Mutual Diffusion Coefficients and Densities at 298.15 K of Aqueous Mixtures of NaCl and Na2SO4for Six Different Solute Fractions at a Total Molarity of 1.500 mol·dm-3and of Aqueous Na2SO4

Isothermal mutual diffusion coeffs. (interdiffusion coeffs.) were measured for ternary aq. mixts. of NaCl and Na2SO4 at a const. total molarity of 1.500 mol·dm-3 at 298.15 K, using Rayleigh interferometry with computerized data acquisition. The expts. were performed at NaCl molarity fractions of z1 = 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 coeffs. were also measured at three addnl. concns. of Na2SO4(aq). Densities of all solns. were measured with a vibrating tube densimeter. At all ternary soln. compns., one cross-term diffusion coeff. has small neg. values whereas the other has larger pos. values. Comparing the results at 0.500, 1.000, and 1.500 mol·dm-3 shows that both main-term diffusion coeffs. decrease with increasing total concn. at any fixed value of z1, whereas both cross-term coeffs. are shifted in a pos. direction. At 1.500 mol·dm-3 both the NaCl main-term and cross-term diffusion coeffs. have a max. as a function of z1, whereas the Na2SO4 cross-term coeff. has a min. Trace diffusion coeffs. D*(Cl-) = (1.145 ± 0.02) × 10-9 m2·s-1 and D*(SO42-) = (0.805 ± 0.015) × 10-9 m2·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.