Vapour pressure of SO2 above solutions of sulphur(IV) oxospecies: the effects of chloride ion and glycerol

A simple spectrophotometric technique to determine the headspace concentration of SO2 above millimolar aqueous solutions of the gas is described. At a given concentration of SO2 in the aqueous phase, the value of Henryʹs constant ([S(IV)]aq/pSO2), where p(SO2) is the pressure of the gas in the headspace above the solution, is increased by the addition of Cl− but markedly decreased on adding 20–30 wt% glycerol. The data are used to obtain a value of 7.2 mol litre−1 for the dissociation constant of SO2 Cl− which has ϵmax = 2000 mol litre−1 cm−1 at 280 nm.