Mechanism of electroreduction of cobalt(II) in thiocyanate solutions at mercury electrodes

Abstract

The process of electroreduction of cobalt(II) in thiocyanate solutions at mercury electrodes has been investigated by cyclic voltammetric, chronoamperometric and polarographic methods. The influences of pH, the concentrations of Co(II) and SCN −, and the reduction products of SCN −, CN − and S 2− on the reduction waves are described. The polarographic pre-wave is an autocatalytic in nature. A mechanism involving an initial reduction of Co(II)—SCN − at a mercury electrode followed by the chemical reduction of thiocyanate ion with the electroreduced metallic cobalt, and taking into account cyanide, sulfide, and hydroxide ions, the latter being produced by the hydrolysis of cyanide ion, is presented. Cobalt sulfide adsorbed at the electrode surface stimulates further reduction of Co(II)—CN − and —SCN − complexes, and depresses the interfering influence of Co(OH) 2, which is reductively desorbed from the electrode surface with giving rise to an additional peak near −1.08 V vs. SCE.