Calculation of the thermodynamic and transport properties of aqueous species at high pressures and temperatures: Aqueous tracer diffusion coefficients of ions to 1000°C and 5 kb

Abstract

Accurate values of diffusion coefficients for aqueous species are a requisite for predicting mass transfer in many geochemical processes. Tracer diffusion coefficients can be calculated from the limiting equivalent conductances of ions using the Nernst-Einstein equation. A corresponding states approach yields an isothermal/isobaric correlation between the limiting equivalent conductances and the standard partial molal entropies of aqueous species and electrolytes. These correlations, together with an equation of state for the standard partial molal entropies of aqueous species ( Tanger and Helgeson, 1987) and a modified Arrhenius representation of the limiting equivalent conductances of aqueous electrolytes, can be used to predict as a function of temperature and pressure the limiting equivalent conductances of many electrolytes of geologic interest for which no high pressure/temperature experimental data are available. Combining these estimates with the linear dependence of the logarithm of the ratio of the anion to cation transference number for NaCl on reciprocal temperature observed by Smith and Dismukes (1964) permits prediction of the limiting equivalent conductances of ions, and therefore tracer diffusion coefficients at temperatures and pressures to 1000°C and 5 kb. Values of these coefficients are given in tables for 30 monovalent anions, monovalent cations, and divalent cations of geologic interest at high temperatures and pressures. The diffusion coefficients increase with increasing temperature by ~two orders of magnitude from 0° to 1000°C. In contrast, they decrease slightly with increasing pressure.