Derivation of the Freundlich Equation for Ion Exchange Reactions in Soils

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Abstract
The Freundlich adsorption isotherm equation is derived rigorously for the trace adsorption of an ion participating in an exchange reaction. The derivation, which is an application of some general results obtained by R. Sips, is based on the assumption that the exchanger surface is heterogeneous and that each class of exchange sites adsorbs individually accordingly to the Langmuir isotherm. It is shown that, under these conditions, the Freundlich isotherm corresponds uniquely to a distribution of relative adsorption site affinities which is essentially log normal and that the empirical parameters in the Freundlich equation may be used to characterize the site distribution function mathematically, thus giving information about surface heterogeneity in the exchanger.
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