The water activities of MgCl2, Mg(NO3)2, MgSO4, and their mixtures

The water activities of aqueous MgCl2, Mg(NO3)(2), and MgSO4 solutions and their mixtures-MgCl2-Mg(NO3)(2) (mole ratio=3:1, 1:1, 1:3), MgCl2-MgSO4 (1:1), Mg(NO3)(2)-MgSO4 (1:1), and MgCl2,-Mg(NO3)(2)-MgSO4 (1:1:1 and 1:1:5)-were measured at room temperature from dilute solution to high supersaturation using an electrodynamic balance. The experimental results are compared with the predictions of the Zdanovskii-Stokes-Robinson (ZSR) and the Kusik and Meissner (KM) equations. Both the ZSR and the KM equations can predict the water activities of these mixtures well with an acceptable error of less than +/-0.02. The ZSR equation is superior to the KM equation as it can give predictions for a larger range of a(w) where single component bulk data are available. It is also slightly more accurate than the KM equation for this system. The average standard deviations of the difference between the data measured and the predictions of the ZSR and KM equations are 0.004 and 0.01, respectively. The ZSR equation is recommended for estimating the water activities of Mg salt solutions.