Isothermal diffusion coefficients for sodium chloride-magnesium chloride-water at 25°C. 2. Low concentrations of sodium chloride with a wide range of magnesium chloride concentrations.

Isothermal ternary interdiffusion coeffs. have been measured by Gouy interferometry for the system NaCl-MgCl2-H2O at 25°. The mean concn. of NaCl, C1, was 0.08M for the lowest total concn. and 0.115M for the rest of the expts. The total mean concns. of NaCl and MgCl2, C1 + C2, for this set of expts. were 0.5, 1.0, 2.0, 3.0, and 3.79 mol dm-3. With increasing total concn., D11 for NaCl decreases sharply, whereas the D22 for MgCl2 first increases and then moderately decreases. Surprisingly, the main-term diffusion coeffs. D11 and D22 cross each other at about 0.8 mol dm-3. The cross-term diffusion coeff. D12 remains fairly small. However, the other cross-term diffusion coeff. D21 increases to about three-fifths the value of D11 at higher concns. When combined with diffusion data for other salt ratios, extrapolation of D11 at const. C1 + C2 to C1 = 0 will give the tracer-diffusion coeff. of Na+ in MgCl2 solns. D. data are reported for all solns. used in this study.