Raman study of cation effect on sulfate vibration modes in solid state and in aqueous solutions

Raman spectra of potassium, sodium, and ammonium sulfates (K 2 SO 4 , Na 2 SO 4 , and (NH 4 ) 2 SO 4 ) are reported and analyzed. These sulfates have been investigated under two states: solid (anhydrous and hydrated) salts and aqueous solutions. The effects of monovalent ions (K + , Na + , and NH 4 + ) and hydration on the position of Raman lines assigned to internal vibrations of sulfate anion SO 4 2− are discussed. In solid salts, the line position of each Raman peak is shown to decrease with increasing radius of the cation. The main ν 1 mode of sulfate molecule is particularly affected. It is emphasized that this sensitivity in solid sulfates vanishes in aqueous solutions. As a consequence, this mode can be probed by Raman spectroscopy as the main signature of SO 4 2− to determine its concentration within a single calibration. Copyright © 2013 John Wiley & Sons, Ltd.