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

Isothermal mutual, diffusion coefficients (interdiffusion coefficients) wer e measured for ternary aqueous mixtures of NaCl and Na2SO4 at a constant Na Cl molarity fraction z(1) = 0.9000 at 298.15 K, using high precision Raylei gh 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 supp lement our earlier ones at total molarities of(0.5000, 1.0002(5), and 1.499 0) mol.dm(-3) with various ratios of NaCl to Na2SO4, which also included co mpositions with z(1) = 0.9000. At nearly all of the ternary solution compos itions, the Na2SO4 cross-term diffusion coefficient is small and negative w hereas the NaCl cross-term diffusion coefficient is larger and positive. Ho wever, at the fixed NaCl molarity fraction z(1) = 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 c orresponding Na2SO4 main-term diffusion coefficient. This indicates that, a t high concentrations, a concentration gradient of Na2SO4 causes the transp ort of more NaCl than of itself In contrast, cotransport of Na2SO4 by a con centration gradient of NaCl is quite small.