Isothermal diffusion coefficients for sodium chloride-magnesium chloride-water at 25.degree.C. 1. Solute concentration ratio of 3:1

Isothermal interdiffusion coefficients have been obtained by Gouy interferometry for the ternary system NaCl-MgCl{sub 2}-H{sub 2}O at 25.00{degree}C and at a 3:l molarity ratio of NaCl to MgCl{sub 2}. Data are reported for total molar concentrations of 0.5, 1 .0, 2.0, 3.0, and 3.79 M. One cross-term diffusion coefficient, D{sub 12}, becomes greater than all the other diffusion coefficients above 3.2 M. This shows that at higher concentrations a flow of NaCl resulting from a concentration gradient of MgCl{sub 2} will be greater than the flow resulting from the same concentration gradient of NaCl itself. Also, a concentration gradient of MgCl{sub 2} will cause a greater flow of NaCl than of MgCl{sub 2} itself. Densities were measured for all the solutions; they were used to calculate partial-molal volumes of all components in the solutions and to interconvert reference frames for diffusion.