Electrochemical study of the oxidation of tin(II) at glassy carbon electrodes

TafantaVol. 28, pp. 192to 194 0 PergamonPressLtd 1981.Printedin Great Britain

0039.9140/8I/030192-03SO2.00/0

ELECTROCHEMICAL STUDY OF THE OXIDATION AT GLASSY CARBON ELECTRODES P. KDXENS, H.

VERPLAETSE

and E.

OF TIN(I1)

TEMMERMAN

Laboratory of Analytical Chemistry, Ghent University, J. Plateaustraat, 22, 9000 Ghent, Belgium (Received 10 June 1980. Accepted 25 September 1980)

voltammetric characteristics of the Sn(IV)/Sn(II) system have been studied at a glassy carbon rotating disk electrode in 4M hydrochloric acid. Anodic oxidation of Sn(I1) produces a single well-defined wave, which has been examined in the temperature range 20-50”, and shows distinct irreversibility. The kinetic parameters of the oxidation reaction have been calculated. Kinetic features of the reduction of Sn(IV) to Sn(I1) were not evaluated because the reduction proceeds directly to tin metal. Summary-The

Only a few workers have reported on the kinetic parameters of the oxidation of Sn(I1) at solid electrodes.‘-’ It is proposed by Lerner and Austin,’ Vetter,’ and Ateya and Austin’ that the oxidation of Sn(I1) proceeds by two consecutive one-electron transfer steps at platinum and graphite electrodes in concentrated hydrochloric acid. The first electron transfer would be rate-determining at large overpo-

tentials and the second at low ones. It is also assumed that the concentration of the intermediate Sn(II1) is very small and constant (steady state), causing no mass-transfer polarization. According to Bishop and Hitchcock, this redox pair is characterized by a slow charge-transfer in bromide plus perchloric or hydrobromic acid media, with a single two-electron transfer step, or two consecutive one-electron steps. However a definite conclusion cannot be reached. From their studies with gold and mercury electrodes in 6M hydrochloric acid, Kadish and Stamp4 concluded that the Sn(IV)/Sn(II) system is quite complex and that chemical reactions, competitive with the electron-transfer steps, should be considered. In this study the kinetics of the anodic oxidation of Sn(I1) have been investigated at a glassy carbon rotating disk electrode in 4M hydrochloric acid. EXPERIMENTAL

Reagents

Experiments were carried out in 4M hydrochloric acid to suppress hydrolysis. Solutions were prepared from analytical grade reagents and water freshly generated by a Mini-Q system (Millipore Inc.) and deaerated with nitrogen containing less than 1 ppm of oxygen. Stock solutions of Sn(l1) were prepared by dissolving SnCl,. 5H,O (Merck) in ‘warm concentrated hydrochloric acid and standardized by titration with ceric sulphate. Because Sn(I1) is very sensitive to air-oxidation, all solutions were carefully deaerated with high-purity nitrogen before Sn(I1) was added. Deaeration was also continued between successive voltammetric runs and a nitrogen atmosphere was maintained above the test solutions. Sn(II) stock solutions were renewed when any significant oxidation was detected. Sn(lV) solutions were prepared by quantitative oxidation of Sn(ll) with hydrogen peroxide, followed by boiling to destroy the excess of peroxide.

Instrumentation The electrolysis cell was a Teflon vessel (about 150 ml capacity) equipped with a glassy carbon rotating disk electrode (Tokai Electrode Mfg. Co., Tokyo, geometric surface area 0.194 cm’), a platinum auxiliary electrode (10 cm’) and a saturated calomel electrode (SCE) to which all potentials are referred, unless otherwise specified. The reference electrode was separated from the cell by a saltbridge, with a capillary junction placed within 2 mm of the edge of the disk electrode, which was centrally placed in the vessel. The salt-bridge was filled with concentrated hydrochloric acid and the connection to the SCE was made by a U-tube containing saturated potassium chloride solution. The cell, salt-bridge and reference electrode system were placed in a water:bath, the temperature of which was controlled by a thermostat to 0.1”. The electrode was rotated by a Brion-Leroux motor (BIROTAX type I) and the speed was strictly controlled with a servo control unit (Tacussel Asservitex). The rotation rate was measured with a proximity probe (Philips PR 9373) and freauencv meter (Philios PM 6601). Potential controlwas maintained by means of a Tacussel sweep generator GSATP and bipotentiostat BI-PAD. Curves were registered with a Hewlett-Packard X-YY recorder, model HY 7046 A. Viscosities were determined with an Ostwald viscometer. A Paar Precision Density Meter type DMAOSD was employed for density measurements. Electrode pretreatment

