Abstract
The kinetics of desorption have been calculated numerically for a semi-infinite body with a strongly concentration-dependent diffusivity and a surface resistance which limits the flux into the gas phase. The chosen values of the parameters were similar to those involved in experiments with glass melts. It is shown that the relation between total flux and time can be described by the equation for a system with constant diffusivity, even though no similarity solution exists. The fit is not perfect but discrepancies are small and lie within the range of probable experimental errors. The parameters (surface resistance and diffusivity) giving the best fit of the constant property model are best considered as empirical curve fitting parameters because they can have different significances over the range of conditions covered; the apparent diffusivity is sometimes lower than applies anywhere within the system. A reliable test of whether diffusivity is concentration-dependent requires measurement of concentration distribution or, at least, interface concentration even when data are available for a range of initial concentrations.
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