Effects of different sized concentration differences across free diffusion boundaries and comparison of Gouy and Rrayleigh diffusion measurements using NaCl?KCl?H2O

Diffusion was systematically studied in the ternary system NaCL (0.5M)-KCl (0.5M)-H2O at 25°C. There were four purposes. First, current methods to ext. Dij from Gouy and Rayleigh interferometry data depend on treating the Dij as effectively const. If concn. differences ΔCi across the boundary are large, this may not be true. To explore this issue, four sets of expts. were performed. Each set had four expts. with approx. the same total no. of fringes J. Each set also had the same corresponding ΔC1/ΔC2 ratios as the other three, but the ΔCi were adjusted such that the four sets had J ≈ 30, 60, 90, and 120, resp. No clear dependence of the Dij on ΔCi was found within their realistic errors. Second, the Gosting diffusiometer can yield both Rayleigh and Gouy fringe patterns during the same expt. Therefore, the Dij from both methods were compared, and agree well. Third, a new method for analyzing Gouy fringe positions (Miller; program TNY) can be compared to the classical one (Fujita-Gosting; program rFG). The Dij from both analyses agree well. Fourth, the authors compared results from the Gosting diffusiometer with those at the same compn. from other diffusiometers; one data set by O'Donnell and Gosting from an older Gouy app., and three Rayleigh sets at different J values from the authors' older Model H diffusiometer. Results from older diffusiometers were more scattered, but corresponding Dij agree within realistic errors. As reported previously, realistic errors are approx. four times the statistical errors obtained by least squares. Recommended Rayleigh and Gouy Dij are presented for this compn.