The Raman spectroscopic study of supersaturation: ν 1 mode of aqueous NH 4H 2PO 4 solution

Abstract

In this work, it is shown that careful contour analysis of the symmetric stretching mode of H 2PO − 4 of ammonium dihydrogen phosphate may reveal certain structural information as the solution varies from the unsaturated state to the supersaturated state. Both the concentration and temperature dependent cases are demonstrated. It is seen that in solutions there can be varying compositions of monomer, dimeric and polymeric aggregates of H 2PO − 4 depending on the concentration and temperature. Apparently, there is a thermodynamic point above which the polymeric aggregates cannot be in thermodynamic equilibrium with the monomeric or dimeric species and the solution is therefore thermodynamically unstable. There is also no discontinuity in the half band width and peak frequency as the solution reaches supersaturation consistent with the refractive index and electrical conductivity measurements.