Calculation of the thermodynamic and transport properties of aqueous species at high pressures and temperatures: Standard partial molal properties of inorganic neutral species

Abstract

Consideration of interactions between neutral aqueous species and H 2O dipoles in terms of effective Born coefficients permits extension of the revised HKF (Helgeson, Kirkham and Flowers, 1981) equations of state (Tanger and Helgeson, 1988) for the standard partial molal properties of ionic species at high pressures and temperatures to include inorganic gases, acids, and other neutral aqueous species. Correlation algorithms similar to those used to estimate equation of state parameters for ions and electrolytes ( shock, and Helgeson, 1988) have also been developed for neutral aqueous species. Calculation of the standard partial molal thermodynamic properties of dissolved inorganic gases as well as other neutral aqueous species as a function of pressure and temperature indicates that the standard partial molal volume (V̄ 0), heat capacity (C̄ 0 p), and entropy (S̄ 0), together with the apparent standard partial molal enthalpy of formation (ΔH̄ 0) of many of these species in the liquid phase minimize with increasing temperature at P SAT ∗ ∗ P SAT represents pressures corresponding to liquid-vapor equilibrium for the system H 2O, except at temperatures <100°C where it refers to the reference pressure of 1 bar. and approach ∞ at the critical point of H 2O. In the case of other neutral aqueous species such as SiO 2(aq), V ̄ 0 , C ̄ 0 p , S ̄ 0 , and Δ H ̄ 0 behave as functions of temperature and pressure like those of electrolytes in the liquid phase and maximize with increasing temperature at P SAT, approaching — ∞ at the critical point of H 2O. Which of these types of behavior is exhibited by V ̄ 0 , C ̄ ) p , S ̄ 0 , and Δ H ̄ 0 for a given neutral aqueous species depends in part on the relative volatility of the aqueous species and the effect of the species on solvent dipole-dipole interaction. Close agreement between predicted and experimentally determined equilibrium constants for gas solubility and inorganic acid dissociation reactions at high temperatures and pressures supports the validity and generality of the equations of state and the predictive algorithms. High temperature/pressure equilibrium constants can be predicted for reactions involving a wide variety of neutral aqueous species for which few or no experimental data are available at temperatures > 25°C. Present capabilities permit such predictions to be made for hydrothermal and magmatic conditions at pressures and temperatures to 5 kb and 1000°C.