Water Ionization Constants at High Temperatures and Pressures Calculated with the “Isochoric” Model

An “isochoric” model is presented for calculating the standard (i.e., at infinite dilution in water) thermodynamic properties of ions at temperatures from 373 to 2000 K and pressures corresponding to water densities from about 250 to 1500 kg m−3. The main correlation variable is the standard partial isochoric heat capacity of an ion/electrolyte Cv,2o, which depends weakly on temperature and is virtually independent of the pressure/density of water at T > 373 K. Data supporting the main assumptions of the model are presented. The combination of Cv,2o with the existing method for calculating standard partial volumes of the ion/electrolyte at T > 373 K allows calculating the increments of the standard partial Gibbs energies G2o, enthalpies H2o, entropies S2o, and isobaric heat capacities Cp,2o from the reference point at 373.15 K and 300 MPa up to 2000 K and 10 GPa. The analysis of experimental data for NaCl, NaOH, and HCl allowed choosing the optimal type of analytical expressions for the “isochoric” model. The best studied at high T and P water ionization constant was chosen as a test. At the saturated water vapor pressure, the agreement between the precise experimental and calculated values of water ionization constants pKw is excellent. Over the entire T–P range of data availability (up to 1273 K and up to a water density of ∼1700 kg m−3), the agreement is at least satisfactory, and the quantity and quality of experimental pKw do not allow more definitive conclusions.