KINETICS AND MECHANISM INTRINSIC TO ELECTROREDUCTION OF THE PALLADIUM(II) GLYCINATE COMPLEXES AT THE DROPPING MERCURY-ELECTRODE

The kinetics of electroreduction at the dropping mercury electrode (DM E) of the Pd(gly)(2) chelate complexes that are present in acid soluti ons is studied. The complexes reduce irreversibly, and the half-wave p otential E-1/2 is independent of the solution pH (2.8-5.7) and the con centration of free glycine, c(gly), in the presence of various support ing electrolytes (NaF, Na2SO4, NaClO4). Upon going from acidic to alka line (pH 9.5-11.7) solutions, the irreversible wave of reduction of th e palladium(II) glycinate complexes shifts in the direction of more ne gative potentials, and the diffusion coefficient diminishes. In alkali ne solutions, E-1/2 is independent of pH and c(gly), but shifts in the negative direction with the growing concentration of the supporting e lectrolyte. The conclusion is drawn that (1) there are no preceding ch emical reactions and (2) the slow electrochemical stage involves adsor bed chelate complexes Pd(gly)(2) in acidic solutions and nonchelate co mplexes Pd(gly)(4)(2-) in alkaline solutions. Reduction of adsorbed pa lladium(II) complexes is inhibited by the supporting electrolyte anion s that become adsorbed in the region of positive charges of DME studie d. The inhibiting effect of the anions grows in the sequence F-<SO42-C lO4-. The effect is explained as follows: the adsorbed anions prevent the stretching of the metal-ligand bonds in adsorbed plane complexes o f palladium(II) localized at the mercury electrode surface, which prec edes the electrochemical stage.