Elovich Equation for the Kinetics of Isotopic Exchange Reactions at Solid–Liquid Interfaces

Abstract

FE(111) phosphate complexes are formed on the surfaces of goethite (α-FeOOH) crystals suspended in dilute aqueous orthophosphate solutions. The rates of isotopic exchange reactions, in which phosphate ligands on the crystal surface exchange with free phosphate ions from the surrounding solution, can be readily measured by conventional techniques, using a small quantity of ³²P-labelled orthophosphate solution added to the equilibrium suspension. No satisfactory method is, however, available to interpret kinetic data for isotopic exchange in heterogeneous systems (two phases). Equations describing diffusion of the isotopic species in the solid phase, used when the extent of fast surface-exchanging reactions is small compared with the amount of the slower diffusion-controlled exchange involving the bulk of the solid^1–3, are not relevant here because phosphate ions react only with the Fe(111) ions exposed at the goethite surfaces. No measurable diffusion of phosphate ions into the bulk of the crystals was observed.