Acid-catalyzed interfacial complexation in the extraction kinetics of palladium(II) with 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol

The extraction kinetic mechanism of palladium(II) in acidic aqueous solution 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-flow method. The rate-determining step in the extraction at pH<3 was the complexation between the Pd(II) complexes, PdCl(n−2)−n (n=2–4), and the protonated 5-Br-PADAP adsorbed at the interface. The rate constants at the interfacial complexation were the same order of magnitude as those in the aqueous phase. The extraction rate was accelerated by a decrease in pH causing an increase in the interfacial concentration of the protonated 5-Br-PADAP. The acid-catalyzed interfacial reaction revealed the advantageous property of 5-Br-PADAP; it could be adsorbed at the 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.