrolyte and providing one of the electroactive species. This work is pri-
l
To whom correspondence
0022-0728/94/%7.00 SSDrOO22-0728(94)03335-Z
should be addressed.
marily concerned with testing the predictions detailed in Oldham’s theory. Quinone undergoes the following reduction in aqueous sulfuric acid solution: quinone + 2H++ 2e- -
hydroquinone
(1) In Oldham’s model, this reduction occurred at a hemispherical microelectrode and was assumed to be reversible and at steady state. He considered a series of experiments in which the concentration of sulfuric acid was lowered relative to that of the quinone and reasoned that concentration polarization could result from exhaustion of either quinone or H+. As a result, the limiting current Zlim would be bipartite, i.e. it would depend on whether reaction (1) was limited by the flux of quinone or the flux of H+. Since our measurements were obtained with disk microelectrodes and were very near steady state, we shall assume that the limiting current at the disk microelectrode is equivalent to the limiting current at a hemispherical microelectrode with a radius of 2rdisk/7r [201. Given this assumption, Oldham’s equations for the limiting current become -zlim
=
12FDH+CW2S0,rclisk
if concentration of H+, or
polarization
-Zli,,, = 8FDQC’Qrdisk
(2)
resulted from exhaustion (3)
0 1994 - Elsevier Science S.A. All rights reserved
R. T. Robertson, B. D. Pendley / Reduction of quinone in aqueous H2S04
174
if it resulted from exhaustion of quinone. In these equations, F is the Faraday constant, rdisk is the radius of the disk microelectrode, CH?sO and Co are the bulk concentrations of sulfuric acid &d quinone respectively, and Do and D,+ are the diffusion coefficients for quinone and H+ respectively. We take this cathodic current as negative. The point at which quinone and H+ have equal concentration polarizations is termed the “critical composition” of the solution and occurs when -= H2SO4
2DQ
CQ
3D,+
Consequently, above the critical composition the limiting current should be given by eqn. (31, while below it the limiting current should be given by eqn. (2). This means that the magnitude of the limiting current for the reduction of quinone is predicted to change depending upon whether the system is above or below the critical composition; below the critical composition it is predicted to be independent of the concentration of quinone. 2. Experimental 2.1. Reagents Sulfuric acid (Fisher Optima) was used as received. 1,4-Benzoquinone (Aldrich, 98%) was recrystallized by dissolving a small quantity in hot petroleum ether, filtering the solution to remove the insoluble black material and allowing the resultant yellow solution to crystallize. The yellow prisms were collected and stored under nitrogen. Water was purified using a Milli-Q Plus water purification system. Buffered solutions were prepared using either pHydrion certified buffers or potassium hydrogen phthalate (Aldrich, primary standard) and sodium hydroxide, and the pH of each solution was measured using a Corning 220 pH meter with a combination electrode. Acetonitrile (Burdick and Jackson Distilled in Glass) was dried over 4 A molecular sieves. Ferrocene (Aldrich, 98%) was used as received. Tetra-n-butylammonium perchlorate (TBAP, GF Smith) was recrystallized three times from ethyl acetate and dried under vacuum. All other reagents were of at least reagent grade quality and were used without further purification. 2.2. Electrochemical measurements Platinum disk microelectrodes were prepared by the following method. A 2 cm length of nominally 2.5 pm radius Pt wire (Goodfellow, 99.9%, purchased as Wollaston wire) was attached to the end of a copper wire (for mechanical support) with silver paint. The silver
coating on the Wollaston wire was removed by dipping the wire in concentrated nitric acid and then rinsing it with water. The Pt wire was sealed inside a soft glass tube (outside diameter, 3 mm) using an oxygen gas flame. Additional electrical contact was made using a drop of mercury. The tip of the electrode was successively polished with 400 and 600 grit sandpaper to expose the Pt wire. Prior to use, the electrode was polished with 1 pm diamond paste (Buehler) and rinsed with acetone and water. Optical microscopy was used to identify electrodes with imperfect seals at the Pt [glass interface. Values of the electrode radii were calculated from the values of the steady-state limiting currents for the oxidation of 1 mM ferrocene in acetonitrile containing 0.1 M tetrabutylammonium perchlorate (TBAP) using the reported value for the diffusivity of ferrocene in acetonitrile (2.4 X 10e5 cm* s-l)
m. Electrochemical measurements were made using a conventional three-compartment cell separated by medium porosity sintered glass disks. Measurements were conducted with the electrochemical cell placed inside a Faraday cage. The solutions were deareated by bubbling prepurified nitrogen through them for about 25 min, and were then kept under a blanket of nitrogen throughout the experiment. All potentials were measured versus a saturated sodium chloride calomel electrode (SSCE) without regard to the liquid junction potential. Cyclic voltammetric measurements were performed using a Bioanalytical Systems CV-27 potentiostat and PA-1 low current module, and were recorded on a Soltec VP-6432s x-y recorder. At least four replicate scans were obtained in each solution. 3. Results and discussion Figure 1 shows a representative cyclic voltammogram obtained using a 2.4 pm radius platinum disk
b-4/-* ~~l~l~l~1111111111l1 +0.60
B
I -0.35
Potential/V vs. SSCE Fig. 1. (A) Cyclic voltammogram recorded using a 2.4 pm radius Pt microelectrode in a 10.0 mM sulfuric acid solution containing 1.2 mM quinone at a scan rate of 20 mV s-l; (B) blank.
