Kinetics of Isotopic Exchange of Phosphate Adsorbed on Gibbsite

Summary An Elovich‐type equation has been used to describe the kinetics of isotopic exchange of phosphate adsorbed on the surface of gibbsite. The equation is image where A and B are parameters; θ= bF /[b+ a (I‐ F ]; a and b are the molar concentrations of the phosphate on the crystal surface and in solution respectively and F is the fraction exchange of the radio‐isotope at time t. First‐order rate constants were obtained from the equation. The reference state for the first‐order rate constants, and the distribution of activation energies, for exchange can be related to the Elovich equation parameters. The kinetic results are consistent with S N 1 dissociation or S N 2 bimolecular solvolysis for the phosphate ligand. The eschange reaction is subject to an acid‐base catalysis the exact nature of which could not be determined from the available data.