Isothermal diffusion coefficients for sodium chloride-magnesium chloride-water at 25°C. 3. Low magnesium chloride concentrations with a wide range of sodium chloride concentrations

Isothermal interdiffusion coeffs. have been measured by Gouy interferometry for the system NaCl(1)-MgCl2(2)-H2O at 25 °C. Diffusion coeffs. have been obtained for 5 sets of total mean concns. of C1 + C2 = 0.536, 1.053, 2.05, 3.05, and 3.80 mol dm-3, for which the concns. C2 of MgCl2 are kept small. The main-term diffusion coeffs. D11 and D22 vary modestly from their values at infinite diln. as the NaCl concn. is increased. However, the cross-term diffusion coeff. is D12, Which relates the coupled flow of NaCl to the concn. gradient of MgCl2, increases sharply as the total concn. increases. It becomes even larger than both main-term diffusion coeffs. at higher NaCl concns. Thus, a gradient of MgCl2 will cause a greater flow of NaCl than will the same gradient of NaCl itself. Also a gradient of MgCl2 will cause NaCl to diffuse faster than MgCl2 itself. When combined with diffusion data for other salt ratios, yield the tracer-diffusion coeff. of Mg2+ in NaCl solns. Densities were also obtained in this study. These diffusion and d. data are part of a systematic study of this system at LLNL.