R.T. Robertson, B.D. Pendley / Reduction of phone
in aqueous H,SO,
175
TABLE 1. Limiting currents above the critical composition Co/mm01 I- ’
C,zso~/mmol
1.1 1.2 1.1 1.0 1.0 1.1 1.1
5.00 2.00 1.00 0.500 0.500 0.100 0.100
I-’
CH2S04/CQ
- I,i,
4.5 1.7 0.91 0.50 0.50 0.091 0.091
2.02 f 0.03 2.13 f 0.02 1.98 + 0.02 2.21 f 0.02 2.16 & 0.01 2.31 f 0.03 2.64 & 0.04
(measured) ‘/nA
-In,,, (predicted) b/nA 2.1, 2.3, 2.1, 1.9, 1.9, 2.1, 2.1,
* f + + f + f
0.2, 0.3, 0.2, 0.2, 0.2, 0.2, 0.2,
a The measured limiting current is reported as the mean value + one standard deviation. b Predicted using eqn. (3). The uncertainty in the value results from the uncertainty in C, and D,.
microelectrode in a 10.0 mM sulfuric acid solution containing 1.2 mM quinone. A single well-developed sigmoidal voltammogram is observed, as expected. From the magnitude of the limiting current of the wave and using eqn. (3), we calculate that Do = 0.0, k 0.1,) x 10e5 cm2 s-l. This mean and standard deviation are based on seven replicate measurements, each performed in a 10.0 mM sulfuric acid solution containing approximately 1 mM quinone. Using this measured value of Do and a value of 9.34 x 10d5 cm2 s-l for D,+ [22], we calculate that the critical composition occurs at Cu2so,/Co = 0.075. We have recorded several voltammograms above this value of the critical composition and found that, in each case, a single well-developed sigmoidal curve is observed whose limiting current agrees with that predicted using eqn. (3). The results of these studies are given in Table 1. In addition, we observed that the reduction of quinone under these conditions is electrochemically irreversible at the platinum microelectrode, as is evidenced by the more extended voltammogram. The Tomes criterion of reversibility (E1,4 - E3,J is predicted to be equal to 28 mV for an electrochemically
reversible two-electron reduction at 25°C [23]. However, we find this value to be approximately 105 mV over a range of sulfuric acid concentrations (lo-O.10 mM) indicating that the reaction is irreversible. Oldham also predicted how the half-wave potential and Tomes criterion would change as the critical composition is transversed. However, since our results indicate that the reduction of quinone in dilute aqueous sulfuric acid solutions at platinum microelectrodes is irreversible, we are unable to make any comparison between Oldham’s predictions for these changes and our results. In contrast with the results obtained above for the critical composition, Fig. 2 shows a representative cyclic voltammogram taken below the critical composition using a 2.4 pm radius platinum disk microelectrode in a 0.0500 mM sulfuric acid solution containing 2.0 mM quinone. Instead of a single voltammetric wave, two waves are observed. The magnitude of the limiting current for the more positive wave agrees with the limiting current predicted using eqn. (2) (Table 21, except at low concentrations of sulfuric acid (i.e. 20 PM and lower) where we observe large deviations in the measured limiting current from one trial to the
TABLE 2. Limiting current for the more positive wave below the critical composition c,/mm011-’
CH2so,/mmol
1.0 2.0 1.2 5.1 2.0 2.3 1.0 1.1 1.0 5.1 5.1 1.1
0.0500 0.100 0.0500 0.200 0.0500 0.0500 0.0200 0.0200 0.0100 0.0500 0.0500 0.0100
I-’
cH,S04/cQ
- &,
0.050 0.050 0.042 0.039 0.025 0.022 0.020 0.018 0.010 0.0098 0.0098 0.0091
1.6, f 0.1, 2.7, f 0.2 a 1.44 + 0.09 5.4, + 0.4, 1.2, f 0.1, 1.3, f 0.2, 0.7, f 0.1, 0.6, + 0.1, 0.5, f 0.1, 1.2, f 0.2, 1.6, f 0.1, 0.81 + 0.09
(measured) a/nA
a The measured limiting current is reported as the mean current f the measurement the value results from the uncertainty in CHzSOI.
