Raman spectroscopic studies of ion association in the Na + , Mg 2+ /Cl − , SO 4 2− /H 2 O system

Raman spectroscopic studies of the ion association of Mg 2+ and SO 4 2− with H 2 O molecules in solutions of different crystallization fields of the quaternary system Na + , Mg 2+ /Cl − , SO 4 2− /H 2 O were carried out in the wavenumber ranges 100–1700 and 2700–4000 cm −1 . The water activity of the examined solutions was calculated at 25 °C using the Pitzer method. Direct correlation between the SO 4 2− ion associations and the water activity in these saturated solutions was established. The ν 1 (SO 4 2− ) Raman band was found to be symmetrical in saturated solutions with a high water activity and low concentration of Mg 2+ ions. With increasing Mg 2+ concentration, asymmetry of the ν 1 (SO 4 2− ) band appeared. Band components at 980, 990, 1003 and 1021 cm −1 were found. We assume that the ν 1 (SO 4 2− ) band components at 980 cm −1 correspond to non‐associated SO 4 2− ions. At 990 cm −1 they are associated with solvent‐separated ionic pairs (Mg 2+ H 2 OSO 4 2− ), at 1003 cm −1 with the direct ionic pair Mg 2+ –OSO 3 2− and at 1021 cm −1 with more strongly deformed SO 4 2− groups. The differences observed in the crystallization kinetics of the salts in this quaternary system are explained by the different associations of the SO 4 2− and Mg 2+ ions and the H 2 O molecules in these saturated solutions. Copyright © 2005 John Wiley & Sons, Ltd.