Temperature effects on ion association and hydration in MgSO4 by dielectric spectroscopy

Abstract

A detailed investigation of aq. solns. of magnesium sulfate has been made by dielec. relaxation spectroscopy (DRS) over a wide range of frequencies (0.2 ≤ ν/GHz ≤ 89), concns. (0.02 ≤ m/mol kg-1 ≤ 2.3) and temps. (5 ≤ .vtheta./°C ≤ 65). Consistent with literature data from other techniques, the spectra show clear evidence at all temps. for the simultaneous existence of double solvent-sepd. (2SIP), solvent-shared (SIP) and contact (CIP) ion pairs and a triple ion (TI), Mg2SO42+ (aq). Increased formation of CIPs and esp. TIs occurs with increasing temp. The overall std. ion-assocn. const., KAc, corresponding to the equil.: Mg2+ (aq) + SO42-(aq).dblharw.MgSO40(aq) is in good agreement with literature data at lower temps. but is overestimated at higher temps. due to processing difficulties. Despite the limited precision of the spectra, anal. of the individual steps in the ion-assocn. 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 nos. derived from the DRS spectra indicate that both Mg2+ and SO42- influence solvent water mols. beyond their first hydration spheres but that MgSO4(aq) is less strongly hydrated than the previously studied CuSO4(aq).