-In,,, (predicted) b/nA 1.30 + 0.03 2.60 k 0.06 1.30 * 0.03 5.2, + 0.1, 1.30 + 0.03 1.30 * 0.03 0.52 + 0.01 0.52 f 0.01 0.26 + 0.01 1.30 f 0.03 1.30 + 0.03 0.26 + 0.01
uncertainty. b Predicted using eqn. (2). The uncertainty in
R. T. Robertson, B. D. Pendley / Reduction of quinone in aqueous H,SO,
176
I
-500pA
II
+0.45
11 11 11 11 11
’
11 11 1 ‘I
-0.50
Potential/V vs. SSCE Fig. 3. (A) Cyclic voltammogram recorded using a 2.4 pm radius Pt microelectrode in a buffered solution (pH 5.00) containing 1.0 mM quinone at a scan rate of 20 mV s-r; (B) blank.
Potential/V vs. SSCE Fig. 2. (A) Cyclic voltammogram recorded using a 2.4 pm radius Pt microelectrode in a 0.0500 mM sulfuric acid solution containing 2.0 mM quinone at a scan rate of 10 mV s-t; (B) blank.
next. This poor reproducibility probably reflects the difficulty in reproducing these small concentrations of H+. In addition, the sum of the magnitude of the limiting current for both waves agrees with the limiting current predicted by using eqn. (3) (Table 3). It should be noted that below the critical composition, the results for similar values of the ratio C, so,/Co are the same as those predicted using eqns. f2) or (3), even though the values of the quinone and sulfuric acid concentration vary considerably. These results suggest that the more positive wave is due to the reduction of quinone as illustrated in reaction (1). As the quinone is reduced, H+ is depleted at the electrode surface until reaction (1) ceases. Below
the critical composition, the solution is unable to replenish the H+ consumed and consequently the magnitude of the limiting current depends on the concentration of sulfuric acid as predicted by Oldham. To support this proposal further, Fig. 3 shows a representative cyclic voltammogram taken below the critical composition using a 2.4 pm radius platinum disk microelectrode in a buffered solution (pH 5.00) containing 1.0 mM quinone. A single voltammetric wave is observed. In this instance, the concentration of H+ at the electrode surface is maintained at a pH of 5.00 by the action of the buffer, and so a single voltammetric wave whose limiting current is given by eqn. (3) is obtained. Similar voltammograms were obtained below the critical composition in buffered solutions at pH 6.38 and pH 7.00. The fact that the sum of the magnitude of the limiting currents for both waves is predicted by eqn. (3) strongly suggests that the more negative wave is also due to the two-electron reduction of quinone. Since the solution near the electrode surface has been de-
TABLE 3. Limiting current for the sum of the two waves below the critical composition Co/mm01 1-l
Crr,,o,/mmol
1.0 2.0 1.2 5.1 2.0 2.3 1.0 1.1 1.0 5.1 5.1 1.1
0.0500 0.100 0.0508 0.200 0.0500 0.0500 0.0208 0.0208 0.0108 0.0500 0.0500 0.0106
1-l
CHZS04/CQ
-Sum
0.050 0.050 0.042 0.039 0.025 0.022 0.020 0.018 0.010 0.0098 0.0098 0.0091
1.86 f 4.01 f 2.12 + 8.93 + 3.44 + 3.9, f 1.82 f 2.12 f 2.26 f 8.85 f 7.92 f 2.25 f
far,, (meas) a/nA 0.01 0.02 0.04 0.05 0.03 0.1, 0.02 0.02 0.03 0.05 0.06 0.04
-Sum 1.9, 3.9, 2.3, 9.9 3.9, 4.5, 1.9, 2.1, 1.9, 9.0, 9.0, 2.1,
In,,, (pred) b/nA
f 0.2, f 0.4, f 0.3, + l., f 0.4, f 0.5, f 0.2, f 0.2, f 0.2, + 1.1 * l., f 0.2,
a The measured limiting current is reported as the mean current f one standard deviation. b Predicted using eqn. (3). The uncertainty in the value results from the uncertainty in C, and D,.
