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O2− redox couple at Hg electrode in the presence of PVC in aprotic media
Electrochimica Acta 50 (2005) 4746–4751
Electrode kinetics of the O2/O2− redox couple at Hg electrode in the presence of PVC in aprotic media Madhu Sudan Saha a,b,1 , Takeo Ohsaka b,∗ b
a Department of Chemistry and Chemical Biology, Northeastern University, 360 Huntington Avenue, Boston, MA 02115, USA Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan
Received 2 December 2004; received in revised form 15 February 2005; accepted 19 February 2005 Available online 1 April 2005
Abstract The kinetic and thermodynamic parameters of the O2 /O2 − redox couple at a mercury electrode in various aprotic solvents have been evaluated by normal pulse polarography and cyclic voltammetry in the presence of poly(vinyl chloride) (PVC) as a maximum suppressor. The polarographic maxima were observed on the rising portion of polarogram for O2 reduction, but they are completely suppressed by the addition of a small amount of PVC. The adsorption behavior of PVC on a hanging mercury drop electrode is examined based on the measurement of the differential capacitance of the electrical double layer. The relevant kinetic and thermodynamic parameters, i.e., the standard rate constant, k◦ , the cathodic transfer coefficient, αc , and the formal potential, E◦ of the O2 /O2 − redox couple were estimated together with the diffusion coefficients of O2 , DO2 . An excellent linear relationship between the formal potential and solvent’s acceptor number was found. © 2005 Elsevier Ltd. All rights reserved. Keywords: Superoxide ion; Hg electrode; Aprotic solvent; Normal pulse polarography; Differential capacitance
1. Introduction In the course of a series of recent studies regarding the current oscillation during the redox reaction of the O2 /O2 − (superoxide ion) redox couple at a mercury (Hg) electrode in usual aprotic solvents [1–5] such as N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO), pyridine (Py) and acetone, we have found that the current oscillation completely disappears in the presence of a small amount of poly(vinyl chloride) (PVC) and normal well-defined polarograms without polarographic maxima can be obtained. This allowed us to examine the electrode kinetics of the O2 /O2 − redox couple at the Hg electrode quantitatively.
∗
Corresponding author. Tel.: +81 45 9245404; fax: +81 45 9245489. E-mail addresses: [email protected] (M.S. Saha), [email protected] (T. Ohsaka). 1 Tel.:+1 6173735630; fax: +1 6173738949. 0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.02.022
The electrochemical reduction of oxygen to superoxide ion in aprotic solvents has been studied since 1965 [6–10] and the kinetic data of the O2 /O2 − electrode reaction in aprotic solvents have been reported by some groups [11–19], but the results lack of consistence even for those obtained under same experimental conditions. In addition, the solvent effect on the electrode reaction has not been reported. In this paper, we report the quantitative investigation the electroreduction of O2 to O2 − at the Hg electrode in various aprotic solvents using normal pulse polarography (NPP) which is suitable for the kinetic studies of the electrode reactions of the redox species dissolved in solution [20,21]. The kinetic and thermodynamic parameters (i.e., the standard rate constant, k◦ , the cathodic transfer coefficient, αc and the formal potential, E◦ ) of the O2 /O2 − redox couple at the Hg electrode were evaluated in the presence of PVC as a maximum suppressor. The differential capacitances of the electrical double layer were also measured for studying adsorption of PVC on the Hg electrode.
M.S. Saha, T. Ohsaka / Electrochimica Acta 50 (2005) 4746–4751
2. Experimental 2.1. Reagents The aprotic solvents, N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO), pyridine (Py), N,N-dimethylacetamide (DMA), and N-methyl-2-pyrrolidone (NMP), used in electrochemical experiments were Analar Grade (Kanto Chemical Co. Ltd.) and dried over molecular sieves 4A1/16 (Wako Pure Chemical Industries) overnight prior to use. All the solutions contained tetraethylammonium perchlorate (TEAP) of reagent grade quality as supporting electrolyte (Tokyo Kasei Co. Ltd.) dried in a vacuum desiccator at 25 ◦ C (at least for 3 days) and stored in a desiccator. Poly(vinyl chloride) (PVC), used as a maximum suppressor, was obtained from Acros Organics Co. The average molecular mass was 1.5 to 2.0 × 105 . Mercury (commercial grade) was washed several times with dilute nitric acid and distilled water and twice distilled under reduced pressure. 2.2. Apparatus and procedures Electrochemical measurements were performed in a conventional two-compartment three-electrode cell at laboratory temperature (25 ± 1 ◦ C). A hanging mercury drop electrode (HMDE; Model 303A, EG&G Princeton Applied Research, area: 0.010 cm2 ) and a static mercury drop electrode (SMDE; Model CGME 900, Bioanalytical System Ins., area: 0.010 cm2 ) were used as working electrodes. The working electrode and counter electrode (a platinum wire) were separated by a porous glass. The reference electrode (Ag|AgCl, NaCl saturated) was connected to the electrochemical cell via an intermediate vessel filled with the supporting electrolyte solution of each aprotic solvent under investigation. Cyclic voltammetry was carried out using a computercontrolled electrochemical system (BAS 100B/W). Normal pulse polarographic measurements were performed with the SMDE working electrode using a computer-controlled electrochemical measuring system (BAS CV-50 W). Normal pulse polarograms for the O2 -saturated sample solutions were corrected for the residual current by subtracting the corresponding polarogram obtained in a deoxygenated solution of supporting electrolyte. The differential double layer capacitance at Hg electrode was measured with an ac series bridge using a computercontrolled electrochemical measuring systems (HECS-311B potentiostat and HECS-322B two-phase lock-in amplifier, Huso Electrochemical system, or SI 1287 electrochemical interface and SI 1260 impedance/gain-phase analyzer, Solartron). Phase sensitive ac voltammograms were measured with a phase angle adjusted to 90◦ , corresponding to the out-of-phase component of the ac current (capacitive current component). The amplitude and frequency of ac voltage applied were 10 mV peak-to-peak and 1 kHz, respectively.
