Kinetic study of Ni(II) and Zn(II) complexation with a pyridylazo extractant by a centrifugal liquid membrane method Original Research Article

The centrifugal liquid membrane method, which provides an ultra-thin two phase liquid membrane system in a rotating glass cell, was successfully applied for the kinetic study of the liquid–liquid extraction of nickel(II) and zinc(II) with 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PADAP) in toluene. The initial extraction rate of the metal(II) complexes depended upon the concentrations of the metal(II) ion and ligand. The rate determining step was revealed as 1:1 chelate formation of Ni(II) or Zn(II) with 5-Br-PADAP adsorbed at the liquid–liquid interface. The interfacial complexation rate constants of Ni(II)–5-Br-PADAP and Zn(II)–5-Br-PADAP were determined as (4.1±0.1)×10 and (2.4±0.6)×103 M−1 s−1 at pH 6.50±0.02, respectively.