Influence of sulfide formation on the reduction of nickel(II) in thiosulfate solutions at mercury electrodes

Abstract

The electrochemical behavior of the Ni(II)/Ni(Hg) system in thiosulfate solutions at mercury electrodes has been investigated by direct current (dc) polarographic, cyclic voltammetric, chronoamperometric and anodic stripping methods. Dc polarogram of 1×10 −3 M nickel(II) in 0.1 M Na 2S 2O 3 and 0.2 M NaClO 4 solution exhibits three consecutive reduction waves with the half-wave potentials of −0.60, −0.87, and −1.08 V vs. SCE respectively. The cyclic voltammogram in the same electrolyte solution at 0.02 V s −1 gives two cathodic peaks near −0.64 and −0.91 V, respectively, the latter being followed by a catalytic hydrogen wave with the peak potential near −1.68 V, and an anodic peak near −0.45 V. On the second and subsequent cycles, the first cathodic peak disappears, and the second peak corresponding to the reduction of hydrated nickel(II) shift to less negative potentials. A mechanism which involves the reduction of Ni(II)−S 2O 3 2− followed by a chemical redox reaction of a zerovalent Ni(O)−S 2O 3 2− in the species giving nickel sulfide and sulfite ion, and takes into account the effect of nickel sulfide on the reduction and reoxidation processes of nickel is proposed. Nickel sulfide adsorbed at the mercury electrode inhibits further reduction of Ni(II)−S 2O 3 2−, while it accelerates the reduction of hydrated nickel(II) and the reoxidation of nickel amalgam.