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The point-by-point measurements are made with 50 mV separation with each measurement requiring approximately 2–3 min. Ferrocene (Fc) was used as an internal standard to measure formal potential, E◦ versus Fc◦ /Fc+ . The potential of the reference electrode was found to be constant through the cyclic voltammograms of Fc◦ /Fc+ redox couple obtained by the addition of a suitable amount of ferrocene to the examined solution at the end of each electrochemical measurement. For the measurements in O2 -saturated media, O2 gas (99.98%) was bubbled directly into the cell in order to obtain a saturated solution and during the measurement O2 gas was flushed over the cell solution. The cell solution was deareated by bubbling N2 gas at least 30 min before the blank experiments.
3. Results and discussion 3.1. Differential capacitance Fig. 1 shows the differential capacitance of the double layer, Cdl , at HMDE as a function of potential for different concentrations of TEAP in DMF solution. The potential scan was performed from positive to negative potential (from 0 to −1.6 V versus Ag|AgCl). The measured capacitive ac component is proportional to the differential double layer capacitance in the potential region where no Faradaic process occurs. As can be seen from Fig. 1, a noticeable minimum in the capacitance was observed at the potential of zero-charge (pzc) of ca. −0.3 V versus Ag|AgCl with decreasing the concentration of TEAP, as expected from the Gony–Chapman–Stern model as a series network of Helmholtz-layer and diffuselayer capacitances. It should be noted that the shape of the capacitance curves obtained in this study are practically similar to those reported previously [22].
Fig. 1. Differential capacitance of the double layer at HMDE in DMF solution of TEAP as a function of potential. Concentration of TEAP: (䊉) 0.1; () 0.05; () 0.01 M. Frequency: 1 kHz. Vertical arrows indicate the potential of zero charge.
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M.S. Saha, T. Ohsaka / Electrochimica Acta 50 (2005) 4746–4751
Fig. 2. Differential capacitance of the double layer at HMDE in DMF solution containing 0.1 M TEAP in the absence (dashed line) and in the presence of different concentrations of PVC: () 0.001%; () 0.005%; (䊉) 0.01%; ( ) 0.04%.
Fig. 2 shows the differential capacitance-potential curves at HMDE in Ar-saturated DMF solutions containing 0.1 M TEAP in the presence of various concentrations of PVC. The capacitance-potential curves depend strongly on the concentration of the PVC added. At potentials close to the pzc the differential capacitance in the presence of PVC is smaller than that in its absence and it decreases with increasing the concentration of PVC. This is due to the adsorption of PVC molecule, which is less polar than DMF solvent molecule. During the adsorption, DMF solvent molecules in the compact layer are replaced by the adsorbed PVC molecules. As the potential is moved away from the pzc, the PVC desorbs gradually and at potentials more negative than ca. −1.4 V, the differential capacitance is not affected by the presence of PVC, because the PVC molecules desorb completely. The strong electric field leads to the replacement of the less polar PVC molecules by polar DMF solvent molecules. This phenomenon is similar to that observed by Miller for unionized polymethacrylic acid on the Hg/water interface [23] as well as the potential-dependent adsorption of electroneutral molecules that are less polar than the solvent [24].
Fig. 3. Typical normal pulse polarograms for the reduction of O2 to O2 − at SMDE in DMF solution containing 0.2 M TEAP. Concentration of PVC: (a) 0%; (b) 0.002%; (c) 0.01%; (d) 0.02%; (e) 0.04%. Sampling time: 67.5 ms.
0.002 to 0.01% (w/v), PVC acted as an effective suppressor and the limiting current was almost the same. At concentration lower than 0.002%, no effect of PVC as suppressor was observed. At concentrations higher than 0.01%, the limiting current was seen to decrease (curves d and e). Similar maxima were also observed during the reduction of O2 at SMDE in the other aprotic solvents. In all the cases, the maxima were suppressed completely in the presence of a very low concentration of PVC (in this case 0.005%). PVC was also reported to function as a maximum suppressor for the reduction of Hg(I)I2 , Hg(II)I3 − , Tl+ , Pb2+ , Cd2+ and Zn2+ in DMF solution [25,26]. Fig. 4 shows the typical normal pulse polarograms for O2 reduction in DMF solution in the presence of 0.005% PVC. The plots of the limiting current (ilim ) of these polarograms against the inverse square root of sampling time (τ s ) gave the straight line which passes through the origin as expected from the Cottrell equation indicating that the pro-
3.2. Normal pulse polarography The suppressor effect of PVC on the polarographic maxima obtained for the reduction of O2 to O2 − at SMDE in DMF solution is shown in Fig. 3. It is well known that in aprotic media, dioxygen is reduced in a reversible or quasireversible one-electron process to produce a stable anion radical, superoxide ion (O2 − ), at various electrode materials. As shown in Fig. 3, a maximum was observed on the diffusion current plateau (curve a) that makes accurate evaluation of the limiting current impossible. But the polarographic maxima were suppressed and normal well-defined polarograms were obtained (curves b–e) by the addition of a small amount of PVC in the test solution. In the concentration range from
Fig. 4. Normal pulse polarograms for O2 reduction at SMDE in DMF solution containing 0.2 M TEAP in the presence of 0.005% PVC. Sampling times: (1) 67.5 ms; (2) 47.5 ms; (3) 32.5 ms; (4) 14.5 ms; (5) 4.5 ms.
