Kinetics of disproportionation of hypoiodous acid at high pH, with an extrapolation to rainwater

The disproportionation of hypoiodous acid into iodide and iodate has been studied in alkaline solution (10 -3 to 1.0 mol l -1 OH - ) in the presence of between 0.01 and 0.10 mol l -1 of iodide, and at an ionic strength of 1.0. The recent model incorporating pre-equilibria between I 2 and I 3 - , HOI, I 2 OH - , OI - and rate-determining steps involving couplets of these has been found to account for most of the observed variation. Whereas, at the higher iodide concentrations, the main reaction involves I 2 OH - and OI - , at lower concentrations, the HOI–HOI interaction predominates. The overall behaviour of disproportionation in hydroxide solutions is the same as that found earlier in borate buffer and, therefore, in both cases the pH of the peak rate is determined largely by the iodide concentration and not by buffer catalysis, as has been generally believed previously. By extrapolation the overall rate constant for disproportionation in rainwater is estimated to be 6×10 4 l mol -1 min -1 .