Catalysis by colloidal gold of the reaction between ferricyanide and thiosulphate ions

Abstract

Citrate-stabilized gold sols catalyse the redox reaction between ferricyanide and thiosulphate ions in aqueous solution. The initial rates of this catalysed reaction have been measured at 25 °C over a wide range of reactant concentrations, while keeping the pH and cation concentrations constant. The kinetic orders of ferricyanide and of thiosulphate were found to be fractions which added up to unity within experimental error. This is consistent with the predictions of the electrochemical theory of such catalysis and points to a catalytic mechanism of electron transfer through the gold particles. Further electrochemical experiments support this conclusion. The magnitude of the heterogeneous rate constant confirms that the catalysis is surface- and not diffusion-controlled.