The kinetics of iodine disproportionation: a system of parallel second-order reactions sustained by a multi-species pre-equilibrium

Phosphate and borate buffers used commonly in the study of the disproportionation of iodine( 0) into iodate (I(+5)) and iodide ( I( - 1)) catalyse the reaction. This offers significant advantages when the reaction is used to contain radio-iodine in a rupturing fission reactor, e.g., Three Mile Island. Here, such buffer catalysis has been investigated in solutions 0.20 mol l(-1) in phosphate, borate, acetate or carbonate, with between 0 - 0.10 mol l(-1) iodide. The reaction mechanism is modelled by spreadsheet, with parallel rate determining steps involving couplets such as HOI with I2OH-, all in pre-equilibrium with each other. At a given iodide concentration the kinetics are characterised by a peak in the graph of apparent rate constant versus pH. The peaks for hydroxide and each of the buffers except carbonate, coincided; carbonate displayed slightly anomalous behaviour at higher concentrations of iodide (0.10 mol l(-1)). The presence of a pK(a) in the pH domain occupied by the peak was not found to have any significant bearing on the result. The apparent rate constant for disproportionation increased linearly with phosphate concentration when the pH was that of the top of the peak. However, with the pH set on the shoulders of the peak catalysis increased initially, but then became saturated. It is not possible to ascribe, as in simpler systems, a single value to the catalytic activity of each buffer, which would apply across the entire reaction domain. In phosphate, a combination of parallel rate determining reactions involving I2OH-/IO-, I2OH-/HOI and HOI/IO- couplets was found to offer the best solution to the simultaneous fitting of the model to several sets of data. The reactivity of these couplets is investigated by means of molecular orbital theory.