SIMPLIFIED PROCEDURES FOR THE ESTIMATION OF THE REVISED-HKF EQUATION OF STATE COEFFICIENTS

The revised-HKF (Helgeson-Kirkham-Flowers) equation of state describes the Gibbs free energy of aqueous electrolytes at elevated temperatures and pressures as a function of standard state Gibbs free energy, standard state entropy, temperature, pressure, and seven species-dependent coefficients. Published procedures for estimation of these coefficients are based on the remarkable correlations among the equation of state coefficients. Unfortunately, many of the procedures are complex and using them efficiently requires considerable familiarity with the theoretical derivation of the revised-HKF equation of state. These algorithms can be greatly simplified and I will present some simplified procedures for the estimation of the revised-HKF equation of state coefficients. But, before we can use empirically derived equation-of-state coefficients with confidence we must consider the following concerns: (1) Does the empirical generation of the equation of state coefficients surpass our ability to extrapolate meaningfully from available experimental data? (2) How sensitive is the Gibbs free energy at elevated temperatures and pressures to errors in estimated coefficients? and (3) What are the cumulative effects of these errors when they are used to determine chemical equilibrium states in geochemical systems? While these concerns must be fully addressed, it is encouraging that we can generate equation of state coefficients using simple empirical procedures. On a practical level, simplified algorithms provide a quick, straightforward estimation of the equation-of-state coefficients that are required to calculate Gibbs free energies at elevated temperatures and pressures. On a less immediately practical level, the simple relationships between the equation of state coefficients suggest an underlying order to thermodynamic data which may provide new insights into the fundamental physical chemistry of geochemical systems and I will also present some thoughts on these insights.