The isopiestic molalities and the water activities have been determined at 273.15 K for the Li₂B₄O₇-LiCl-H₂O system by isopiestic method, and the vapor pressures and the osmotic coefficient have been calculated from the measurements. The experimental data have been represented by the different forms of the ion interaction model based on the primary Pitzer model of osmotic coefficient. The Pitzer ion interaction parameters were evaluated from re-cent measurements by using multiple regression and iterative methods. The activity coefficients of LiCl and Li₂B₄O₇ for the system Li₂B₄O₇-LiCl-H₂O were calculated by the ion interac-tion model parameterized at low concentrations. The experimental data of osmotic coefficients measured and calculated by using the Pitzer model are in reasonable agreements. These studies are useful for improving the thermodynamic model for salt lake brine complex systems containing lithium and borate.

Key words: Li₂B₄O₇-LiCl-H₂O system, isopiestic method, osmotic coefficient, Pitzer model