Kinetics of disproportionation of hypoiodous acid

Abstract

The disproportionation of hypoiodous acid into iodide and iodate has been studied at pHs between 7 and 13 and without added iodide. The second-order behaviour of the reaction velocity with respect to total molecular iodine (∑ I₂), well established for highly alkaline conditions, has been found to hold at these lower pHs provided the solutions are not brown with iodine. It is shown that the kinetics can be modelled as a second-order reaction in [HOI] influenced by two opposing pre-equilibria involving I₂ and HOI, and HOI and IO^–, respectively. The equilibrium between I₂ and I₃^– is shown to be of low significance when the background iodide concentration is less than about 10^–3 mol I^–1. The model predicts that in highly alkaline solutions additional reactions involving I₂OH^– are necessary to comply with the first-order behaviour with respect to iodide reported in historical rate data. The reliance of the model on iodine speciation also obviates the need for a reciprocal dependence of the reaction rate upon OH^– concentration in alkaline solutions. The sluggishness of the reaction at pHs less than about 8 (brown solutions) is consistent with either the rate-limiting step changing to one involving IO₂^–, or the retarding effect of iodide generated during the reaction.