R. T. Robertson, B.D. Pet&y
/ Reduction of quinone in aqueous H2S04
pleted of H+ due to reaction (0, the quinone must now obtain protons from either the hydroquinone formed in reaction (1) and/or the water. Reaction with the hydroquinone appears more likely since it is a stronger acid than water. This type of reaction was not considered in Oldham’s theory but can occur since the quinone is not exhausted at the electrode surface. In conclusion, we find that the reduction of quinone in dilute aqueous solutions of sulfuric acid above the critical composition is in excellent agreement with the predictions of Oldham, but below the critical composition agreement is good only for the more positive wave. Thus the concept of concentration polarization resulting from depletion of either of two electroactive species and leading to a bipartite current has been verified. However, we observed the subsequent reduction of quinone after depletion of H+, an aspect not addressed in Oldham’s theoretical analysis. Acknowledgments
This research was supported by a Faculty Development Grant through Rhodes College. The authors wish to thank Dr. Robert G. Mortimer for helpful discussions, and Dr. Fred C. Anson for a helpful comment made during a presentation of this work at the 184th National Meeting of The Electrochemical Society. References 1 A.M. Bond, M. Fleischmann and J. Robinson, J. Electroanal. Chem., 168 (1984) 299. 2 J.O. Howell and R.M. Wightman, Anal. Chem., 56 (1984) 524.
3 L. Geng, A.G. Ewing, J.C. Jernigan
177
and R.W. Murray, Anal. Chem., 58 (1986) 852. 4 T. Dibble, S. Bandyopadhyay, J. Ghoroghchian, J.J. Smith, F. Sarfarazi, M. Fleischmann and S. Pons, J. Phys. Chem., 90 (1986) 5275. 5 A.M. Bond and P.A. Lay, J. Electroanal. Chem., 199 (1986) 285. 6 M.J. Peiia, M. Fleischmann and N. Garrard, J. Electroanal. Chem., 220 (1987) 31. 7 M. Ciszkowska, Z. Stojek and J. Osteryoung, Anal. Chem., 62 (1990) 349. 8 G.N. Kamau and J.F. Rusling, J. Electroanal. Chem., 292 (1990) 187. 9 J.D. Norton, W.E. Benson, H.S. White, B.D. Pendley and H.D. Abrufia, Anal. Chem., 63 (1991) 1909. 10 C. Lee and F.C. Anson, J. Electroanal. Chem., 323 (1992) 381. 11 J.D. Norton and H.S. White, J. Electroanal. Chem., 325 (1992) 341. 12 A.M. Bond, M. Fleischmann and J. Robinson, J. Electroanal. Chem., 172 (1984) 11. 13 K.B. Oldham, J. Electroanal. Chem., 250 (1988) 1. 14 R.B. Morris, K.F. Fisher and H.S. White, J. Phys. Chem., 92 (1988) 5306. 15 J.D. Norton, H.S. White and SW. Feldberg, J. Phys. Chem., 94 (1990) 6772. 16 K.B. Oldham, J. Electroanal. Chem., 337 (1992) 91. 17 B.D. Pendley, H.D. Abruiia, J.D. Norton, W.E. Benson and H.S. White, Anal. Chem., 63 (1991) 2766. 18 J.B. Cooper and A.M. Bond, J. Electroanal. Chem., 315 (1991) 143. 19 SM. Drew, R.M. Wightman and CA. Amatore, J. Electroanal. Chem., 317 (1991) 117. 20 A.M. Bond, K.B. Oldham and C.G. Zoski, J. Electroanal. Chem., 216 (1988) 177. 21 T. Kuwana, D.E. Bublitz and D.E. Hoh, J. Am. Chem. Sot., 82 (1960) 5811. 22 I.M. Kolthoff and J.J. Lingane, Polarography (2nd edn.), Vol. 1, Interscience, New York, 1952, p. 52. 23 A.J. Bard and L.R. Faulkner, Electrochemical Methods, Wiley, New York, 1980, p. 160.
