Temperature Effects on Ion Association and Hydration in MgSO₄ by Dielectric Spectroscopy

A detailed investigation of aqueous solutions of magnesium sulfate has been made by dielectric relaxation spectroscopy (DRS) over a wide range of frequencies (0.2<or=nu/GHz<or=89), concentrations (0.02<or=m/mol kg(-1)<or=2.3) and temperatures (5<or=theta/ degrees C<or=65). Consistent with literature data from other techniques, the spectra show clear evidence at all temperatures for the simultaneous existence of double solvent-separated (2SIP), solvent-shared (SIP) and contact (CIP) ion pairs and a triple ion (TI), Mg(2)SO(4) (2+)(aq). Increased formation of CIPs and especially TIs occurs with increasing temperature. The overall standard ion-association constant, K(A) degrees , corresponding to the equilibrium: Mg(2+)(aq)+SO(4) (2) (-)(aq) <==> MgSO(4) (0)(aq) is in good agreement with literature data at lower temperatures but is overestimated at higher temperatures due to processing difficulties. Despite the limited precision of the spectra, analysis of the individual steps in the ion-association process is possible for the first time. The 2SIPs are formed with little disturbance to their hydration shells, the (partial) destruction of which appears to occur mostly during the formation of SIPs. Effective hydration numbers derived from the DRS spectra indicate that both Mg(2+) and SO(4) (2-) influence solvent water molecules beyond their first hydration spheres but that MgSO(4)(aq) is less strongly hydrated than the previously studied CuSO(4)(aq).