Raman spectra of aqueous solutions of strong electrolytes: evolving-factor analysis

Abstract

Ion association in aqueous solutions of varied concentrations of LiNO₃, Mg(NO₃)₂, Co(NO₃)₂, Li₂SO₄ and MgSO₄ was studied by means of Raman spectrometry assisted by principal-components (PCA) and evolving-factor (EFA) analyses. Formation of one Raman-active associated species, {M···L^(R)} (M=Li⁺, Mg²⁺, Co²⁺; L=NO₃⁻, SO₄²⁻), was detected at higher salt concentrations (≳2 mol dm⁻³ for LiNO₃, ≳1 mol dm⁻³ for Co(NO₃)₂, ≳0.2 mol dm⁻³ for Li₂SO₄, ≳0.5 mol dm⁻³ for MgSO₄). Spectral profiles of L(aq) and {M···L^(R)} species were computed by means of EFA, as well as their equilibrium concentrations in each solution. Maximum fractions of anions engaged in {M···L^(R)} species amounted (at highest salt concentrations) to ≈0.6 (LiNO₃), ≈0.4 (Co(NO₃)₂), ≈0.60 (Li₂SO₄) and ≈0.3 (MgSO₄). Since, for a given salt concentration, the eqilibrium concentration of Raman-active species, [M···L^(R)], was always lower than the concentration of analogous UV-active species, [M···L^(UV)] (reported in a previous paper) it is suggested that {M···L^(UV)} could be a precursor of {M···L^(R)}. In all instances, apparent stability constants of {M···L^(R)} species, falling into the range of (0.01≲K₁/dm³ mol⁻¹≲1), have a progressively upward trend with increasing salt concentration.