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Reconsideration of the diaphragm cell method of measuring diffusion coefficients.

University of British Columbia

The diaphragm cell technique for measurement of diffusion coefficients has been subjected to detailed analysis with the particular view to obtaining measurements on the various solutions involved in solvent extraction of uranium and its compounds. Consideration of types of cells used previously has led to an improved diaphragm cell design. An apparatus accommodating and magnetically stirring six cells simultaneously was designed and constructed. The result of errors of analysis of solutions on the diffusion coefficients has been determined as a function of the experimental conditions. The effect that volume changes occurring in real solutions during diffusion have on the value of the diffusion coefficient D, as computed by the standard logarithmic formula, was considered. Computations carried out in a first approximation showed that although the error committed by the neglect of volume changes is appreciably larger than estimated by previous workers it is not large enough to account for the full amount of some discrepancies reported in the literature. A numerical procedure is given for pursuing the analysis in a second and a third approximation involving decreasing dependence on the simplifying assumptions always made in previous calculations using this technique. This procedure can be programmed on an electronic computer. It is proposed that in situ concentration determinations at the beginning and end of a diffusion experiment be used based on comparison with an outside standard. Combined with the numerical method suggested this will allow considerable reduction in the concentration difference between the two solutions in the cell without significant loss in accuracy or increase in experimental time, and also make the measured diffusion coefficient a more "true" value.

Text

2012-01-12

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http://hdl.handle.net/2429/40037

Chemical and Biological Engineering

University of British Columbia

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