Reduction and reoxidation processes of the system Fe(II)/Fe(Hg) in thiosulfate solutions at mercury electrodes

Abstract

The electrochemical behavior of the system Fe(II)/Fe(Hg) in thiosulfate solutions at stationary mercury electrodes has been investigated by cyclic voltammetric, chronoamperometric, and anodic stripping methods. The cyclic voltammogram of 1×10 −3 M Fe(II) in 0.1 M Na 2S 2O 3 and 0.2 M NaClO 4 solution at 0.05 V s −1 exhibits a cathodic peak at −1.02 V vs. SCE with a hysteresis on the reversal, and an anodic wave with the principal peak at −0.63 V and small peaks near −0.73 and −0.39 V, respectively. The cathodic wave is kinetic in nature. Controlled potential reduction of Fe(II) indicated that thiosulfate ion was also reduced during the course of the electroreduction of Fe(II). A mechanism involving an initial reduction of Fe(II)-S 2O 3 2− produced in an activation step at a mercury electrode, followed by the chemical redox reaction of Fe(0)S 2O 3 2− in the species, and taking into account the influence of the reduction products of thiosulfate ion, sulfide and sulfite ions, on the reduction and reoxidation processes of iron is presented. Iron sulfide adsorbed at the mercury electrode stimulates further reduction and reoxidation of iron. The hysteresis of the cathodic wave on the first cycle arises from the fact that Fe(II) is reduced more easily at the mercury electrode covered with iron sulfide than at a pure mercury electrode.