M.S. Saha, T. Ohsaka / Electrochimica Acta 50 (2005) 4746–4751
Fig. 5. Representative plots of
χ
1.75+χ2 (1+exp ζ)2 1−χ(1+exp χ)
1/2 vs. E for the
normal pulse polarograms shown in Fig. 4. Sampling times are indicated on each straight line.
cess is diffusion-controlled [20]. Another information is that the half-wave potential of the voltammograms shifted to the negative direction of potential with decreasing τ s . Thus, the kinetic parameters of the electrode reaction of the O2 /O2 − redox couple were evaluated from an analysis of the rising part of the current-potential curves (Fig. 4), associated with the shifts of half-wave potential, using the following equation [20,21]. 1.75+χ2 (1+expζ)2 1/2 2.303RT (1) E = E∗ − log χ αc nF 1−χ(1+expζ) with r E∗ = E1/2 +
χ=
2.303RT log αc nF
√ 4 k ◦ τs √ √ , D 3
i , (id )Cott
∗ 1/2 (id )Cott = nFACO DO (πτs )−1/2 , 2 2
(2) (3) (4)
nF r (E − E1/2 ), RT
(5)
D = (DO2 )1−αc (DO2 − )αc
(6)
ζ=
where E is the electrode potential, i the normal pulse voltammetric current, (id )Cott the diffusion current expressed by Cottrell equation, DO2 and DO2 − are the diffusion coefficients of O2 and O2 − , respectively and other symbols having their usual meanings. Fig. 5 shows the modified log plots of the normal pulse polarograms in which the logarithm of the second term of the right-hand side of Eq. (1) is plotted against E for the reduction of O2 . These plots gave straight lines, the slopes of which were constant at the different sampling times for each solution, within the experimental errors. The intercepts (E* ) shifted to more negative potential from the formal potential
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(E◦ ) with decreasing τ s as seen from Eq. (2). Thus, from the slopes of the straight lines shown in Fig. 5 and the intercept of these lines with the X-axis, the values of k◦ and αc were estimated by assuming that DO2 ≈ DO2 − and using the r , D and τ . Here, with the assumption known values of E1/2 O2 s r is equal to E◦ which is estimated as that DO2 ≈ DO2 − , E1/2 the average of the anodic and cathodic peak potentials of the cyclic voltammogram for the O2 /O2 − redox couple (mentioned below). The results obtained in different solvents are summarized in Table 1, which also contains the values of diffusion coefficients obtained from the normal pulse polarograms using the Cottrell equation. The concentrations of oxygen in these solvents were taken from the literature [27]. The obtained values of k◦ are comparable to those obtained at GC electrode in the absence of PVC using the same experimental technique in the same aprotic solvents containing 0.1 M TEAP as used in this study (typically [28] k◦ = 3.2 × 10−2 , 2.5 × 10−2 , 5.9 × 10−2 cm s−1 in DMF, DMSO and Py, respectively). 3.3. Cyclic voltammetry Fig. 6(A) shows the cyclic voltammograms (CVs) obtained at HMDE in O2 -saturated DMF solution in the presence of 0.005% PVC at various potential scan rates (ν). In the absence of PVC, a current oscillation behavior was observed in the anodic scan [1–5], while in the presence of 0.005% PVC the oscillation disappeared and the ordinary well-defined redox peak was obtained. Here it should be noted that a HMDE is a spherical electrode and thus the measured current is expressed as i(plane) + i(spherical correction), where i(plane) is the current under a semi-infinite linear diffusion and i(spherical correction) is the correction term for a spherical diffusion [29,30]. For the present case in which the radius of the HMDE is 0.028 cm, the diffusion coefficients of O2 are 2 to 6 × 10−5 cm2 s−1 and the potential scan rates used are 0.05–0.5 V s−1 , the i(plane) term is much larger than the spherical correction term and the electrode can be considered to be planar under these conditions. The separation between the anodic and cathodic peak potentials, Ep , was ca. 85 mV at a scan rate of 0.1 V s−1 with essentially equal anodic and cathodic peak currents, indicating that the redox reaction of the O2 /O2 − couple is reversible or quasireversible under the experimental condition used here. This conclusion is supported by the correlation obtained between the cathodic peak current Ipc and ν (Fig. 6(B)): Ipc is proportional to ν1/2 at ν < 200 mV s−1 and the Ipc versus ν1/2 plot is slightly curved at higher scan rates. Similar cyclic voltammetric measurements were also carried out in the other solvents in the presence of 0.005% PVC and similar results were obtained except that the peak current changed with solvent because of the differences in the solubility and diffusion coefficient of oxygen as well as in the electrode reaction rate. The E◦ values were estimated as the average of the anodic and cathodic peak potentials with respect to Fc◦ /Fc+ internal standard redox couple and are summarized in Table 1.
