Ion-ion-solvent interactions in solution. Aqueous solutions of nitrates of cations from Groups 2A and 3A

Abstract

Association equilibria in aqueous solutions of the salts Be(NO₃)₂, Ca(NO₃)₂ Sr(NO₃)₂, Ba(NO₃)₂ and Al(NO₃)₃ have been studied through band shape and component band analysis of the non-degenerate, Raman-active V₁ vibrational band of the nitrate ion. Band contributions due to the aquated nitrate ion, solvent-separated ion pair, and contact ion pair were made for all salts. The nature of the contact ion pair was shown to be different in the presence of Be²⁺ and Al³⁺ from that seen for the other salts. This difference was associated with a strongly directional perturbation of the nitrate ion. For all salts the solvent-separated ion pair species was present in the lowest concentration solution studied (as low as 0.05 M). At intermediate concentrations the solvent separated ion pair species was the dominant solution species while at the higher concentrations the species with anion and cation in contact becomes appreciable. For solutions of Ca(NO₃)₂ some evidence for a more extensive ion aggregate was found at the highest concentration (5M). Equilibrium quotients calculated from the spectroscopic components are in reasonable agreement with previous values determined by equilibrium and transport measurements, and it is suggested that these previous determinations detected the presence of solvent-separated ion pairs rather than contact ion pairs.