THE STATE OF SULFUR-DIOXIDE AT HIGH-CONCENTRATION AND LOW WATER ACTIVITY

The apparent pK of HSO-3 is lowered by the addition of salts (NaCl, NaNO3, Na2SO4) but increased by addition of ethanol, glycerol or polyethylene glycol. Sucrose (0-60 wt%) has little or no effect. The pK of sulphurous acid is similarly affected by salts (NaCl, NaNO3) but only ethanol and polyethylene glycol give rise to a substantial increase in value. The effect of salts can be modelled by the extended Debye-Huckel theory. In the case of the 1:1 electrolytes, graphs of pK versus ionic strength pass through minima; for sulphurous acid the pK in the presence of 3 M NaCl is actually greater than that at zero ionic strength for this reason. The equilibrium constant for the formation of the ion-pair NaSO3- is estimated to be of the order 6 M-1 and evidence is given for the formation of a weak complex between SO2 and Cl-. The nucleophilic reactivity of sulphur (IV) oxospecies in equilibrium with HSO3- was assessed kinetically by measuring rates of reaction with malachite green. The data indicate that S2O5(2)- ion does not behave as a nucleophile in comparison with the nucleophilicity of SO3(2)-. The relevance of the results to the use of 'sulphur dioxide' as a preservative in concentrated foods and in the presence of humectants is considered.