Ion Association and Hydration in Aqueous FeSO4 and MnSO4 Solutions

Abstract Ion association in aqueous electrolyte solutions depends critically on the interplay between ion hydration and electrostatic attraction such that different ion-pair species have been detected in aqueous 2:2 electrolytes with sulfate as the prototypical divalent anion. Dielectric relaxation (DR) spectroscopy has the potential to resolve and quantify the various ion-pair species, yet literature studies are too scarce to systematically address the effect of the nature of the cation on ion association. Here, we report a DR study of aqueous solutions of MnSO4 and FeSO4 at 25 °C, at frequencies 0.2 ≤ ν/GHz ≤ 115 and concentrations up to near-saturation (∼2.7 and ∼0.8 mol·L–1 respectively). The observed spectra were consistent with the presence of five Debye modes: two at higher frequencies (ν > ca. 10 GHz) that correspond to bulk-water processes, and three solute-related modes at lower frequencies. The slowest mode, centered at ν ≈ 0.3 GHz, was consistent with overlapping contributions from the rotation of double-solvent-separated ion pairs (2SIPs) and hydrated triple ions (M2SO42+, T+) as has been found for some other divalent metal sulfate solutions. The other lower frequency modes, centered at ν ≈ 1.5 GHz and ∼8 GHz, were assigned (respectively) to the rotation of solvent-shared (SIPs) and contact (CIPs) ion pairs, with the latter mode possibly also incorporating an unresolved contribution from “slow” bulk-water molecules. Effective total cation solvation numbers, Zt(M2+, aq), derived from the water-related modes, indicated that both Mn2+ and Fe2+ immobilize approximately 11 water molecules on the DR time scale, independent of solute concentration. Combination of the present results with literature data revealed a strong correlation between the types of ion aggregates detected by DR spectroscopy and the hydration enthalpy of the divalent metal cations (excepting Zn2+). Concentrations of the various species derived from the solute-related modes were used to calculate formation constants for each species that were, where comparisons were possible, in reasonable agreement with literature values obtained by other techniques.