Thermodynamic study of the NaCl+MgCl2+H2O mixed system by emf measurements at different temperatures

Mean activity coefficients of NaCl in NaCl+MgCl2+H2O mixed system were determined from the emf measurements of galvanic cells without transport and at 15, 25, 35 and 45 degrees C. The measurements were carried out at total ionic strengths of 0.1, 0.3, 0.5, 1, 2, 3, 4 and 6 mol.kg(-1) and at ionic strength fractions for MgCl2 between 0 and 0.8 mol.kg(-1). These activity coefficient values were analysed by using the Harned rule and the treatments of Scatchard-Rush-Johnson and Fitter. From these, it can be deduced that binary ionic interactions are only significant in this mixed system and hence triple ionic interactions are negligible (except for the mixture of 6 mol.kg(-1) for which these high-order interactions probably cannot be considered irrelevant). Moreover, a decrease in the ion-ion interaction (and therefore an increase in the ion-solvent interaction) takes place by the simultaneous presence of Na+ and Mg2+ ions in the mixture, specially when the ionic strength increases, probably as a consequence of the larger hydration of the Mg2+ ions. Finally, the excess properties of the mixtures were determined and qualitatively analysed against the composition of the mixed system.