Thermodynamics of anion exchange equilibria in aqueous and non-aqueous media. I–/NO –3, Br–/NO –3, Cl–/NO –3 and I–/Cl– exchanges in Dowex 1 × 8 resin in water and methanol

Abstract

I^–/NO^–₃, Br^–/NO^–₃, Cl^–/NO^–₃ and I^–/Cl^– exchange equilibria have been studied at an ionic strength of 0.1 or 0.05 mol dm^–3 on Dowex 1 × 8 resin in water and methanol at 298 K. Thermodynamic equilibrium constants in water (K_w) and methanol (K_s) have been obtained for these exchanges by correcting the integrated selectivity coefficients, derived from the selectivity coefficient against resin composition plots, for the activity coefficients of electrolytes in the solution phase. K_s and K_w values for the Br^–/Cl^– and I^–/Br^– exchanges were calculated from these, using the triangle rule. Equilibrium constants in both media follow the ionic selectivity sequence I^– > NO^–₃ > Br^– > Cl^–, K_s being generally lower than K_w. K_s values have also been computed for these anion-exchange reactions for the first time based on the thermodynamic theory of ion-exchange equilibria in non-aqueous solvents using the available values of ionic free energies of transfer [ΔG^°_t(ion)] and the free energies of transfer of resinates [ΔG^°_t(resin)] from water to methanol. These computed K_s values are in good agreement with the observed ones, establishing the validity of the theory for anion-exchange systems. ΔG^°_t(ion) values are seen to be responsible for lowering the equilibrium constant values in methanol. Selectivity reversals in favour of Cl^– and Br^– against I^– in DMSO on Dowex 1 × 8 as observed by Phipps apparently arise due to the large differences in ΔG^°_t(ion) values (water to DMSO) between I^– and Cl^–, and I^– and Br^– ions.