Raman spectroscopic study of ion association in aqueous magnesium sulphate solutions

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Abstract
A Raman spectroscopic study of aqueous solutions of MgSO4as function of concentration and temperature was performed. From the analysis of the band profile of the internal vibrations of the sulphate ion, the spectroscopic ionic association constant was calculated at different temperatures. It was found that the association is solvent separated in the ranges of concentration and temperature studied. The large differences observed between the values obtained for the spectroscopic association constant and those obtained from macroscopic measurements were interpreted on the basis of a four‐step solvent‐separated dehydration mechanism. It is, assumed that the Raman spectra reflect only the short‐distance force acting on the sulphate anions. The contribution of the long‐range forces for the consecutive steps were calculated using the Bjerrum equation. Good agreement between the overall association constant thus calculated and the macroscopic value was found in the concentration and temperature ranges studied.