Raman Spectral Studies of Aqueous Zinc Nitrate Solution at High Temperatures and at a High Pressure of 30 MPa

Abstract

We present the first direct measurement of dynamic behavior of ions in an aqueous solution at high temperatures and pressure using Raman spectroscopy. We have studied the N−O symmetric stretching mode at high temperatures up to 340 °C and at a high pressure of 30 MPa. The Raman spectra of 1.3 M aqueous zinc nitrate solution have been analyzed by curve fitting. The zinc ion forms two species. In one species Zn2+ is bound more strongly to the NO3-, and in the other Zn2+ is bound more strongly to H2O. The ratio of the former to the latter remains unaltered with temperatures below 300 °C, but above 300 °C the ratio increases significantly. The average number of water molecules bound to Zn2+ (nH2O) is estimated using the intensity of peak frequency of the symmetric stretching mode of the haxaaquazinc(II) cation. As the temperature increases, the nH2O gradually decreases, but above 300 °C it shows a large decrease, suggesting the displacement of water molecules from the first solvation shell around Zn2+ and the...