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Measurements have been made of the rate of infusion of caffeine into distilled water from medium roast Kenyan Arabica coffee beans and from eight sieved fractions of the ground beans at 25.8"C. The first-order rate constants increased dramatically as the particle size decreased. For one of the size fractions the rate constants were then measured at various temperatures up to 84.1"C and were found to rise eight-fold over this temperature interval. The partition coefficients of caffeine between ground beans and water were also determined. These results, interpreted by a new steady-state theory of extraction, show that the rate-determining step in the infusion is diffusion of caffeine through the swollen coffee particles. The low magnitude of the diffusion coefficient and its high activation energy demonstrate that caffeine diffusion within the bean particles is a hindered process.

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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.