Vincent Post has incorporated the Millero equations for calculating the density of a solution in PHREEQC 2.16 (see release.txt). The figure compares the calculated density of evaporating seawater with measured data from Baseggio (1974) and Brantley et al. (1984), cited by Chambers et al. (1993). The agreement is quite good upto halite saturation and perhaps beyond (PHREEQC input file dens, uses database Pitzer.dat extended with the Millero equations distributed with Phreeqc for Windows).
The calculated electrical conductivity of the evaporated solution is plotted on the secondary Y-axis. The calculated EC is accurate initially, but probably less so when the SO42- concentration increases above 0.1 mol/L where gypsum starts to precipitate (see PHREEQC's calculation of the specific conductance).
References
Chambers, L.A., Wadsley, M.W. and Brereton, G.J., 1993. Modelling halite formation and brine densities: comparison of non-marine and seawater brines. Seventh Symp. Salt, 1, 533-538, Elsevier.
Millero, F. J., 2000. The equation of state of lakes. Aquatic Geochem. 6, 1-17.
Millero, F. J., 2001. The physical chemistry of natural waters, Wiley and Sons, 680 p.
Millero, F. J., Lawson, D. and Gonzalez, A., 1976. The density of artificial river and estuarine waters, J. Geophys. Res. 81, 1177–1179.