Thermodynamic properties of aqueous sodium sulfate solutions to 773 K and 3 GPa derived from acoustic velocity measurements in the diamond anvil cell.

The thermodynamic properties of a 1 m Na(2)SO(4) solution have been determined to 773 K and 3 GPa from acoustic velocity measurements in externally heated diamond anvil cell using Brillouin spectroscopy. The measured acoustic velocities were inverted to obtain the density of the aqueous electrolyte solution with an accuracy of 0.3%-0.5%, and an equation of state (EoS) valid in the 293-773 K and 0.4-3 GPa range is proposed. The new EoS reproduces the experimental acoustic velocity data with a maximal deviation of 1.5% and allows deriving all thermodynamic properties of the aqueous solution, including isobaric heat capacity (C(P)), thermal expansion (α(P)), and compressibility (β) with an accuracy better than 3%-8%. The addition of dissolved sulfate species decreases the compressibility of water, consistent with the structure-maker character of SO(4)(2-) ions in solution that enhance the hydrogen-bond network of the solvent.