ACID-CATALYZED INTERFACIAL COMPLEXATION IN THE EXTRACTION KINETICS OFPALLADIUM(II) WITH 2-(5-BROMO-2-PYRIDYLAZO)-5-DIETHYLAMINOPHENOL

The extraction kinetic mechanism of palladium(II) in acidic aqueous so lution with 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PADAP) in toluene was investigated by the high-speed stirring method, along with the complexation kinetics in the aqueous phase by the stopped-how method. The rate-determining step in the extraction at pH<3 was the c omplexation between the Pd(II) complexes, PdCln(n-2-) (n=2-4), and the protonated 5-Br-PADAP adsorbed at the interface. The rate constants a t the interfacial complexation were the same order of magnitude as tho se in the aqueous phase. The extraction rate was accelerated by a decr ease in pH causing an increase in the interfacial concentration of the protonated 5-Br-PADAP. The acid-catalyzed interfacial reaction reveal ed the advantageous property of 5-Br-PADAP; it could be adsorbed at th e interface by protonation at the diethylamino group under the acidic condition of pH<2, but did not lose reactivity to the metal ion since its pyridyl group was not protonated. (C) 1998 Elsevier Science B.V.