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

A detailed investigation of aq. solns. of MgSO4 has been made by dielec. relaxation spectroscopy over a wide range of frequencies (0.2 ≤ ν/GHz ≤ 89) and concns. (0.017 ≤ c/M ≤ 2.24). Detailed anal. of the spectra shows conclusively, as has long been inferred from ultrasonic absorption studies, the simultaneous presence of double solvent sepd. (2SIP), solvent-shared (SIP), and contact (CIP) ion pairs. The consts. derived for the stepwise formation of each ion pair type and for the overall assocn. are in excellent agreement with literature ests. based on other kinds of measurements. In addn., evidence has been obtained for the existence of a triple ion, Mg2SO42+(aq), or possibly a more aggregated species, at high electrolyte concns. (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 quant. models of the thermodn. behavior of higher-valent electrolytes are briefly discussed