Prediction of the thermodynamic properties of aqueous metal complexes to 1000 degrees C and 5 kb

A large number of aqueous metal complexes contribute significantly to hydrothermal metamorphic, and magmatic processes in the crust of the Earth. Nevertheless, relatively few thermodynamic data other than dissociation constants (K) for a few dozen of these complexes have been determined experimentally at elevated temperatures and pressures. The calculations summarized below are intended to supplement these experimental data by providing interim predictions of the thermodynamic properties of supercritical aqueous metal complexes using the revised HKF (Helgeson et al., 1981) equations of state for aqueous species (Tanger and Helgeson? 1988; Shock et al., 1992) and correlations among equations of state parameters and standard partial molal proper ties at 25 degrees C and 1 bar (Shock and Helgeson, 1988, 1990; Shock et al., 1989). These equations and correlations permit retrieval of the conventional standard partial molal entropies (<(S)over bar (0)>), volumes (<(V)over bar (0)>), and heat capacities (<(C)over bar (0)(P)>) of aqueous metal complexes at 25 degrees C and 1 bar from published values of log K in the supercritical region and the limited number of experimental dissociation constants available in the literature over relatively short ranges of elevated temperature at P-SAT (P-SAT and SAT are used in the present communication to refer to pressures corresponding to liquid-vapor equilibrium for the system H2O except at temperatures <100 degrees C, where they refer to the reference pressure of 1 bar). The standard partial molal properties computed in this way can then be used to generate corresponding values of <Delta(S)over bar (0)>, <Delta(V)over bar (0)>, and <Delta($)over bar (0)(P)> of association, which for similar complexes correlate linearly with <(S)over bar (0)>, <(V)over bar (0)>, and <(C)over bar (0)(P)>, respectively, of the constituent cations and ligands at 25 degrees C and 1 bar. Generalizing these correlations and combining them with the equations of state permits prediction of the temperature and pressure dependence of log K and other thermodynamic properties of a large number of aqueous metal complexes. As a consequence, it is possible to retrieve values of log K at 25 degrees C end 1 bar from the results of hydrothermal experiments at higher temperatures and pressures or to predict values of log K at hydrothermal conditions when no experimental data are available at temperatures and pressures above 25 degrees C and 1 bar. Such predictions can be made for temperatures and pressures from 0 degrees C and 1 bar to 1000 degrees C and 5000 bars. Copyright (C) 1997 Elsevier Science Ltd.