Complexity in “Simple” Electrolyte Solutions:  Ion Pairing in MgSO₄(aq)

A detailed investigation of aqueous solutions of MgSO4 has been made by dielectric relaxation spectroscopy over a wide range of frequencies (0.2 ≤ ν/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 behavior of higher-valent electrolytes are briefly discussed.

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