Raman spectral studies of concentrated aqueous solutions of strong electrolytes

(Bell Telephone Laboratories, Incorporated. Murray Hill, New Jersey.), U.S.A.

Abstract

The intermolecular hydrogen-bond stretching and librational motions of pure water give rise to broad Raman bands near 170 cm^-1, and between 450- 720 cm^-1, respectively, at 25 °C. When strong electrolytes such as chlorides and bromides are added to water the Raman intensity near 170 cm^-1 is observed to decrease rapidly, whereas a marked increase in intensity near 450-600 cm^-1 is observed. The decrease in intensity at 170 cm^-1 is consistent with breakage of O — H ... O units, but the increase near 450-600 cm^-1 is indicative of hydration, particularly of anions, and the increase is observed to be much larger for Br- than for Cl^-.

When the temperature of pure water is increased, the intermolecular hydrogen-bond stretching and librational Raman intensities decrease rapidly. The intermolecular Raman intensity from a concentrated chloride or bromide solution, however, decreases only slightly near 450-600 cm^-1 with increasing temperature, even to temperatures approaching the boiling points. Such observations indicate that primary hydration of anions is not greatly disrupted even at relatively high temperatures.