The effect of anion adsorption on the kinetics of the Fe 3+/Fe 2+ reaction on Pt and Au electrodes in HclO 4

Abstract

The kinetics of the Fe 3+/Fe 2+ reaction was studied on Pt and Au rotating disk electrodes in 0.5 M HClO 4 solutions. It was found that the rate constant is trongly affected by adsorption of traces of anionic impurities. Consequently, the solution was purified by adsorption on a large platinum electrode. The measured current was corrected for the effect of the 2 potential. The actual rate constant value for the Fe 3+/Fe 2+ reaction on a Pt electrode was 1.5×10 −3 cm s −1 and on an Au electrode, 7.9×10 −3 cm s −1; the corresponding reorganization energy values were 120 kJ mol −1, respectively. The effect of the adsorption of chloride, bromide and sulphate anions in increasing the rate of the reaction was studied. The rate constant of the reaction increases approximately exponentially with the coverage of the electrode by anions. The mechanism by which the anions increase the reaction rate was discussed.