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

Raman spectra of potassium, sodium, and ammonium sulfates (K2SO4, Na2SO4, and (NH4)2SO4) 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 NH4+) and hydration on the position of Raman lines assigned to internal vibrations of sulfate anion SO42− are discussed. In solid salts, the line position of each Raman peak is shown to decrease with increasing radius of the cation. The main ν1 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 SO42− to determine its concentration within a single calibration. Copyright © 2013 John Wiley & Sons, Ltd. For sulfates in solid state, the ν1 mode was shown to be the most affected by increasing radius of the cation. 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 SO42−.