Mutual diffusion coefficients and densities at 298.15 K of aqueous mixtures of NaCl and Na2SO4 for six different solute fractions at a total molarity of 1.500 mol•dm-3 and of aqueous Na2SO4

Isothermal mutual diffusion coefficients (interdiffusion coefficients) were measured for ternary aqueous mixtures of NaCl and Na2SO4 at a constant total molarity of 1.500 mol.dm(-3) at 298.15 K, using Rayleigh interferometry with computerized data acquisition. The experiments were performed at NaCl molarity fractions of z(1) = 1, 0.90, 0.75, 0.50, 0.25, and 0. These measurements supplement our earlier results at total molarities of 0.500 and 1.000 mol.dm(-3). Diffusion coefficients were also measured at three additional concentrations of Na2SO4(aq). Densities of all solutions were measured with a vibrating tube densimeter. At all ternary solution compositions, one cross-term diffusion coefficient has small negative values whereas the other has larger positive values. Comparing the results at 0.500, 1.000, and 1.500 mol.dm(-3) shows that both main-term diffusion coefficients decrease with increasing total concentration at any fixed value of z(1), whereas both cross-term coefficients are shifted in a positive direction. At 1.500 mol.dm(-3) both the NaCl main-term and cross-term diffusion coefficients have a maximum as a function bf z(1), whereas the Na2SO4 cross-term coefficient has a minimum. Trace diffusion coefficients D*(Cl-) = (1.14(5) +/- 0.02) x 10(-9) m(2).s(-1) and D*(SO42-) = (0.80(5) +/- 0.01(5)) x 10(-9) m(2).s(-1) were extrapolated from these results for the Cl-(aq) ion in 1.500 mol.dm(-3) Na2SO4(aq) and for the SO42-(aq) ion in 1.500 mol.dm(-3) NaCl(aq). Values of D*(Cl-) in Na2SO4(aq) and in NaCl(aq) were found to be essentially identical, as were D*(SO42-) in these same two electrolytes, provided the comparisons are made at the same volumetric ionic strengths.