Mutual Diffusion Coefficients and Densities at 298.15 K of Aqueous Mixtures of NaCl and Na2SO4 at High Concentrations with NaCl Solute Fractions of 0.9000

Abstract

Isothermal mutual diffusion coefficients (interdiffusion coefficients) were measured for ternary aqueous mixtures of NaCl and Na2SO4 at a constant NaCl molarity fraction z1 = 0.9000 at 298.15 K, using high precision Rayleigh interferometry with computerized data acquisition. The experiments were performed at total molarities of (2.0000, 3.0003, 4.0010, and 5.0071) mol·dm-3. Diffusion coefficients of NaCl(aq) were also measured at essentially these same four concentrations, those of Na2SO4(aq) at (1.3500 and 1.6998) mol·dm-3, as were the densities of all solutions. These measurements supplement our earlier ones at total molarities of (0.5000, 1.00025, and 1.4990) mol·dm-3 with various ratios of NaCl to Na2SO4, which also included compositions with z1 = 0.9000. At nearly all of the ternary solution compositions, the Na2SO4 cross-term diffusion coefficient is small and negative whereas the NaCl cross-term diffusion coefficient is larger and positive. However, at the fixed NaCl molarity fraction z1 = 0.9000, the Na2SO4 cross-term diffusion coefficient changes from negative to positive values as the total concentration exceeds about 4.0 mol·dm-3, and above 3.0 mol·dm-3 the size of the NaCl cross-term diffusion coefficient exceeds that of the corresponding Na2SO4 main-term diffusion coefficient. This indicates that, at high concentrations, a concentration gradient of Na2SO4 causes the transport of more NaCl than of itself. In contrast, cotransport of Na2SO4 by a concentration gradient of NaCl is quite small.