Heterogeneous catalysis in solution. Part 23.—Kinetics of a redox system showing complete mass-transport control: the hexacyanoferrate(III)+ iodide reaction at a rotating-platinum-disc catalyst

Abstract

The rate of the reaction between Fe(CN)^3–₆ and I^– has been measured from 5 to 30 °C in 1 mol dm^–3 aqueous KNO₃ solution both homogeneously and in the presence of a large anodically preconditioned platinum-disc catalyst rotating from 100 to 2000 rev. min^–1. The catalytic rate agreed quantitatively with the mixture current (converted to a rate) at the point where the current–voltage curves of the two separate couples intersected; the mixture potential at this point also agreed well with the potential adopted by the catalysing disc. This confirmed the electrochemical mechanism of the platinum catalysis. That the catalytic process had become wholly mass-transport controlled was shown by the fact that the catalytic rates always varied proportionately with the square root of the angular velocity (ω) of the disc, while the catalyst potential remained independent of ω. Other predictions of the theory in the preceding paper to be fulfilled within experimental error were the kinetic orders of the heterogeneous reaction, the marked changes in these orders on adding the product Fe(CN)^4–₆ to the initial mixture, and the negative Arrhenius activation energy associated with the catalytic rate constant.