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Table 1 Formal potentials of the O2 /O2 − redox couple and the kinetic parameters for the electroreduction of O2 to O2 − at Hg electrode in aprotic solvents in the presence of 0.005% PVC at 25 ◦ C Solventa
E◦ /V vs. Fc◦ /Fc
×102 k◦ (cm s−1 )
αc
×105 DO2 (cm2 s−1 )
DMF Py DMA NMP DMSO
−1.23 −1.26 −1.27 −1.29 −1.17
2.6 ± 0.3 1.6 ± 0.5 1.9 ± 0.4 1.0 ± 0.7 1.8 ± 0.6
0.31 ± 0.04 0.28 ± 0.03 0.33 ± 0.05 0.24 ± 0.02 0.39 ± 0.05
6.2 ± 0.3 5.7 ± 0.3 5.1 ± 0.4 1.7 ± 0.3 1.9 ± 0.1
a
Containing 0.2 M TEAP.
3.4. Solvent effects on formal potential Relying on the usual assumption that the redox potential of the Fc◦ /Fc+ couple is independent of the solvents used [31] should permit one to directly evaluate the solvent effect on the O2 /O2 − redox reaction. The anionic character of superoxide ion (O2 − ) results in a strong dependence of formal potential on the Gutmann acceptor number (AN) [32,33], which is a
Fig. 7. Dependence of formal potential vs. Fc◦ /Fc+ , of the O2 /O2 − redox couple in various solvents upon Gutmann’s acceptor number of the solvents.
measure of the solvent acidity, that is, its ability to solvate anions. The E◦ values (versus Fc◦ /Fc+ ) of the O2 /O2 − redox couple were plotted against AN and are shown in Fig. 7. An excellent linear relationship between E◦ and AN was obtained with a correlation coefficient, r of 0.992. The formal potential depends on the relative solvation of the oxidized and reduced forms of a redox couple. The fact that E◦ becomes more positive with an increase in AN may suggest that O2 − is more strongly solvated in more acidic solvent. In other words, the E◦ values of the O2 /O2 − couple are determined primarily by the degree of solvation of O2 − , as an anion, and the solvation energies of O2 are probably small or almost the same in different solvents used here.
4. Conclusions
Fig. 6. (A) Typical cyclic voltammograms obtained at HMDE for the redox reaction of the O2 /O2 − couple in DMF solution containing 0.2 M TEAP in the presence of 0.005% PVC. Potential scan rates: (1) 0.5 V s−1 ; (2) 0.3 V s−1 ; (3) 0.2 V s−1 ; (4) 0.1 V s−1 ; (5) 0.05 V s−1 . (B) Ipc vs. ν1/2 plot for the data in Fig. 6(A).
In the practical analytical work, the occurrence of polarographic maxima is a nuisance, but fortunately it can be generally eliminated by the addition of a suitable maximum suppressor. We have found that the polarographic maxima observed for the reduction of oxygen in aprotic solvents can be completely suppressed by the addition of a very low concentration of PVC and consequently well-defined S-shaped polarograms are obtained. The thermodynamic and kinetic parameters of the O2 /O2 − electrode reaction have for the first time been evaluated at the Hg electrode in aprotic solvents in
M.S. Saha, T. Ohsaka / Electrochimica Acta 50 (2005) 4746–4751
the presence of PVC as a maximum suppressor using normal pulse polarography and cyclic voltammetry. A good linear relationship was obtained between E◦ (versus Fc◦ /Fc+ ) and AN, reflecting the degree of solvation of O2 − .
Acknowledgements The present work was financially supported by Grant-inAids for Scientific Research on Priority Areas (No. 417), Scientific Research (No. 12875164) and Scientific Research (A) (No. 10305064) to T. Ohsaka from the Ministry of Education, Culture, Sports, Science and Technology, Japan. M.S.S. gratefully acknowledges the Government of Japan for the Monbusho Fellowship.
