The Ionization Constant of Water over Wide Ranges of Temperature and Density

A semitheoretical approach for the ionization constant of water, KW , is used to fit the available experimental data over wide ranges of density and temperature. Statistical thermodynamics is employed to formulate a number of contributions to the standard state chemical potential of the ionic hydration process. A sorption model is developed for calculating the inner-shell term, which accounts for the ion–water interactions in the immediate ion vicinity. A new analytical expression is derived using the Bragg–Williams approximation that reproduces the dependence of a mean ion solvation number on the solvent chemical potential. The proposed model was found to be correct at the zerodensity limit. The final formulation has a simple analytical form, includes seven adjustable parameters, and provides good fitting of the collected KW data, within experimental uncertainties, for a temperature range of 0 – 800 °C and densities of 0 – 1.2 g cm . © 2006 American Institute of Physics. DOI: 10.1063/1.1928231