Thermodynamic Properties of Ternary Aqueous Solutions with the Common Magnesium Cation {Mg/Cl/NO₃/SO₄}(aq) at T = 298.15 K

In this investigation, ternary aqueous electrolytes of chloride, nitrate, and sulfate with the common magnesium cation have been studied using the hygrometric method at 298.15 K. The water activities of the ternary systems { y MgCl 2 + (1 − y )MgSO 4 }(aq) and { y Mg(NO 3 ) 2 + (1 − y )MgSO 4 }(aq) are measured at total molalities from 0.35 mol·kg -1 to saturation for different ionic strength fractions ( y ) of MgCl 2 or Mg(NO 3 ) 2, respectively, with y = 0.20, 0.50, and 0.80. The obtained data allow the deduction of osmotic coefficients of the solvent. From these measurements, mixing ionic parameters are determined and used to predict the solute activity coefficients in the mixture, using Pitzer's model. The parameters θ MN and ψ MNX are evaluated for these systems. The thermodynamic properties of the ternary aqueous systems nitrate, chloride, or sulfate with magnesium, {Mg/Cl/NO 3 /SO 4 }(aq), are compared with the data obtained for the system [MgCl 2 + Mg(NO 3 ) 2 ](aq) already published in a previous work. 4 The comparison shows that the hydration is affected by the nature of the solute, MgSO 4, MgCl 2, or Mg(NO 3 ) 2, in the mixed aqueous solutions. These effects might be influenced by ion pairing, with no contact in Mg(NO 3 ) 2 (aq) and with ion association in MgSO 4 (aq).