The water activities and osmotic coefficients have been determined at 25℃, over the range of concentrations from diluted up to saturated with respect to the solute of NaCl for the synthetic salt lake brine Li-Na-K-Mg-Cl-SO_4-H_2O systems by isopiestic method using an improved apparatus. The two synthetic brine stock solutions were prepared respectively in accordance with the ionic strength fractions of the main ion components of Yi Li-Ping and Dong Tai. salt lake interstitial brines on Qinghai-Tibet plateau of China. The thermodynamic properties were compared between the two brines, and a cause of formation of the differences in concentrations between the two brines has been indicated physicochemically. The calculated osmotic coefficients using the ioninteraction model of the salt lake brine systems agree with the experimental data obtained in this work, with standard deviations of 0. 0078 nad 0.0145 for Yi Li-Ping and Dong Tai. synthetic brines respectively. The saturation indices [ln (K'/K)] were calculated for the salt components which may crystallize out from the brines for Dong Tai. interstitial brine and its concentrated brines at different evaporation stages in the isothermal evaporation-crystallization expeirment at 25 ℃, over the range of ionic strengths from 9.766 to 20.52 mol·kg~(-1). The calculated results show the change rules in saturation indices of the salts in the brines during the evaporating of the brine and agree reasonably with the field observation result and with the salt sequences of crystallizations from the brine in the evaporation experiment. These studies demonstrate well the applicability of the model of the salt lake brine systems extended from Pitzer ion-interaction model for the predictions of the thermodynamic properties and the mineral sequences to the salt lake brine systems containing lithium with high ionic strengths.

Key words: LITHIUM SULFURIC ACID, SALT LAKES, BRINE, OSMOTIC COEFFICIENT, SODIUM SULFATE, POTASSIUM SULFATE, LITHIUM CHLORIDE, POTASSIUM CHLORIDE, MAGNESIUM CHLORIDE, THERMODYNAMIC PROPERTIES