The glassy carbon working electrode was polished by standard metallographic techniques. Final polishing to a mirror-like surface was performed with O.OQm alumina on Buehler microcloth. The electrode was rinsed with water and activated by cycling the potential between 1.2 and - 1.OV vs. SCE (evolving oxygen and hydrogen for cleaning) for 10 min at a scan-rate of 0.030 V/secin 1M hvdrochloric acid. The electrode was then held successivelv at -0.5 and 0.0 V I’S.SCE for 5 min to remove any oxide film possibly formed and metallic impurities that might have been deposited at -0.5 V. During this pretreatment nitrogen was passed through the solution. Finally the electrodes were transferred into the previously deaerated test solution containing Sn(II) and/or Sn(lV). Procedure

Generally the current-potential curves were recorded from 0.0 V as initial potential, first in the anodic direction up to 1.2 V and then returning to -0.4 V before going back to zero potential. Only curves from scans in the anodie direction, on a reduced surface, were taken into account. The scan rate was 0.005 V/set. 192

SHORT

193

COMMUNICATIONS

RESULTS AND DBCUSSION

Voltammetric

experiments

Current-voltage curves for the oxidation of Sn(I1) in 4M hydrochloric acid were recorded as a function of rotation speed, Sn(I1) concentration and temperature between 20 and 50”. The reduction of Sn(IV) gave a single wave, not completely separated from the hydrogen reduction wave. Well-defined tin oxidation peaks always appeared during the subsequent anodic scan when a significant cathodic reduction current was observed previously. A somewhat better detined Sn(IV) reduction wave appeared when 9M hydrachloric acid medium was used, although still without a distinct limiting current plateau. It is known6 that only with a very large chloride concentration will a well-developed doublet wave be produced. Figure 1 depicts current-voltages curves for the oxidation of 7.OmM Sn(II) at 20” in 4M hydrochloric acid. The currents at the foot of the curves were rather irreproducible, and depended on whether the curves were recorded with increasing or decreasing potential. Convective-diffusion

controlled

oxidation

Fig. I. Current-voltage curves for oxidation of 7.0mM Sn(II) in 4M HCI, on a glassy carbon rotating disk eleo trode at 20°C. Each curve is labelled with the rotation speed (rad/sec).

I.5

of Sn(li)

It was found that the limiting currents were proportional to the square root of the rotation speed, with zero intercept for all concentrations and temperatures. From this it is clear that the Levich equation’ for a convective-diffusion limited process was obeyed. Figure 2 illustrates the dependence of the limiting current, IL, on wl” for 3.5mM Sn(II) at various temperatures. From the increase in the limiting currents with temperature. Arrhenius plots were constructed, from which the experimental activation energy related to the diffusion process’ was found to be 9.5 f 1 kJ/mole, a value in the usual range. According to Milner’ the temperature coefficient of the diffusion coefficient is generally about 2-3% per “C. The experimental results were in agreement with this figure. At 50”, the IL values were 45-50x larger than at 25”. Table 1 assembles the temperature-dependence of the diffusion coefficient, density and kinematic viscosity for’7.OmM Sn(I1). From these data the radius of the reducing species was calculated to be 0.23 f 0.05 run, from the Einstein-Stokes relation,” with an Einstein-Stokes ratio, On/T, almost constant over the range 2045”.

II)

as

w-2

I

Fig 2. Dependence of limiting currents on e~r’s for various temperatures: (x ) 20, (0) 30, (A) 40 and (0) 50°C; [Sri(D)] = 3.5mM.

