Isopiestic studies of thermodynamic properties

The osmotic coefficients were determined for LiCl-H_2O,Li_2SO_4-H_2O and LiCl-Li_2SO_4-H_2O at 273.15K by the isopiestic method established for low temperatures using an improved equipment in the molality ranges of 0.5-9.2 mol kg~(-1),0.3-2.5 mol kg~(-1) and in the ionic strength range of 0.5-9.5 mol kg~(-1) respectively.The experimental iso-water activity lines obey the Zdanovskii rule for non-ideal mixtures well with the standard deviation of 0.008.The relations among experienced isopiestic molalities obey the extended Zdanovskii rule with the standard deviation of 0.0027 in the high ionic strength range in which the Li_2SO_4 pure solutions became saturation and were taken away from the isopiestic measurements.The parameters of Pitzer ion-interaction model were evaluated from the measured data in this work and the data in the references by using multiple regression analsis.The standard deviations between the calculated and the experimental osmotic coefficients were reported.These studies reveal that the osmotic coefficients increase with increasing of ionic strengths and with decreasing of temperatures,and those decrease with increasing of ionic concentration fractions of Li_2SO_4,and indicate Pitzer ion-interaction model can describe the thermodynamic properties for the studied systems in the wider molality ranges at low temperature.