173
Microelectrodes as probes in low electrolyte solutions: the reduction of quinone in aqueous sulfuric acid solution Rebecca T. Robertson
and Bradford D. Pendley
l
Department of Chemistry, Rhodes College, 2000 North Parkway, Memphis, TN 38112 (USA)
(Received 10 September 1993; in revised form 21 December 1993)
There has been considerable interest in the use of steady-state voltammetry at microelectrodes to study electrochemical reactions in solutions to which little electrolyte has been added [l-11]. Investigations in such low electrolyte media are possible because the “ohmic” potential drop is much smaller at microelectrodes than at conventionally sized electrodes, leading to sigmoidally shaped voltammograms with minimal distortion. Several researchers have presented models that predict the steady-state voltammetric response of a microelectrode in solutions with small amounts of added electrolyte [12-161. In particular, Oldham predicted the voltammetric behavior for the oxidation of ferrocene and the reduction of quinone in aqueous sulfuric acid solution [13]. While several other researchers have subsequently tested the predictions for ferrocene [17-191, no report of the behavior of the quinone system has appeared, even though this system is significantly different from the ferrocene system in that sulfuric acid serves the dual role of electrolyte and providing one of the electroactive species. This work is pri-
l
To whom correspondence
0022-0728/94/%7.00 SSDrOO22-0728(94)03335-Z
should be addressed.
marily concerned with testing the predictions detailed in Oldham’s theory. Quinone undergoes the following reduction in aqueous sulfuric acid solution: quinone + 2H++ 2e- -
hydroquinone
(1) In Oldham’s model, this reduction occurred at a hemispherical microelectrode and was assumed to be reversible and at steady state. He considered a series of experiments in which the concentration of sulfuric acid was lowered relative to that of the quinone and reasoned that concentration polarization could result from exhaustion of either quinone or H+. As a result, the limiting current Zlim would be bipartite, i.e. it would depend on whether reaction (1) was limited by the flux of quinone or the flux of H+. Since our measurements were obtained with disk microelectrodes and were very near steady state, we shall assume that the limiting current at the disk microelectrode is equivalent to the limiting current at a hemispherical microelectrode with a radius of 2rdisk/7r [201. Given this assumption, Oldham’s equations for the limiting current become -zlim
=
12FDH+CW2S0,rclisk
if concentration of H+, or
polarization
-Zli,,, = 8FDQC’Qrdisk
(2)
resulted from exhaustion (3)
0 1994 - Elsevier Science S.A. All rights reserved
R. T. Robertson, B. D. Pendley / Reduction of quinone in aqueous H2S04
174
if it resulted from exhaustion of quinone. In these equations, F is the Faraday constant, rdisk is the radius of the disk microelectrode, CH?sO and Co are the bulk concentrations of sulfuric acid &d quinone respectively, and Do and D,+ are the diffusion coefficients for quinone and H+ respectively. We take this cathodic current as negative. The point at which quinone and H+ have equal concentration polarizations is termed the “critical composition” of the solution and occurs when -= H2SO4
2DQ
CQ
3D,+
Consequently, above the critical composition the limiting current should be given by eqn. (31, while below it the limiting current should be given by eqn. (2). This means that the magnitude of the limiting current for the reduction of quinone is predicted to change depending upon whether the system is above or below the critical composition; below the critical composition it is predicted to be independent of the concentration of quinone. 2. Experimental 2.1. Reagents Sulfuric acid (Fisher Optima) was used as received. 1,4-Benzoquinone (Aldrich, 98%) was recrystallized by dissolving a small quantity in hot petroleum ether, filtering the solution to remove the insoluble black material and allowing the resultant yellow solution to crystallize. The yellow prisms were collected and stored under nitrogen. Water was purified using a Milli-Q Plus water purification system. Buffered solutions were prepared using either pHydrion certified buffers or potassium hydrogen phthalate (Aldrich, primary standard) and sodium hydroxide, and the pH of each solution was measured using a Corning 220 pH meter with a combination electrode. Acetonitrile (Burdick and Jackson Distilled in Glass) was dried over 4 A molecular sieves. Ferrocene (Aldrich, 98%) was used as received. Tetra-n-butylammonium perchlorate (TBAP, GF Smith) was recrystallized three times from ethyl acetate and dried under vacuum. All other reagents were of at least reagent grade quality and were used without further purification. 2.2. Electrochemical measurements Platinum disk microelectrodes were prepared by the following method. A 2 cm length of nominally 2.5 pm radius Pt wire (Goodfellow, 99.9%, purchased as Wollaston wire) was attached to the end of a copper wire (for mechanical support) with silver paint. The silver
