Raman Spectral Studies of Aqueous Strontium Nitrate Solution Up to 450 °C and 40 MPa

Abstract

Abstract We have studied the structure of aqueous strontium nitrate solution (2.8 M) at temperatures up to 450 °C and pressures of 22.6, 30.0, and 40.0 MPa using Raman spectroscopy technique. The strontium ion forms two species; one is the bound species in contact more strongly with the NO3− like the monovalent cation, and the other is the free species separated from the NO3−. The relative ratio of the bound species to the free ones remains unaltered with an increase in temperature up to 375 °C; above this temperature, however, the ratio significantly increases. The half-width at half peak height (hwhh) for the bound species is larger than that for the free species; it increases at 375 °C or above, suggesting that the bound species relaxes faster than does the free species. The relaxation for the bound species becomes significantly faster at the critical temperature. The number of water molecules of hydration around Sr2+, nH2O, is estimated by the comparison of Raman intensity and previous XAFS data. As the temperature increases, the nH2O gradually decreases. The value shows a remarkable decrease above 370 °C, indicating the displacement of water molecules from the solvation shell around Sr2+ and the concomitant entry of NO3− into the shell at higher temperatures.