The speciation equilibrium coupling with phase equilibrium in the H2O–CO2–NaCl system from 0 to 250 °C, from 0 to 1000 bar, and from 0 to 5 molality of NaCl

A model is developed for the calculation of the speciation equilibrium of H+, OH−, HCO3−, CO32−, and CO2(aq) in aqueous solutions coupled with liquid-vapor phase equilibrium in the CO2–H2O–NaCl system from 273 to 523 K, from 0 to 1000 bar (possibly to 2000 bar) , and from 0 to 5.0 molality (m) of NaCl. The first and second ionization constants of carbonic acid, pH values in aqueous solutions, and their dependence on temperature (T), pressure (P) and NaCl concentrations can be calculated with accuracy close to those of experiments in the stated T–P–m range. Using the specific ion-interaction equations of Pitzer to calculate activity coefficients, the model is also able to accurately predict the difference between dissociation constants and apparent dissociation constants, and the concentrations of H+, OH−, HCO3−, CO32−, and CO2(aq) at a given T–P–m condition. Compared to previous models, this model covers larger T–P–m space and much more closely reproduces experimental results, especially in the elevated pressure region. Even without fitting to the experimental data for pH values, dissociation constants and apparent dissociation constants at pressures above the saturation pressures of pure water, this model can accurately predicted these properties up to high pressures. A computer program based on this model is developed and online calculation is made available at: www.geochem-model.org/models.htm.