coating on the Wollaston wire was removed by dipping the wire in concentrated nitric acid and then rinsing it with water. The Pt wire was sealed inside a soft glass tube (outside diameter, 3 mm) using an oxygen gas flame. Additional electrical contact was made using a drop of mercury. The tip of the electrode was successively polished with 400 and 600 grit sandpaper to expose the Pt wire. Prior to use, the electrode was polished with 1 pm diamond paste (Buehler) and rinsed with acetone and water. Optical microscopy was used to identify electrodes with imperfect seals at the Pt [glass interface. Values of the electrode radii were calculated from the values of the steady-state limiting currents for the oxidation of 1 mM ferrocene in acetonitrile containing 0.1 M tetrabutylammonium perchlorate (TBAP) using the reported value for the diffusivity of ferrocene in acetonitrile (2.4 X 10e5 cm* s-l)
m. Electrochemical measurements were made using a conventional three-compartment cell separated by medium porosity sintered glass disks. Measurements were conducted with the electrochemical cell placed inside a Faraday cage. The solutions were deareated by bubbling prepurified nitrogen through them for about 25 min, and were then kept under a blanket of nitrogen throughout the experiment. All potentials were measured versus a saturated sodium chloride calomel electrode (SSCE) without regard to the liquid junction potential. Cyclic voltammetric measurements were performed using a Bioanalytical Systems CV-27 potentiostat and PA-1 low current module, and were recorded on a Soltec VP-6432s x-y recorder. At least four replicate scans were obtained in each solution. 3. Results and discussion Figure 1 shows a representative cyclic voltammogram obtained using a 2.4 pm radius platinum disk
b-4/-* ~~l~l~l~1111111111l1 +0.60
B
I -0.35
Potential/V vs. SSCE Fig. 1. (A) Cyclic voltammogram recorded using a 2.4 pm radius Pt microelectrode in a 10.0 mM sulfuric acid solution containing 1.2 mM quinone at a scan rate of 20 mV s-l; (B) blank.
R.T. Robertson, B.D. Pendley / Reduction of phone
in aqueous H,SO,
175
TABLE 1. Limiting currents above the critical composition Co/mm01 I- ’
C,zso~/mmol
1.1 1.2 1.1 1.0 1.0 1.1 1.1
5.00 2.00 1.00 0.500 0.500 0.100 0.100
I-’
CH2S04/CQ
- I,i,
4.5 1.7 0.91 0.50 0.50 0.091 0.091
2.02 f 0.03 2.13 f 0.02 1.98 + 0.02 2.21 f 0.02 2.16 & 0.01 2.31 f 0.03 2.64 & 0.04
(measured) ‘/nA
-In,,, (predicted) b/nA 2.1, 2.3, 2.1, 1.9, 1.9, 2.1, 2.1,
* f + + f + f
0.2, 0.3, 0.2, 0.2, 0.2, 0.2, 0.2,
a The measured limiting current is reported as the mean value + one standard deviation. b Predicted using eqn. (3). The uncertainty in the value results from the uncertainty in C, and D,.
microelectrode in a 10.0 mM sulfuric acid solution containing 1.2 mM quinone. A single well-developed sigmoidal voltammogram is observed, as expected. From the magnitude of the limiting current of the wave and using eqn. (3), we calculate that Do = 0.0, k 0.1,) x 10e5 cm2 s-l. This mean and standard deviation are based on seven replicate measurements, each performed in a 10.0 mM sulfuric acid solution containing approximately 1 mM quinone. Using this measured value of Do and a value of 9.34 x 10d5 cm2 s-l for D,+ [22], we calculate that the critical composition occurs at Cu2so,/Co = 0.075. We have recorded several voltammograms above this value of the critical composition and found that, in each case, a single well-developed sigmoidal curve is observed whose limiting current agrees with that predicted using eqn. (3). The results of these studies are given in Table 1. In addition, we observed that the reduction of quinone under these conditions is electrochemically irreversible at the platinum microelectrode, as is evidenced by the more extended voltammogram. The Tomes criterion of reversibility (E1,4 - E3,J is predicted to be equal to 28 mV for an electrochemically
reversible two-electron reduction at 25°C [23]. However, we find this value to be approximately 105 mV over a range of sulfuric acid concentrations (lo-O.10 mM) indicating that the reaction is irreversible. Oldham also predicted how the half-wave potential and Tomes criterion would change as the critical composition is transversed. However, since our results indicate that the reduction of quinone in dilute aqueous sulfuric acid solutions at platinum microelectrodes is irreversible, we are unable to make any comparison between Oldham’s predictions for these changes and our results. In contrast with the results obtained above for the critical composition, Fig. 2 shows a representative cyclic voltammogram taken below the critical composition using a 2.4 pm radius platinum disk microelectrode in a 0.0500 mM sulfuric acid solution containing 2.0 mM quinone. Instead of a single voltammetric wave, two waves are observed. The magnitude of the limiting current for the more positive wave agrees with the limiting current predicted using eqn. (2) (Table 21, except at low concentrations of sulfuric acid (i.e. 20 PM and lower) where we observe large deviations in the measured limiting current from one trial to the
TABLE 2. Limiting current for the more positive wave below the critical composition c,/mm011-’
CH2so,/mmol
1.0 2.0 1.2 5.1 2.0 2.3 1.0 1.1 1.0 5.1 5.1 1.1
0.0500 0.100 0.0500 0.200 0.0500 0.0500 0.0200 0.0200 0.0100 0.0500 0.0500 0.0100
I-’
cH,S04/cQ
- &,
0.050 0.050 0.042 0.039 0.025 0.022 0.020 0.018 0.010 0.0098 0.0098 0.0091
1.6, f 0.1, 2.7, f 0.2 a 1.44 + 0.09 5.4, + 0.4, 1.2, f 0.1, 1.3, f 0.2, 0.7, f 0.1, 0.6, + 0.1, 0.5, f 0.1, 1.2, f 0.2, 1.6, f 0.1, 0.81 + 0.09
(measured) a/nA
a The measured limiting current is reported as the mean current f the measurement the value results from the uncertainty in CHzSOI.
