Ternary mutual diffusion coefficients and densities of the system {z1NaCl +(1 –z1)Na2SO4}(aq) at 298.15 K and a total molarity of 0.5000 mol dm–3

Abstract

Isothermal ternary mutual diffusion coefficients (interdiffusion coefficients) have been measured for the system {z₁NaCl +(1 –z₁)Na₂SO₄}(aq) at a constant total molarity of 0.5000 mol dm^–3 and 298.15 K using either Rayleigh or Gouy interferometry. Measurements were performed at NaCl molarity fractions of z₁= 1, 0.90, 0.75, 0.50, 0.25 and 0. Densities of all solutions used in the diffusion experiments were measured using pycnometers and/or a vibrating densimeter. Trace diffusion coefficients have been evaluated from these results for the Cl^–(aq) ion in 0.5 mol dm^–3 Na₂SO₄(aq) and for the SO₄^2–(aq) ion in 0.5 mol dm^–3 NaCl(aq). The resulting values are D*(Cl^–)=(1.681 ± 0.002)× 10^–9 m² s^–1 and D*(SO₄^2–)=(0.900 ± 0.006)× 10^–9 m² s^–1, respectively. At all compositions, (D₂₁)_v, the cross-term diffusion coefficient of Na₂SO₄ due to a concentration gradient of NaCl, was found to be negative, whereas (D₁₂)_v, the cross-term diffusion coefficient of NaCl due to a concentration gradient of Na₂SO₄, was found to be positive.