Voltammetric study of cobalt(II) in thiosulfate solutions at stationary mercury electrodes

Abstract

The electrochemical behavior of cobalt (II) has been investigated in thiosulfate solutions at stationary mercury electrodes. In 0.1 M Na2S2O3 and 0.2 M NaClO4 solution containing 1×10−3 M cobalt(II), cyclic voltammogram on the first scan at 0.02 V s−1 gives a cathodic peak at −0.85 V vs. SCE, with hysteresis on the reversal, and an anodic wave with the peak at −0.42 V and with two shoulders near −0.5 and −0.39 V, respectively. A multicyclic voltammogram under the same conditions gives a cathodic peak at −0.82 V, and two peaks at −0.48 and −0.43 V, respectively. Controlled potential reduction of cobalt(II) indicated that thiosulfate ion was also reduced to sulfide ion during the course of the reduction of cobalt(II). A mechanism involving a chemical reaction of thiosulfate ion with the electroreduced metallic cobalt and taking into account the effect of sulfide and sulfite ions on the reduction and reoxidation processes of cobalt(II) is proposed. The hysteresis of the cathodic wave is attributable to the fact that the rate of reduction of cobalt(II) is faster at the mercury electrode covered with cobalt sulfides than at the pure mercury electrode. The chemical reduction of thiosulfate ion by the electroreduced metallic cobalt has been estimated by the reaction of Raney cobalt with thiosulfate ions.