-In,,, (predicted) b/nA 1.30 + 0.03 2.60 k 0.06 1.30 * 0.03 5.2, + 0.1, 1.30 + 0.03 1.30 * 0.03 0.52 + 0.01 0.52 f 0.01 0.26 + 0.01 1.30 f 0.03 1.30 + 0.03 0.26 + 0.01
uncertainty. b Predicted using eqn. (2). The uncertainty in
R. T. Robertson, B. D. Pendley / Reduction of quinone in aqueous H,SO,
176
I
-500pA
II
+0.45
11 11 11 11 11
’
11 11 1 ‘I
-0.50
Potential/V vs. SSCE Fig. 3. (A) Cyclic voltammogram recorded using a 2.4 pm radius Pt microelectrode in a buffered solution (pH 5.00) containing 1.0 mM quinone at a scan rate of 20 mV s-r; (B) blank.
Potential/V vs. SSCE Fig. 2. (A) Cyclic voltammogram recorded using a 2.4 pm radius Pt microelectrode in a 0.0500 mM sulfuric acid solution containing 2.0 mM quinone at a scan rate of 10 mV s-t; (B) blank.
next. This poor reproducibility probably reflects the difficulty in reproducing these small concentrations of H+. In addition, the sum of the magnitude of the limiting current for both waves agrees with the limiting current predicted by using eqn. (3) (Table 3). It should be noted that below the critical composition, the results for similar values of the ratio C, so,/Co are the same as those predicted using eqns. f2) or (3), even though the values of the quinone and sulfuric acid concentration vary considerably. These results suggest that the more positive wave is due to the reduction of quinone as illustrated in reaction (1). As the quinone is reduced, H+ is depleted at the electrode surface until reaction (1) ceases. Below
the critical composition, the solution is unable to replenish the H+ consumed and consequently the magnitude of the limiting current depends on the concentration of sulfuric acid as predicted by Oldham. To support this proposal further, Fig. 3 shows a representative cyclic voltammogram taken below the critical composition using a 2.4 pm radius platinum disk microelectrode in a buffered solution (pH 5.00) containing 1.0 mM quinone. A single voltammetric wave is observed. In this instance, the concentration of H+ at the electrode surface is maintained at a pH of 5.00 by the action of the buffer, and so a single voltammetric wave whose limiting current is given by eqn. (3) is obtained. Similar voltammograms were obtained below the critical composition in buffered solutions at pH 6.38 and pH 7.00. The fact that the sum of the magnitude of the limiting currents for both waves is predicted by eqn. (3) strongly suggests that the more negative wave is also due to the two-electron reduction of quinone. Since the solution near the electrode surface has been de-
TABLE 3. Limiting current for the sum of the two waves below the critical composition Co/mm01 1-l
Crr,,o,/mmol
1.0 2.0 1.2 5.1 2.0 2.3 1.0 1.1 1.0 5.1 5.1 1.1
0.0500 0.100 0.0508 0.200 0.0500 0.0500 0.0208 0.0208 0.0108 0.0500 0.0500 0.0106
1-l
CHZS04/CQ
-Sum
0.050 0.050 0.042 0.039 0.025 0.022 0.020 0.018 0.010 0.0098 0.0098 0.0091
1.86 f 4.01 f 2.12 + 8.93 + 3.44 + 3.9, f 1.82 f 2.12 f 2.26 f 8.85 f 7.92 f 2.25 f
far,, (meas) a/nA 0.01 0.02 0.04 0.05 0.03 0.1, 0.02 0.02 0.03 0.05 0.06 0.04
-Sum 1.9, 3.9, 2.3, 9.9 3.9, 4.5, 1.9, 2.1, 1.9, 9.0, 9.0, 2.1,
In,,, (pred) b/nA
f 0.2, f 0.4, f 0.3, + l., f 0.4, f 0.5, f 0.2, f 0.2, f 0.2, + 1.1 * l., f 0.2,
a The measured limiting current is reported as the mean current f one standard deviation. b Predicted using eqn. (3). The uncertainty in the value results from the uncertainty in C, and D,.
