KINETICS OF DISPROPORTIONATION OF HYPOIODOUS ACID AT HIGH PH, WITH ANEXTRAPOLATION TO RAINWATER

The disproportionation of hypoiodous acid into iodide and iodate has b een 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 ioni c strength of 1.0. The recent model incorporating pre-equilibria betwe en I-2 and I-3(-), HOI, I2OH-, OI- and rate-determining steps involvin g couplets of these has been found to account for most of the observed variation. Whereas, at the higher iodide concentrations, the main rea ction involves I2OH- and OI-, at lower concentrations, the HOI-HOI int eraction predominates. The overall behaviour of disproportionation in hydroxide solutions is the same as that found earlier in berate buffer and, therefore, in both cases the pH of the peak rate is determined l argely 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 x 1 0(4) 1 mol(-1) min(-1).