Isopiestic Investigation of the Osmotic and Activity Coefficients of {yMgCl2+(1−y)MgSO4}(aq) and the Osmotic Coefficients of Na2SO4⋅MgSO4(aq) at 298.15 K

Abstract

Isopiestic vapor-pressure measurements were made for {yMgCl2+(1−y)MgSO4}(aq) solutions with MgCl2 ionic strength fractions of y=(0,0.1997,0.3989,0.5992,0.8008, and 1) at the temperature 298.15 K, using KCl(aq) as the reference standard. These measurements for the mixtures cover the ionic strength range I=0.9794 to 9.4318 mol⋅kg−1. In addition, isopiestic measurements were made with NaCl(aq) as reference standard for mixtures of {xNa2SO4+(1−x)MgSO4}(aq) with the molality fraction x=0.5000 that correspond to solutions of the evaporite mineral bloedite (astrakanite), Na2Mg(SO4)2⋅4H2O(cr). The total molalities, m T=m(Na2SO4)+m(MgSO4), range from m T=1.4479 to 4.4312 mol⋅kg−1 (I=5.0677 to 15.509 mol⋅kg−1), where the uppermost concentration is the highest oversaturation molality that could be achieved by isothermal evaporation of the solvent at 298.15 K. The parameters of an extended ion-interaction (Pitzer) model for MgCl2(aq) at 298.15 K, which were required for an analysis of the {yMgCl2+(1−y)MgSO4}(aq) mixture results, were evaluated up to I=12.075 mol⋅kg−1 from published isopiestic data together with the six new osmotic coefficients obtained in this study. Osmotic coefficients of {yMgCl2+(1−y)MgSO4}(aq) solutions from the present study, along with critically-assessed values from previous studies, were used to evaluate the mixing parameters of the extended ion-interaction model.