  An empirical model of the concentration dependence of the excess Gibbs free
energy Gex for saline solutions is proposed. Our simple analytical form of Gex
allows obtaining equations of state of saline solutions equally precise in the
whole range of the salt concentrations, from dilute solutions up to the limit
of solubility. Our equation for Gex includes one term responsible for
concentration dependence of Gex at low salt concentrations and two terms of
Margules type dependent on powers of mole fractions of the components. These
terms contain four parameters dependent on temperature and pressure. As an
example of application of the proposed model we took the system H2O-NaCl. For
fixed T and P, our four-parameter form of Gex allows precisely reproduce
experimental data for NaCl water solutions, including both regions of low and
high concentrations. An introduction of temperature dependence of the
parameters allowed us to build a precise analytic fit of experimental data on
osmotic coefficients of NaCl solutions for saturation pressures of NaCl at
temperatures 423.15K-573.15K. On the basis of PVTx data by Driesner (2007) the
model was extended towards pressures up to 5kbar. Our form of Gex allows
obtaining the molar volumes, activities of components and other thermodynamic
properties of NaCl-H2O solutions in the range of temperatures 423.15K-573.15K
and pressures from saturation up to 5kbar in the whole range of concentrations
possible for H2O-NaCl solutions (from zero up to approximately 10 molal at
573.15K).
