Raman Spectroscopic Study of Ion Pairing of Alkali Metal Ions with Carbonate and Sulfate in Aqueous Solutions

Raman spectra of carbonate and sulfate ions were recorded in aqueous solutions in the presence of large excesses of M′X (M′+ = Na+, K+ or Cs+; X– = OH– or Cl–) at 25°C. The positions of the v1(A1′) symmetric stretching vibrations for both CO32– (at ~1060 cm–1) and SO42– (at ~980 cm–1) shift, by up to 10 cm–1, to lower frequencies and broaden (by up to 25%) with increasing concentrations of M′X. The shifts are slightly smaller for SO42– than for CO32– and also differ somewhat between M′OH and M′Cl solutions. The changes in the peak positions, which are indicative of contact ion pairing, show a clear dependence on the nature of the cation. However, the observed order, Na+ < K+ < Cs+, is directly opposite to the magnitudes of the (overall) ion-pairing constants. This anomaly is thought to be due to decreased hydration of Cs+ cf. Na+ in these highly concentrated, water-deficient solutions. Some implications of this for understanding ionic interactions in concentrated electrolyte solutions are suggested.