Raman Spectral Studies of Molten Potassium Bisulfate and Vibrational Frequencies of S2O7 Groups

Abstract

Raman intensities and frequencies of molten potassium bisulfate, obtained in the temperature range of about 300° to 700°C, provide evidence for the reactions 2HSO4−→S2O72−+H2O and S2O72−→SO42−+SO3. Between 300° and 620°C the HSO4− concentration, inferred from Raman intensities, decreases rapidly. Simultaneously, the S2O72− concentration increases and becomes maximal at 500°C. Above 500°C the S2O72− concentration decreases, and Raman lines of SO42− appear. Analyses of gaseous and solidified products indicate that the production of SO42− is accompanied by the evolution of SO3. Raman frequencies and polarizations of HSO4− ion obtained at temperatures ranging from the melting point to 620°C have been compared with the corresponding quantities obtained from studies of aqueous solutions. Differences observed are discussed in terms of hydrogen bonding. Raman spectra of molten potassium bisulfate obtained at ∼620°C provide vibrational frequencies of S2O72−. The number of Raman frequencies observed, polarizations of the Raman bands, and comparisons of the Raman frequencies with infrared frequencies of the corresponding solid exclude structures of the dihedral point groups and favor a structure of C2 symmetry (a slightly distorted C2v structure). Vibrational assignments for frequencies of S2O72− were made, and a comparison of those frequencies with frequencies of the related species HS2O7− and H2S2O7 indicates many close correspondences.