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

Isopiestic vapor-pressure measurements were made for {yMgCl(2) + (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 {xNa(2)SO(4) + (1-x)MgSO4}(aq) with the molality fraction x = 0.5000 that correspond to solutions of the evaporite mineral bloedite (astrakanite), Na2Mg(SO4)(2).4H(2)O(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 {yMgCl(2) + (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 {yMgCl(2) + (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.