Direct spectrophotometric measurements of acid-catalyzed complexation of palladium(II) with 2-(5-bromo-2-pyridylazo)5-diethylaminophenol at the heptane/water interface by a centrifugal liquid membrane method

The centrifugal liquid membrane (CLM) method, designed for the rapid sample injection, was applied to the kinetic study of the complexation of palladi um(II) with 2-(5-bromo-2-pyridylazo)-5-diethyaminophenol (5-Br-PADAP) in th e heptane/water system. The formation rates of Pd(II)-5-Br-PADAP complex, w hich existed only at the heptane/water interface, could be directly measure d by the CLM method combined with transmission spectrophotometry. We found that the formation rates of Pd(II)-5-Br-PADAP complex were accelerated by t he protonation of 5-Br-PADAP at the diethylamino-group that did not coordin ate to Pd(II) ion and that the rate constant for the reaction of protonated 5-Br-PADAP at the interface was close to that in the aqueous phase. The pr esent study demonstrated that the CLM method was easily applicable for the measurements of relatively fast interfacial reactions.