KINETICS OF ISOTOPIC EXCHANGE OF PHOSPHATE ADSORBED ON GIBBSITE

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 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_N1 dissociation or S_N2 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.