Mutual Diffusion Coefficients and Densities at 298.15 K of Aqueous Mixtures of NaCl and Na_2SO_4 with NaCl Solute Fractions of 0.9500, Trace Diffusion Coefficients of SO_4~(2-) in NaCl(aq), and Trace Refractive Index Increments and Partial Molar Volu

摘要

Isothermal mutual diffusion coefficients (interdiffusioncoefficients) and refractive index increments were measured for ternary aqueous mixtures of NaCl and Na_2SO_4 at constant NaCl molarity fraction z_1 = 0.9500 at 298.15 K, using high precision Rayleigh interferometry with computerized data acquisition. The densities of these solutions were measured using vibratin tube densimetry. The experiments were performed at seven total molarities ranging from 90.499 92 to 4.978 34) mol·dm~(-3). These measurements supplement our earlier ones at other compositions and composition fractions. The Na_2SO_4 cross-term diffusion coefficient D_(21) is small and negative at nearly all of the ternary solutioncompositions, whereas the NaCl cross-term diffusion coefficient D_(12) is larger and positive. However, at z_1 = 0.9500, D_(21) changes sign as the total concentration exceeds 4.3 mol·dm~(-3), and above 3.2 mol·dm(-3) the value of D_(12) exceeds that of the corresponding Na_2SO_4 main-term diffusion coefficient D_(22). Thus, at high concentrations, a concentration gradient of Na_2SO_4 causes the transport of moreNaCl than of itself. At each fixed total concentration, extrapolationof na_2SO_4 main-term diffusion coefficients from these andour earlier results, to zero concentration fraction of Na_2SO_4 main-term diffusioncoefficients from these and our earlier results, to zero concentraction fraction of Na_2SO_4 (i.e., z_1 —> 1), yielded the trace diffusion coefficient of the SO_4~(2-) ion D~(tr)(SO_4~(2-)) in NaCl(aq). Comparing these D~(tr)(SO_4~(2-)) values in NaCl(aq) to published D~(tr)(SO_4~(2-)) values in Na_2SO_4(aq) shows agreement to 1-8%, when the comparisons are made at equal volumetric ionic strength. Similar extrapolations of the volumetric and refractive index increment results yielded the trace partial molar volume and trace refractive index increment, respectively. Densities of NaCl(aq) at 298.15 K were critically reanalyzed.