| Abstract
| - Isothermal mutual diffusion coefficients (interdiffusion coefficients) and refractive index increments weremeasured for ternary aqueous mixtures of NaCl and Na2SO4 at constant NaCl molarity fraction z1 =0.9500 at 298.15 K, using high precision Rayleigh interferometry with computerized data acquisition.The densities of these solutions were measured using vibrating tube densimetry. The experiments wereperformed at seven total molarities ranging from (0.499 92 to 4.978 34) mol·dm-3. These measurementssupplement our earlier ones at other compositions and composition fractions. The Na2SO4 cross-termdiffusion coefficient D21 is small and negative at nearly all of the ternary solution compositions, whereasthe NaCl cross-term diffusion coefficient D12 is larger and positive. However, at z1 = 0.9500, D21 changessign as the total concentration exceeds 4.3 mol·dm-3, and above 3.2 mol·dm-3 the value of D12 exceedsthat of the corresponding Na2SO4 main-term diffusion coefficient D22. Thus, at high concentrations, aconcentration gradient of Na2SO4 causes the transport of more NaCl than of itself. At each fixed totalconcentration, extrapolation of Na2SO4 main-term diffusion coefficients from these and our earlier results,to zero concentration fraction of Na2SO4 (i.e., z1 → 1), yielded the trace diffusion coefficient of the SO42-ion Dtr(SO42-) in NaCl(aq). Comparing these Dtr(SO42-) values in NaCl(aq) to published Dtr(SO42-) valuesin Na2SO4(aq) shows agreement to 1−8%, when the comparisons are made at equal volumetric ionicstrength. Similar extrapolations of the volumetric and refractive index increment results yielded thetrace partial molar volume and trace refractive index increment, respectively. Densities of NaCl(aq) at298.15 K were critically reanalyzed.
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