R. T. Robertson, B.D. Pet&y
/ Reduction of quinone in aqueous H2S04
pleted of H+ due to reaction (0, the quinone must now obtain protons from either the hydroquinone formed in reaction (1) and/or the water. Reaction with the hydroquinone appears more likely since it is a stronger acid than water. This type of reaction was not considered in Oldham’s theory but can occur since the quinone is not exhausted at the electrode surface. In conclusion, we find that the reduction of quinone in dilute aqueous solutions of sulfuric acid above the critical composition is in excellent agreement with the predictions of Oldham, but below the critical composition agreement is good only for the more positive wave. Thus the concept of concentration polarization resulting from depletion of either of two electroactive species and leading to a bipartite current has been verified. However, we observed the subsequent reduction of quinone after depletion of H+, an aspect not addressed in Oldham’s theoretical analysis. Acknowledgments
This research was supported by a Faculty Development Grant through Rhodes College. The authors wish to thank Dr. Robert G. Mortimer for helpful discussions, and Dr. Fred C. Anson for a helpful comment made during a presentation of this work at the 184th National Meeting of The Electrochemical Society. References 1 A.M. Bond, M. Fleischmann and J. Robinson, J. Electroanal. Chem., 168 (1984) 299. 2 J.O. Howell and R.M. Wightman, Anal. Chem., 56 (1984) 524.
3 L. Geng, A.G. Ewing, J.C. Jernigan
177
and R.W. Murray, Anal. Chem., 58 (1986) 852. 4 T. Dibble, S. Bandyopadhyay, J. Ghoroghchian, J.J. Smith, F. Sarfarazi, M. Fleischmann and S. Pons, J. Phys. Chem., 90 (1986) 5275. 5 A.M. Bond and P.A. Lay, J. Electroanal. Chem., 199 (1986) 285. 6 M.J. Peiia, M. Fleischmann and N. Garrard, J. Electroanal. Chem., 220 (1987) 31. 7 M. Ciszkowska, Z. Stojek and J. Osteryoung, Anal. Chem., 62 (1990) 349. 8 G.N. Kamau and J.F. Rusling, J. Electroanal. Chem., 292 (1990) 187. 9 J.D. Norton, W.E. Benson, H.S. White, B.D. Pendley and H.D. Abrufia, Anal. Chem., 63 (1991) 1909. 10 C. Lee and F.C. Anson, J. Electroanal. Chem., 323 (1992) 381. 11 J.D. Norton and H.S. White, J. Electroanal. Chem., 325 (1992) 341. 12 A.M. Bond, M. Fleischmann and J. Robinson, J. Electroanal. Chem., 172 (1984) 11. 13 K.B. Oldham, J. Electroanal. Chem., 250 (1988) 1. 14 R.B. Morris, K.F. Fisher and H.S. White, J. Phys. Chem., 92 (1988) 5306. 15 J.D. Norton, H.S. White and SW. Feldberg, J. Phys. Chem., 94 (1990) 6772. 16 K.B. Oldham, J. Electroanal. Chem., 337 (1992) 91. 17 B.D. Pendley, H.D. Abruiia, J.D. Norton, W.E. Benson and H.S. White, Anal. Chem., 63 (1991) 2766. 18 J.B. Cooper and A.M. Bond, J. Electroanal. Chem., 315 (1991) 143. 19 SM. Drew, R.M. Wightman and CA. Amatore, J. Electroanal. Chem., 317 (1991) 117. 20 A.M. Bond, K.B. Oldham and C.G. Zoski, J. Electroanal. Chem., 216 (1988) 177. 21 T. Kuwana, D.E. Bublitz and D.E. Hoh, J. Am. Chem. Sot., 82 (1960) 5811. 22 I.M. Kolthoff and J.J. Lingane, Polarography (2nd edn.), Vol. 1, Interscience, New York, 1952, p. 52. 23 A.J. Bard and L.R. Faulkner, Electrochemical Methods, Wiley, New York, 1980, p. 160.