Ion–ion–solvent interactions in solution. Part 5.—Influence of added halide, change in temperature and solvent deuteration on ion association in aqueous solutions of nitrate salts

Abstract

Band-component analysis of the band due to symmetrical stretching of the nitrate ion in aqueous solutions has been used to study association equilibria. Variation of anion composition at constant ionic strength indicates that the association equilibria are weakly dependent on the nature and concentration of counter-ions. Variation of ionic strength at constant anion concentration produced large changes in the associated species. Association was found to be dependent on both the competitive association equilibria with other anions and of the structural disturbance of the solution. The measurement of the concentration of associated species as a function of temperature in solutions of NaNO₃ showed that the formation of both solvent-separated ion pairs and contact ion pairs is favoured by a reduction in temperature. Values of the enthalpy change associated with the equilibria Na⁺(aq)+ NO^–₃(aq) [graphic omitted] Na⁺· H₂O · NO^–₃(aq) [graphic omitted] Na⁺· NO^–₃(aq) were found to be ΔH₁=–42.2 kJ mol^–1 and ΔH₂=–64.56 kJ mol^–1. In a comparison of association equilibria in H₂O and D₂O solutions of LiNO₃, NaNO₃ and KNO₃ the association equilibria were all altered in a way compatible with the solvent D₂O having weaker hydrogen bonding and aquating properties than H₂O.