Evaluation

of kinetic

parameters

When log I/(1, - I) was plotted vs. potential, which is the Tafel plot corrected for masstransport effects, straight tines were obtained for currents from O.lZLto 0.95Zr. From the gradients we determined an anodic transfer coefficient, fl, of 0.35 + 0.02, which was inde pendent of temperature and rotation rate. This is a rather low value and is reflected in the anodic curves in which a high overpotential is necessary before the potential-independent plateau is reached. The value of the transfer coefficient suggests the first electron-

Table 1. Physical characteristics of 7.OmM Sn(I1) in 4M HCl at different temperatures

TN.. 28/3-E

Temperature, “C

Density, @n3

Kinematic viscosity, cm2/sec

20 25 30 35 40 45

1.107 1.105 1.103 1.100 1.098 1.096

0.012660 0.011551 0.010550 0.009966 0.009381 0.008940

Diffusion coefficient of Sn(II), cm2/sec 7.0 x 7.3 x 7.9 x 8.6 x 9.3 x 10.0 x

1o-6 lo-” 10-6 10-6 lo-” 1o-6

194

SHORT

COMHUNlCATIONS

also shift to more negative potentials at higher temperatures. To determine the apparent energy of activation, E,, rate constants at different temperatures were evaluated and plotted against the reciprocal of the absolute temperature (T). The potential of the reference electrode was corrected for the temperature change according to the values tabulated by Ives and Janz.16 From the linear plots and the relation

a [1 1

E, = - 2.3R -

b k:tm

a(l/T)

E

the energy of activation was found to be 40.5 f 5 kJ/mole on glassy carbon which has weak anisotropic properties.” 0

005

0 IO

015

i”*

(red/secjl’2

Fig. 3. Dependence of 1-r on (1)-r” for 7.Orm%fSri(H) on glassy carbon at 20°C. (0) 0.330; (x) 0.360; (0) 0.390; (0) 0.420; (m) 0.450; (A) 0.480; (A) 0.510 V us. SCE.

REFERENCES

I. H. Lerner and L. G. Austin, J. Electrochem. Sot.. 1965,

112, 636.

2. K. J. Vetter, Electrochemical Kinetics, pp. 152, 481. Academic Press, New York, 1967.

transfer to be rate-determining” in that potential range. Analogous values of fl were given by Bishop and Hitchcock while Ateya and Austin’ found /I to be 0.41. To calculate the rate of the process, the reciprocals of the currents preceding the limiting plateau were plotted against o- li2. The linear relationships obtained are shown in Fig. 3 and are also characteristic of a first-order process. Extrapolation to infinite rotation speed allowed us to evaluate the rate constants. This technique was first used by Frumkin and Tedoradze” and by Jahn and Vielstich.’ 3 As the Sn(IV)/Sn(II) oxidation-reduction potential evaluated by Huey and Tartar14 depended on the acidity and the couple was found to behave irreversibly, rate constants were evaluated at E = 0.0 V vs. SHE (standard hydrogen electrode), which is close to the standard potential. This gives an apparent rate constant, k&, of 2.0 x lo-’ cm .sec at 20”. It is concluded that this redox system is distinctly irreversible. This is also confirmed by the dependence of the half-wave potentials on the rotation speed;” they become more anodic at higher rates of rotation. They

3. E. Bishop and P. H. Hitchcock, Analyst, 1973,98,635. 4. K. M. Kadish and J. Stamp, Anal. Letr., 1973, 6,909: 5. B. G. Ateya and L. G. Austin, 1. Electroanal. Chem., 1974, 51, 85. 6. J. J. Lingane, J. Am. Chem. Sot., 1945, 67. 919. 7. V. G. Levich, Phvsicochemical Hvdrodvnamics. p. 69. Prentice Hall, EnRlewood Cliffs, N. J., i962. 8. D. Moller and K.-H. Heckner, J. EIectroanaL Chem., 1972, 36, 277. 9. G. W. C. Milner, Polarography

and

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and Applications

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Sot.,

1962,

Sot., 1934, 56.2585. 15. J. Koryta, J. Dvoiak and V. BohPEkov& Electra chemistry, p. 269. Methuen, London, 1970. 16. D. J. G. Ives and G. J. Janz, Reference Electrodes, and Practice, p. 161. Academic Press, New

17.

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