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[9] E.C. Johnson, K.H. Pool, R.E. Hamm, Anal. Chem. 39 (1967) 888. [10] D.T. Sawyer, A. Sobkowiak, J.L. Roberts, Electrochemistry for Chemists, 2nd ed., Wiley/Interscience, New York, 1995 (Chapter 9). [11] D.T. Sawyer, J.L. Roberts Jr., J. Electroanal. Chem. 12 (1966) 90. [12] D.T. Sawyer, G. Chiericato, C.T. Angelis, E.J. Nanni, T. Tsuchiya, Anal. Chem. 54 (1987). [13] P. Tissot, A.K. Yadav, Electrochim. Acta 31 (1986) 71. [14] R.A. Petersen, E.H. Evans, J. Electroanal. Chem. 222 (1987) 129. [15] M.S. Hossain, D. Tryk, E. Yeager, Electrochim. Acta 34 (1989) 1733. [16] D.S. Shin, N. Doddapaneni, S.M. Park, Inorg. Chem. 31 (1992) 4060. [17] D. Vasudevan, H. Wendt, J. Electroanal. Chem. 392 (1995) 69. [18] J.F. Wu, Y. Che, T. Okajima, F. Matsumoto, K. Tokuda, T. Ohsaka, Anal. Chem. 71 (1999) 4056. [19] D. Zhang, T. Okajima, F. Matsumoto, T. Ohsaka, J. Electrochem. Soc. 151 (2004) 31. [20] H. Matsuda, Bull. Chem. Soc. Jpn. 53 (1980) 3439. [21] T. Ohsaka, N. Oyama, S. Yamaguchi, H. Matsuda, Bull. Chem. Soc. Jpn. 54 (1981) 2475. [22] W.R. Fawcett, B.M. Ikeda, J.B. Sellan, Can. J. Chem. 57 (1979) 2268. [23] I.R. Miller, Trans. Faraday Soc. 57 (1961) 301. [24] J. Koryta, J. Dvorak, L. Kavan, Principles of Electrochemistry, 2nd ed., Wiley, 1987 (Chapter 4). [25] S. Musha, T. Wasa, K. Tani, Rev. Polarography 11 (1963) 169. [26] Y. Matsui, Y. Kurosaki, Y. Date, Bull. Chem. Soc. Jpn. 43 (1970) 1707. [27] M. Tsushima, K. Tokuda, T. Ohsaka, Anal. Chem. 66 (1994) 4551. [28] Y. Che, Doctoral Thesis, Tokyo Institute of Technology, Japan, 1998. [29] A.J. Bard, L.R. Faulkner, Electrochemical Methods-Fundamentals and Applications, 2nd ed., Wiley, 2001 (Chapter 6). [30] W.H. Reinmuth, J. Am. Chem. Soc. 79 (1957) 6358. [31] G. Gritzner, J. Kuta, Pure Appl. Chem. 56 (1984) 461. [32] W.R. Fawcett, Lagmuir 5 (1989) 661. [33] T.M. Krygowski, W.R. Fawcett, J. Am. Chem. Soc. 97 (1975) 2143.
Electrode kinetics of the O2/O2− redox couple at Hg electrode in the presence of PVC in aprotic media Madhu Sudan Saha a,b,1 , Takeo Ohsaka b,∗ b
a Department of Chemistry and Chemical Biology, Northeastern University, 360 Huntington Avenue, Boston, MA 02115, USA Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan
Received 2 December 2004; received in revised form 15 February 2005; accepted 19 February 2005 Available online 1 April 2005
Abstract The kinetic and thermodynamic parameters of the O2 /O2 − redox couple at a mercury electrode in various aprotic solvents have been evaluated by normal pulse polarography and cyclic voltammetry in the presence of poly(vinyl chloride) (PVC) as a maximum suppressor. The polarographic maxima were observed on the rising portion of polarogram for O2 reduction, but they are completely suppressed by the addition of a small amount of PVC. The adsorption behavior of PVC on a hanging mercury drop electrode is examined based on the measurement of the differential capacitance of the electrical double layer. The relevant kinetic and thermodynamic parameters, i.e., the standard rate constant, k◦ , the cathodic transfer coefficient, αc , and the formal potential, E◦ of the O2 /O2 − redox couple were estimated together with the diffusion coefficients of O2 , DO2 . An excellent linear relationship between the formal potential and solvent’s acceptor number was found. © 2005 Elsevier Ltd. All rights reserved. Keywords: Superoxide ion; Hg electrode; Aprotic solvent; Normal pulse polarography; Differential capacitance
1. Introduction In the course of a series of recent studies regarding the current oscillation during the redox reaction of the O2 /O2 − (superoxide ion) redox couple at a mercury (Hg) electrode in usual aprotic solvents [1–5] such as N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO), pyridine (Py) and acetone, we have found that the current oscillation completely disappears in the presence of a small amount of poly(vinyl chloride) (PVC) and normal well-defined polarograms without polarographic maxima can be obtained. This allowed us to examine the electrode kinetics of the O2 /O2 − redox couple at the Hg electrode quantitatively.
∗
Corresponding author. Tel.: +81 45 9245404; fax: +81 45 9245489. E-mail addresses: [email protected] (M.S. Saha), [email protected] (T. Ohsaka). 1 Tel.:+1 6173735630; fax: +1 6173738949. 0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.02.022
The electrochemical reduction of oxygen to superoxide ion in aprotic solvents has been studied since 1965 [6–10] and the kinetic data of the O2 /O2 − electrode reaction in aprotic solvents have been reported by some groups [11–19], but the results lack of consistence even for those obtained under same experimental conditions. In addition, the solvent effect on the electrode reaction has not been reported. In this paper, we report the quantitative investigation the electroreduction of O2 to O2 − at the Hg electrode in various aprotic solvents using normal pulse polarography (NPP) which is suitable for the kinetic studies of the electrode reactions of the redox species dissolved in solution [20,21]. The kinetic and thermodynamic parameters (i.e., the standard rate constant, k◦ , the cathodic transfer coefficient, αc and the formal potential, E◦ ) of the O2 /O2 − redox couple at the Hg electrode were evaluated in the presence of PVC as a maximum suppressor. The differential capacitances of the electrical double layer were also measured for studying adsorption of PVC on the Hg electrode.
