Interfacial versus aqueous phase reaction sites in the solvent extraction of metal ions: The ambaguity of correlation plots

Correlation plots of rate constant versus a non-kinetic parameter such as limiting extractant interfacial area or extractant distribution coefficient have been used in the literature as a means of establishing the locale of the rate-determining step in metal-solvent extraction processes. A critical analysis of this approach is presented by considering the pertinent interfacial and acqueous phase equilibria as well as kinetics. It is shown that when the adsorption density of the extractant is below its saturation value, the adsorption density and the distribution coefficient of the extraction reagent are interrelated in such a way that in terms of the relevant mathematical expressions, an interfacial reaction can mimic a bulk aqueous phase reaction and vice verse. Thus, in order to meaningfully access the relative importance of aqueous phase and interfacial reaction sites, it is imperative that the extractant distribution and adsorption data be available over a wide range of extractant concentration.