Kinetics of disproportionation of hypoiodous acid in phosphate and borate buffer at pH < 8.5 modelled using iodide feedback

Abstract

The kinetics of disproportionation of hypoiodous acid has been studied in phosphate and borate buffers at pH < 8.5, and without added iodide. In this region, after an initial rapid phase of reaction lasting tens of minutes, the reaction becomes very sluggish sometimes continuing for weeks at 25 °C. The region offered a further opportunity to test the applicability of the mechanism in which the rate-determining step of the formation of the IO₂^– ion is preceded by a pre-equilibria involving species such as I₂, I₂OH^–, etc.

The model, after it was modified to include a feedback loop which compensates for the gradual increase in iodide concentration generated internally as a product of disproportionation, predicts reaction curves of the correct shape for reaction in both buffers. In borate buffer, the rate of reaction was found to be first order in respect of buffer concentration, as described earlier. However, in phosphate buffer the rate was second order in respect of buffer concentration.