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 soluti ons in the presence of large excesses of M'X (M'(+) = Na+, K+ or Cs+; X- = OH- or Cl-) at 25 degreesC. The positions of the v(1)(A(1)') symmetric stre tching vibrations for both CO32- (at similar to 1060 cm(-1)) and SO42- (at similar to 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 ar e slightly smaller for SO42- than for CO32- and also differ somewhat betwee n M'OH and M'Cl solutions. The changes in the peak positions, which are ind icative of contact ion pairing, show a clear dependence on the nature of th e 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 con centrated, water-deficient solutions. Some implications of this for underst anding ionic interactions in concentrated electrolyte solutions are suggest ed.