Voltammetric Analysis for Electrochemical Reactions of Various Ions in Molten Sodium Borate

Abstract

The cathodic reactions of simple ions such as Cd²⁺, Ni²⁺ and Zn²⁺ and redox ions such as Fe³⁺/Fe²⁺, Cr⁶⁺/Cr³⁺ and As⁵⁺/As³⁺ in molten sodium borate (28.7Na₂O⋅71.3B₂O₃) were investigated by potential sweep voltammetry with platinum electrodes. The electrolytic currents were measured when linearly changing potentials were applied to the electrodes. Potential sweep rates were selected between 10 and 0.02V/s. The electrode reactions and reversible electrode potentials of various ions were determined by analysis of the peak potential and the peak current in the current-potential curve at 950°, 900°, 850° and 790°C.
Cadmium, zinc and nickel ions at lower temperature were reduced reversibly to metallic state with 2-electrons process without deposition on the electrode. On the other hand, lead and thallium ions were reduced reversibly to metallic state and deposited on the electrode. The latter results agree with the fact that these metals are preferentially adsorped on platinum electrode by Faradaic adsorption process in acidic aqueous solution.
Such redox ions as Fe³⁺/Fe²⁺, Cu²⁺/Cu⁺, Sb⁵⁺/Sb³⁺ and As⁵⁺/As³⁺ were reduced stepwisely. In the cases of former two ions, 1-electron steps were followed by the second reduction steps of 2-electrons or 1-electron, respectively, while in the cases of latter two ions 2-electrons steps were followed by the second 3-electrons steps. Chromium, cerium and manganese which are expected to be reduced stepwisely, showed only one reduction step, respectively, because the second step became obscure by disturbance of the large residual current of the base glass. The reduction of tin ions shows the first step caused by adsorption phenomenon and is followed by the second step caused by the reduction of Sn⁴⁺ to Sn²⁺. On many ions the results obtained by potential sweep method were almost compatible with the previous studies by chronopotentiometry. Potential sweep method was found to be more suitable than chronopotentiometry in terms of detailed study on the electrode reaction.
On the basis of these results, electrochemical series were established. For example, at 950°C the order of increase of oxidizing power was Cr⁶⁺/Cr³⁺>Mn³⁺/Mn²⁺≅Sb⁵⁺/Sb³⁺>Ce⁴⁺/Ce³⁺>Cu²⁺/Cu⁺>As⁵⁺/As³⁺>Sb³⁺/Sb>Sn⁴⁺/Sn²⁺>Fe³⁺/Fe²⁺>Pb²⁺/Pb>Cd²⁺/Cd>Tl⁺/Tl>Zn²⁺/Zn>Fe²⁺/Fe. The order of increase of oxidizing power among Cr, Mn and Ce ions coincided with the result by Kühl et al., but the order between As⁵⁺/As³⁺ and Sb⁵⁺/Sb³⁺ was reverse. The electrochemical series supports the results of measurement on the free energies of antimony and arsenic oxides in molten glass by Baak and Hornyak.
Some equilibrium constants K between different redox pairs were calculated from the electrochemical series. The K value between iron and cerium ions approximately coincided with the value obtained by chemical analysis. In comparison with the results on mutual interaction of different redox pairs obtained from optical absorption measurement and chemical analysis, the reactions were shown to proceed toward the same directions in the cases of chromium-iron, manganese-arsenic and chromium-arsenic. In the case of chromium-cerium, however, the reverse direction of reaction was obtained. On manganese-cerium the K values obtained by these two different methods roughly agreed.