Concentration and temperature dependence of ultrasonic velocity and isentropic compressibility in aqueous sodium nitrate and sodium thiosulfate solutions

Abstract

Ultrasonic velocities (u/m s ^-1 ) of aqueous sodium nitrate and sodium thiosulfate solutions have been measured as functions of temperature (278.15⩽T/K⩽323.15) and concentration (0.006⩽m/mol kg ^-1 ⩽10.522). Isentropic compressibilities (κ _S ), apparent molal isentropic compressibilities (K _S,φ ) and hydration numbers (n _h ) for both systems have been calculated. Sodium thiosulfate is more highly hydrated than sodium nitrate; the primary hydration numbers of NO ₃ ^- and S ₂ O ₃ ^2- ions have been estimated to be 6 and 13, respectively. The values of κ _S , K _S,φ and n _h indicate that a strong structural effect exists in the sodium thiosulfate system arising from strong ion–solvent and ion–ion interactions in different concentration regions.