Complexity in 'Simple' Electrolyte Solutions: Ion Pairing in MgSO4(aq)
R. Buchner, T. Chen and G. Hefter,
J. Phys. Chem. B 108 (2004) 2365-2375
Abstract
A detailed investigation of aqueous solutions of MgSO4 has been made by dielectric relaxation spectroscopy over a
wide range of frequencies (0.2 <=n/GHz <=89) and concentrations (0.017 <= c/M <= 2.24). Detailed analysis of the
spectra shows conclusively, as has long been inferred from ultrasonic absorption studies, the simultaneous presence
of double solvent separated (2SIP), solvent-shared (SIP) and contact (CIP) ion pairs. The constants derived for the
stepwise formation of each ion pair type and for the overall association are in excellent agreement with literature
estimates based on other kinds of measurements. In addition, evidence has been obtained for the existence of a triple
ion, Mg2SO42+(aq), or possibly a more aggregated species, at high electrolyte concentrations (c > 1 M). Support for
the presence of CIPs, SIPs, and the triple ion is provided by Raman spectroscopy. The implications of the present
findings for quantitative models of the thermodynamic behaviour of higher-valent electrolytes are briefly discussed.
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