Linear Sweep Voltammetry of Ni(II), Co(II), Cd(II), and Pb(II) at Glassy Carbon Electrodes in Molten Lithium Chloride-Potassium Chloride Eutectic

Abstract

The reduction of Ni(II), Co(II), Cd(II), and Pb(II) was studied at glassy carbon electrodes in molten lithium chloride-potassium chloride eutectic at 450~ using the technique of linear sweep voltammetry. Peak-shaped voltammograms were obtained at scan rates of 10 mV to 1 V/sec. Linear dependence of peak currents on metal ion concentration and the square root of voltage scan rate was obtained for all the cations studied. The diffusion coefficients of Ni(II), Co(II), Cd(II), and Pb(II) were calculated from the experimental data and found to be 1.26, 1.02, 0.98, and 1.29 x 10 -~ cm~/sec, respectively.The polarographic method has been extensively applied to a study of the solute species dissolved in molten salts. These studies have been carried out both at dropping metal electrodes (mercury, bismuth, lead) and solid (platinum, tungsten, graphite) microelectrodes. The subject has been thoroughly covered in a number of reviews (1-4). At elevated temperatures, the polarographic method suffers from the use of large polarization times so that it is extremely difficult to completely eliminate the convective transport of the electroactive species to the electrode surface. Further, the metal ions which result in the deposition of a solid metal at the solid microelectrodes show an "increasing limiting current phenomenon" (1, 2, 5, 6) due to an increase in the electrode area. These difficulties can be eliminated by using the techniques of chronopotentiometry or linear sweep voltammetry (oscillographic polarography) where polarization times of only a few seconds or less are employed. The chronopotentiometric method has been widely used (1, 2) in molten salts in the past whereas the technique of linear sweep voltammetry has attracted attention only recently.Mamantov et al. employed this technique to study the voltammetry of several cations in molten fluorides (7-12) at platinum wire, pyrolytic graphite, or glassy carbon electrodes and in molten NaNO~-KNO3 eutectic (13) at platinum wire electrodes. Francini et al. used the dropping mercury electrode to study the voltammetric behavior of several cations (14, 15), halide ions (16) and oxygen and its ions (17) in low melting nitrates. The reduction of Ag(I) at platinum wire electrodes was studied by Hills, Oxley, and Turner (18) in molten LiC1-KC1 eutectic and by Kawamura (19) in mixtures of molten NaNOa-KNO3. Behl and Gaur (20) studied the reduction of Ag(I) and Pb(II) at platinum wire electrodes in molten NaNO3-Ba(NO3)2 eutectic. Naryshkin and co-workers studied the reduction of several cations at platinum microelectrodes and determined their diffusion coefficients in molten NaC1-KCI (21) and LiC1-KC1 (22) eutectic. They also used this method to study (23) the temperature dependence of diffusion coefficients in the latter solvent.It was the purpose of this study to examine the applicability of the theory (24, 25) of linear sweep voltammetry to the reduction of Ni(II), Co(II), Cd(II), and Pb(II) at glassy carbon electrodes in molten * Electrochemical Society Active Member. Key word...

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