M.S. Saha, T. Ohsaka / Electrochimica Acta 50 (2005) 4746–4751
2. Experimental 2.1. Reagents The aprotic solvents, N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO), pyridine (Py), N,N-dimethylacetamide (DMA), and N-methyl-2-pyrrolidone (NMP), used in electrochemical experiments were Analar Grade (Kanto Chemical Co. Ltd.) and dried over molecular sieves 4A1/16 (Wako Pure Chemical Industries) overnight prior to use. All the solutions contained tetraethylammonium perchlorate (TEAP) of reagent grade quality as supporting electrolyte (Tokyo Kasei Co. Ltd.) dried in a vacuum desiccator at 25 ◦ C (at least for 3 days) and stored in a desiccator. Poly(vinyl chloride) (PVC), used as a maximum suppressor, was obtained from Acros Organics Co. The average molecular mass was 1.5 to 2.0 × 105 . Mercury (commercial grade) was washed several times with dilute nitric acid and distilled water and twice distilled under reduced pressure. 2.2. Apparatus and procedures Electrochemical measurements were performed in a conventional two-compartment three-electrode cell at laboratory temperature (25 ± 1 ◦ C). A hanging mercury drop electrode (HMDE; Model 303A, EG&G Princeton Applied Research, area: 0.010 cm2 ) and a static mercury drop electrode (SMDE; Model CGME 900, Bioanalytical System Ins., area: 0.010 cm2 ) were used as working electrodes. The working electrode and counter electrode (a platinum wire) were separated by a porous glass. The reference electrode (Ag|AgCl, NaCl saturated) was connected to the electrochemical cell via an intermediate vessel filled with the supporting electrolyte solution of each aprotic solvent under investigation. Cyclic voltammetry was carried out using a computercontrolled electrochemical system (BAS 100B/W). Normal pulse polarographic measurements were performed with the SMDE working electrode using a computer-controlled electrochemical measuring system (BAS CV-50 W). Normal pulse polarograms for the O2 -saturated sample solutions were corrected for the residual current by subtracting the corresponding polarogram obtained in a deoxygenated solution of supporting electrolyte. The differential double layer capacitance at Hg electrode was measured with an ac series bridge using a computercontrolled electrochemical measuring systems (HECS-311B potentiostat and HECS-322B two-phase lock-in amplifier, Huso Electrochemical system, or SI 1287 electrochemical interface and SI 1260 impedance/gain-phase analyzer, Solartron). Phase sensitive ac voltammograms were measured with a phase angle adjusted to 90◦ , corresponding to the out-of-phase component of the ac current (capacitive current component). The amplitude and frequency of ac voltage applied were 10 mV peak-to-peak and 1 kHz, respectively.
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The point-by-point measurements are made with 50 mV separation with each measurement requiring approximately 2–3 min. Ferrocene (Fc) was used as an internal standard to measure formal potential, E◦ versus Fc◦ /Fc+ . The potential of the reference electrode was found to be constant through the cyclic voltammograms of Fc◦ /Fc+ redox couple obtained by the addition of a suitable amount of ferrocene to the examined solution at the end of each electrochemical measurement. For the measurements in O2 -saturated media, O2 gas (99.98%) was bubbled directly into the cell in order to obtain a saturated solution and during the measurement O2 gas was flushed over the cell solution. The cell solution was deareated by bubbling N2 gas at least 30 min before the blank experiments.
3. Results and discussion 3.1. Differential capacitance Fig. 1 shows the differential capacitance of the double layer, Cdl , at HMDE as a function of potential for different concentrations of TEAP in DMF solution. The potential scan was performed from positive to negative potential (from 0 to −1.6 V versus Ag|AgCl). The measured capacitive ac component is proportional to the differential double layer capacitance in the potential region where no Faradaic process occurs. As can be seen from Fig. 1, a noticeable minimum in the capacitance was observed at the potential of zero-charge (pzc) of ca. −0.3 V versus Ag|AgCl with decreasing the concentration of TEAP, as expected from the Gony–Chapman–Stern model as a series network of Helmholtz-layer and diffuselayer capacitances. It should be noted that the shape of the capacitance curves obtained in this study are practically similar to those reported previously [22].
Fig. 1. Differential capacitance of the double layer at HMDE in DMF solution of TEAP as a function of potential. Concentration of TEAP: (䊉) 0.1; () 0.05; () 0.01 M. Frequency: 1 kHz. Vertical arrows indicate the potential of zero charge.
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Fig. 2. Differential capacitance of the double layer at HMDE in DMF solution containing 0.1 M TEAP in the absence (dashed line) and in the presence of different concentrations of PVC: () 0.001%; () 0.005%; (䊉) 0.01%; ( ) 0.04%.
Fig. 2 shows the differential capacitance-potential curves at HMDE in Ar-saturated DMF solutions containing 0.1 M TEAP in the presence of various concentrations of PVC. The capacitance-potential curves depend strongly on the concentration of the PVC added. At potentials close to the pzc the differential capacitance in the presence of PVC is smaller than that in its absence and it decreases with increasing the concentration of PVC. This is due to the adsorption of PVC molecule, which is less polar than DMF solvent molecule. During the adsorption, DMF solvent molecules in the compact layer are replaced by the adsorbed PVC molecules. As the potential is moved away from the pzc, the PVC desorbs gradually and at potentials more negative than ca. −1.4 V, the differential capacitance is not affected by the presence of PVC, because the PVC molecules desorb completely. The strong electric field leads to the replacement of the less polar PVC molecules by polar DMF solvent molecules. This phenomenon is similar to that observed by Miller for unionized polymethacrylic acid on the Hg/water interface [23] as well as the potential-dependent adsorption of electroneutral molecules that are less polar than the solvent [24].
