Thermodynamic Properties of Aqueous Mixtures of Magnesium and Ammonium Sulfates

The mixture { y MgSO 4 + (1 − y )(NH 4 ) 2 SO 4 }(aq) has been studied with the hygrometric method at the temperature 298.15 K. The relative humidities of this system are measured at total molalities from 0.2 mol·kg -1 to about saturation for different ionic-strength fractions of MgSO 4 with y = 0.2, 0.5, and 0.8. The results allow the deduction of new water activities and osmotic coefficients. The experimental results are compared with the predictions of the ECA law (extended composed additivity) and the Zdanovskii−Stokes−Robinson (ZSR), Pitzer, and LS II models. From these measurements, the new Pitzer mixing ionic parameters are determined and used to predict the solute activity coefficients in the mixture.