Ion association in aqueous solutions of strong electrolytes: a UV–Vis spectrometric and factor-analytical study

Abstract

Ion association in aqueous solutions of varied concentrations of Co(NO₃)₂ and LiNO₃ has been studied by means of UV–Vis absorption spectrometry assisted by principal-components and evolving-factor analyses. Formation of [Co(NO₃)]⁺ and [LiNO₃]⁰ associated ion pairs was detected at higher salt concentrations (>0.1 mol dm^-3 for Co(NO₃)₂, >0.7 mol dm^-3 for LiNO₃). Absorption spectra of ion pairs were computed by means of EFA, as well as their equilibrium concentrations and formation constants in each solution. The fractions of ion pairs (relative to total salt concentration) amounted to ca. 0.6 ([Co(NO₃)]⁺) or ca. 0.5 ([LiNO₃]⁰), at the highest salt concentrations. Differences in the absorption spectra of ion pairs, suggesting different kinds of cation–anion interaction, were tentatively interpreted as a weak coordination in the case of [Co(NO₃)]⁺ and as a classical electrostatic ion association (probably with intervening water molecule) in [LiNO₃]⁰. No association was observed with Mg(NO₃)₂ solutions in the experimental concentration range (c⩽2 mol dm^-3).