Heterogeneous catalysis in solution. Part 24.—Kinetics of a redox system exhibiting three separate diffusion-controlled regions: the iron(III)+ iodide reaction at a rotating platinum disc catalyst

Abstract

The rate of the reaction between Fe³⁺ and I^– ions, in aqueous 0.05 mol dm^–3 HClO₄ made up to an ionic strength of 0.1 mol dm^–3, has been determined from 5 to 20 °C both homogeneously and in the presence of a large reduced platinum rotating disc catalyst. At low iodide concentrations the catalytic rate was first order in iodide and zero order in iron(III), at low iron(III) concentrations it was first order in iron(III) and zero order in iodide, and at intermediate and comparable concentrations of reactants both kinetic orders were fractional. In all three concentration regions the catalytic rate was proportional to the square root of the disc rotation speed, indicative of mass transport control. These results, as well as the variation in the potential adopted by the catalyst and other findings, agreed with the predictions of the electrochemical mechanism of the catalysis and with the current–potential curves measured for each of the two couples at the same platinum disc.