Fig. 3. Typical normal pulse polarograms for the reduction of O2 to O2 − at SMDE in DMF solution containing 0.2 M TEAP. Concentration of PVC: (a) 0%; (b) 0.002%; (c) 0.01%; (d) 0.02%; (e) 0.04%. Sampling time: 67.5 ms.
0.002 to 0.01% (w/v), PVC acted as an effective suppressor and the limiting current was almost the same. At concentration lower than 0.002%, no effect of PVC as suppressor was observed. At concentrations higher than 0.01%, the limiting current was seen to decrease (curves d and e). Similar maxima were also observed during the reduction of O2 at SMDE in the other aprotic solvents. In all the cases, the maxima were suppressed completely in the presence of a very low concentration of PVC (in this case 0.005%). PVC was also reported to function as a maximum suppressor for the reduction of Hg(I)I2 , Hg(II)I3 − , Tl+ , Pb2+ , Cd2+ and Zn2+ in DMF solution [25,26]. Fig. 4 shows the typical normal pulse polarograms for O2 reduction in DMF solution in the presence of 0.005% PVC. The plots of the limiting current (ilim ) of these polarograms against the inverse square root of sampling time (τ s ) gave the straight line which passes through the origin as expected from the Cottrell equation indicating that the pro-
3.2. Normal pulse polarography The suppressor effect of PVC on the polarographic maxima obtained for the reduction of O2 to O2 − at SMDE in DMF solution is shown in Fig. 3. It is well known that in aprotic media, dioxygen is reduced in a reversible or quasireversible one-electron process to produce a stable anion radical, superoxide ion (O2 − ), at various electrode materials. As shown in Fig. 3, a maximum was observed on the diffusion current plateau (curve a) that makes accurate evaluation of the limiting current impossible. But the polarographic maxima were suppressed and normal well-defined polarograms were obtained (curves b–e) by the addition of a small amount of PVC in the test solution. In the concentration range from
Fig. 4. Normal pulse polarograms for O2 reduction at SMDE in DMF solution containing 0.2 M TEAP in the presence of 0.005% PVC. Sampling times: (1) 67.5 ms; (2) 47.5 ms; (3) 32.5 ms; (4) 14.5 ms; (5) 4.5 ms.
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Fig. 5. Representative plots of
χ
1.75+χ2 (1+exp ζ)2 1−χ(1+exp χ)
1/2 vs. E for the
normal pulse polarograms shown in Fig. 4. Sampling times are indicated on each straight line.
cess is diffusion-controlled [20]. Another information is that the half-wave potential of the voltammograms shifted to the negative direction of potential with decreasing τ s . Thus, the kinetic parameters of the electrode reaction of the O2 /O2 − redox couple were evaluated from an analysis of the rising part of the current-potential curves (Fig. 4), associated with the shifts of half-wave potential, using the following equation [20,21]. 1.75+χ2 (1+expζ)2 1/2 2.303RT (1) E = E∗ − log χ αc nF 1−χ(1+expζ) with r E∗ = E1/2 +
χ=
2.303RT log αc nF
√ 4 k ◦ τs √ √ , D 3
i , (id )Cott
∗ 1/2 (id )Cott = nFACO DO (πτs )−1/2 , 2 2
(2) (3) (4)
nF r (E − E1/2 ), RT
(5)
D = (DO2 )1−αc (DO2 − )αc
(6)
ζ=
where E is the electrode potential, i the normal pulse voltammetric current, (id )Cott the diffusion current expressed by Cottrell equation, DO2 and DO2 − are the diffusion coefficients of O2 and O2 − , respectively and other symbols having their usual meanings. Fig. 5 shows the modified log plots of the normal pulse polarograms in which the logarithm of the second term of the right-hand side of Eq. (1) is plotted against E for the reduction of O2 . These plots gave straight lines, the slopes of which were constant at the different sampling times for each solution, within the experimental errors. The intercepts (E* ) shifted to more negative potential from the formal potential
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(E◦ ) with decreasing τ s as seen from Eq. (2). Thus, from the slopes of the straight lines shown in Fig. 5 and the intercept of these lines with the X-axis, the values of k◦ and αc were estimated by assuming that DO2 ≈ DO2 − and using the r , D and τ . Here, with the assumption known values of E1/2 O2 s r is equal to E◦ which is estimated as that DO2 ≈ DO2 − , E1/2 the average of the anodic and cathodic peak potentials of the cyclic voltammogram for the O2 /O2 − redox couple (mentioned below). The results obtained in different solvents are summarized in Table 1, which also contains the values of diffusion coefficients obtained from the normal pulse polarograms using the Cottrell equation. The concentrations of oxygen in these solvents were taken from the literature [27]. The obtained values of k◦ are comparable to those obtained at GC electrode in the absence of PVC using the same experimental technique in the same aprotic solvents containing 0.1 M TEAP as used in this study (typically [28] k◦ = 3.2 × 10−2 , 2.5 × 10−2 , 5.9 × 10−2 cm s−1 in DMF, DMSO and Py, respectively). 3.3. Cyclic voltammetry Fig. 6(A) shows the cyclic voltammograms (CVs) obtained at HMDE in O2 -saturated DMF solution in the presence of 0.005% PVC at various potential scan rates (ν). In the absence of PVC, a current oscillation behavior was observed in the anodic scan [1–5], while in the presence of 0.005% PVC the oscillation disappeared and the ordinary well-defined redox peak was obtained. Here it should be noted that a HMDE is a spherical electrode and thus the measured current is expressed as i(plane) + i(spherical correction), where i(plane) is the current under a semi-infinite linear diffusion and i(spherical correction) is the correction term for a spherical diffusion [29,30]. For the present case in which the radius of the HMDE is 0.028 cm, the diffusion coefficients of O2 are 2 to 6 × 10−5 cm2 s−1 and the potential scan rates used are 0.05–0.5 V s−1 , the i(plane) term is much larger than the spherical correction term and the electrode can be considered to be planar under these conditions. The separation between the anodic and cathodic peak potentials, Ep , was ca. 85 mV at a scan rate of 0.1 V s−1 with essentially equal anodic and cathodic peak currents, indicating that the redox reaction of the O2 /O2 − couple is reversible or quasireversible under the experimental condition used here. This conclusion is supported by the correlation obtained between the cathodic peak current Ipc and ν (Fig. 6(B)): Ipc is proportional to ν1/2 at ν < 200 mV s−1 and the Ipc versus ν1/2 plot is slightly curved at higher scan rates. Similar cyclic voltammetric measurements were also carried out in the other solvents in the presence of 0.005% PVC and similar results were obtained except that the peak current changed with solvent because of the differences in the solubility and diffusion coefficient of oxygen as well as in the electrode reaction rate. The E◦ values were estimated as the average of the anodic and cathodic peak potentials with respect to Fc◦ /Fc+ internal standard redox couple and are summarized in Table 1.
