Mutual Diffusion Coefficients and Densities at 298.15 K of Aqueous Mixtures of NaCl and Na2SO4 with NaCl Solute Fractions of 0.9500, Trace Diffusion Coefficients of SO42- in NaCl(aq), and Trace Refractive Index Increments and Partial Molar Volumes of Na2SO4 and NaCl

Abstract

Isothermal mutual diffusion coefficients (interdiffusion coefficients) and refractive index increments were measured 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 were performed at seven total molarities ranging from (0.499 92 to 4.978 34) mol·dm-3. These measurements supplement our earlier ones at other compositions and composition fractions. The Na2SO4 cross-term diffusion coefficient D21 is small and negative at nearly all of the ternary solution compositions, whereas the NaCl cross-term diffusion coefficient D12 is larger and positive. However, at z1 = 0.9500, D21 changes sign as the total concentration exceeds 4.3 mol·dm-3, and above 3.2 mol·dm-3 the value of D12 exceeds that of the corresponding Na2SO4 main-term diffusion coefficient D22. Thus, at high concentratio...