Experimental study of dissociation of HCl from 350 to 500°C and from 500 to 2500 bars: Thermodynamic properties of HCl°(aq)

Abstract

New values of the dissociation constants of HCl° referring to low density supercritical solutions and near-critical temperatures of water (350–500°C and 500–2500 bars) have been obtained based on comparison of AgCl(s) solubility in NaCl or KCl solutions with Ag(s) solubility in HCl + NaCl or KCl solutions at controlled hydrogen fugacities. During the course of this study the thermodynamic properties of AgCls− were refined with the aid of the revised HKF equation of state (Tanger and Helgeson, 1988). The dissociation constants of HCl° obtained in the present work are higher than that found from electrical conductance measurements (Frantz and Marshall, 1984) by more than an order of magnitude at pressures of about 500 bars, but the difference becomes smaller as the pressure increases. Both sets of dissociation constants agree well at high pressures where the isothermal compressibility of water is less than about 1.42·10−4 bar−1.To reliably compare experimental data obtained by different methods, the Redlikh-Kwong equation of state was applied to available literature data as well as to the results of this study. Finally, the standard state thermodynamic properties and HKF parameters for HCl°(aq) were established. These results allow extrapolation of the thermodynamic properties of HCl°(aq) and consequently the HCl dissociation constant up to 700°C and 5000 bars.