Oxidation of iodide ion by iron(III) ion in aqueous solution

The kinetics of the redox reaction between Fe3+ and I– ion have been reinvestigated over a wide range of reactant concentrations using the method of initial rates. The first-order dependence on Fe3+ and second-order dependence on I– have been confirmed, the third-order rate constant being 16·0 ± 1·0 I2 mol–2 s–1 at 25°C and ionic strength of 1·0M. In the presence of added ferrous ion, however, the previously derived rate laws do not adequately describe the reaction under all conditions. It is believed that the inner-sphere complex Fel2+ does not form to a significant extent, but that an outer-sphere ion-pair between Fe3+ and I– is a likely intermediate. A di-iodo-species may also be involved, and stationary-state assumptions, leading to relatively simple rate expressions, are not valid under all conditions.