Influence of sulfide and cyanide ions on the electrochemical behavior of the Fe(II)/Fe(Hg) system in thiocyanate solutions at mercury electrodes

Abstract

The reduction and reoxidation processes of the Fe(II)/Fe(Hg) system in thiocyanate solutions at stationary mercury electrodes have been investigated by cyclic voltammetric, anodic stripping and controlled potential electrolysis methods. In 0.1 M NaSCN and 0.4 M NaClO 4 solution containing 1×10 −3 M Fe(II), the voltammogram on the first cycle at. 0.05 V s −1 gives two consecutive cathodic peaks near −1.2 and −1.39 V with a hysteresis on the reversal, and an anodic wave with two large peaks near −0.58 and −0.05 V and two small peaks near −0.52 and −0.43 V, respectively. The multicyclic voltammogram under the same conditions in the potential region between 0.00 and −1.50 V gives a cathodic wave with a principal peak near −1.02 V and two small peaks near −0.02 and −0.53 V, respectively, and an anodic wave with a principal peak near −0.72 V, three small peaks near −0.64, −0.52 and −0.40 V, and with a shoulder near −0.05 V, respectively. The variation of the shape of the voltammogram on the second and subsequent runs is due to the formation of S 2− and CN − during the process of electroreduction of Fe(II). A mechanism is proposed which involves an initial reduction of Fe(II)−SCN − produced in an activation step at a mercury electrode, followed by the chemical redox reaction of a part of Fe(0)−SCN − in the species giving FeS and CN − , and takes into account the influence of FeS and CN − on the further reduction and reoxidation of iron. Both FeS and CN − stimulate 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 FeS than at a pure mercury electrode.