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Table 1 Formal potentials of the O2 /O2 − redox couple and the kinetic parameters for the electroreduction of O2 to O2 − at Hg electrode in aprotic solvents in the presence of 0.005% PVC at 25 ◦ C Solventa
E◦ /V vs. Fc◦ /Fc
×102 k◦ (cm s−1 )
αc
×105 DO2 (cm2 s−1 )
DMF Py DMA NMP DMSO
−1.23 −1.26 −1.27 −1.29 −1.17
2.6 ± 0.3 1.6 ± 0.5 1.9 ± 0.4 1.0 ± 0.7 1.8 ± 0.6
0.31 ± 0.04 0.28 ± 0.03 0.33 ± 0.05 0.24 ± 0.02 0.39 ± 0.05
6.2 ± 0.3 5.7 ± 0.3 5.1 ± 0.4 1.7 ± 0.3 1.9 ± 0.1
a
Containing 0.2 M TEAP.
3.4. Solvent effects on formal potential Relying on the usual assumption that the redox potential of the Fc◦ /Fc+ couple is independent of the solvents used [31] should permit one to directly evaluate the solvent effect on the O2 /O2 − redox reaction. The anionic character of superoxide ion (O2 − ) results in a strong dependence of formal potential on the Gutmann acceptor number (AN) [32,33], which is a
Fig. 7. Dependence of formal potential vs. Fc◦ /Fc+ , of the O2 /O2 − redox couple in various solvents upon Gutmann’s acceptor number of the solvents.
measure of the solvent acidity, that is, its ability to solvate anions. The E◦ values (versus Fc◦ /Fc+ ) of the O2 /O2 − redox couple were plotted against AN and are shown in Fig. 7. An excellent linear relationship between E◦ and AN was obtained with a correlation coefficient, r of 0.992. The formal potential depends on the relative solvation of the oxidized and reduced forms of a redox couple. The fact that E◦ becomes more positive with an increase in AN may suggest that O2 − is more strongly solvated in more acidic solvent. In other words, the E◦ values of the O2 /O2 − couple are determined primarily by the degree of solvation of O2 − , as an anion, and the solvation energies of O2 are probably small or almost the same in different solvents used here.
4. Conclusions
Fig. 6. (A) Typical cyclic voltammograms obtained at HMDE for the redox reaction of the O2 /O2 − couple in DMF solution containing 0.2 M TEAP in the presence of 0.005% PVC. Potential scan rates: (1) 0.5 V s−1 ; (2) 0.3 V s−1 ; (3) 0.2 V s−1 ; (4) 0.1 V s−1 ; (5) 0.05 V s−1 . (B) Ipc vs. ν1/2 plot for the data in Fig. 6(A).
In the practical analytical work, the occurrence of polarographic maxima is a nuisance, but fortunately it can be generally eliminated by the addition of a suitable maximum suppressor. We have found that the polarographic maxima observed for the reduction of oxygen in aprotic solvents can be completely suppressed by the addition of a very low concentration of PVC and consequently well-defined S-shaped polarograms are obtained. The thermodynamic and kinetic parameters of the O2 /O2 − electrode reaction have for the first time been evaluated at the Hg electrode in aprotic solvents in
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the presence of PVC as a maximum suppressor using normal pulse polarography and cyclic voltammetry. A good linear relationship was obtained between E◦ (versus Fc◦ /Fc+ ) and AN, reflecting the degree of solvation of O2 − .
Acknowledgements The present work was financially supported by Grant-inAids for Scientific Research on Priority Areas (No. 417), Scientific Research (No. 12875164) and Scientific Research (A) (No. 10305064) to T. Ohsaka from the Ministry of Education, Culture, Sports, Science and Technology, Japan. M.S.S. gratefully acknowledges the Government of Japan for the Monbusho Fellowship.
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