PVT properties of concentrated aqueous electrolytes. III. Volume changes for mixing the major sea salts at I=1.0 and 3.0 at 25°C

The densities of mixtures of the six possible combinatons of the major sea salts (NaCl, Na₂SO₄, MgSO₄, and MgCl₂) were determined at constant ionic strengths of I=1.0 and I=3.0 at 25°C. The results are used to determine the volume changes for mixing (V _m ) the major sea salts. The values of V _m were fit to equations of the form V _m where y _i is the molal ionic strength fraction of solute i, and ₀ and ₁ are parameters related to the interaction of like-charged ions. The cross-square rule was found to hold at both ionic strengths. Density estimates were made without and with the addition of volume of mixing terms to Young's Rule and compared to the experimental values. The densities calculated with the addition of volume of mixing terms gave better estimates, demonstrating that the densities of concentrated brines can be more accurately estimated using V _m terms. The equations of Reilly and Wood which include the cross-square rule were used to estimate the densities of the cross mixtures (NaCl–MgSO₄ and MgCl₂–Na₂SO₄). The estimated densities agree with the measured values to within ±30 ppm at I=1.0 and ±125 ppm at I=3.0.