Vibrational Spectral Studies of Solutions at Elevated Temperatures and Pressures. IV. Raman Spectra of Aqueous Zinc Nitrate Solutions

Abstract

Raman spectra of 5.196 m aqueous zinc nitrate solution have been measured at a pressure of 11 MPa and temperatures ranging from 25 to 300°C. A band at 386 cm⁻¹, assigned to the hexaaquazinc(II) cation, decreases in intensity and increases in freqeuncy as the temperature rises. These observations are consistent with the replacement of water molecules in the primary solvation shell of Zn²⁺ by a nitrate ion. Changes in the spectrum of nitrate ion support this interpretation. Evidence is also presented which suggests that the octahedral configuration around Zn²⁺ is being changed to a tetrahedral configuration. Estimates are made of the populations of inner sphere and outer sphere nitrate; from the plots of log Q_m vs 1/T estimates are made of ΔH and ΔS for the ion association. The plots are remarkably linear over 275 degrees of temperature.