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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.

It has been shown that the flux of material diffusing to the surface of a colloidal particle is greater than to a plane surface of the same area and diffusion layer thickness (6). Around a small sphere of radius R the effective Nernst layer thickness deff is given by

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