Studies on the effects of inorganic ions and their concentrations on raman spectra of water

Raman spectra of aqueous solutions containing ClO^-₄, SO^2-₄, Li⁺, Na⁺, K⁺, Cs⁺ were measured. The effects of the concentrations and the properties of these ions on the frequencies, band shape and relative strength of the stretching and bending vibration bands of water molecules were compared in detail. With increasing the concentration of ClO^-₄, a typical stucture-breaking anion, the strengh of the O-H stretching vibration peaks at lower wavenumber (3244 and 3396 cm^-1) decreases and a new peak at higher wave number (3552 cm^-1) appears and is becoming the strongest one at the 8 mol·L^-1 NaClO₄ solution, this is obviously different from the temperaure-dependent Raman spectra of pure water in that the O-H stretching vibration frequency continously shifts to higher wavenumber with increasing temperature, indicating that the process of hydrogen-bond breaking by ClO^-₄ between water molecules could be expressed by the mixture model. For SO^2-₄, however, the concentration effect on the Raman spectra of water is very weak because SO^2-₄ is a structure-making anion which can form hydrogen bond with water molecules. It has been also found that Li⁺ and Cs⁺ have similar effects on the Raman spectra of water, while the effects of Na⁺ and K⁺ are very weak.