Kinetics of complexation of uranium with 2-(5-Bromo-2-pyridylazo)-5-(diethylamino) phenol (Br-PADAP) in a 1:1 water-ethanol medium

2-(5-Bromo-2-pyridylazo)-5-(diethylamino) phenol (Br-PADAP) forms a 1: 1 complex with the uranyl ion in the presence of sulphosalicylic acid, which acts as stabilizer for this complex in the triethanol amine/perchloric acid buffer system. A change in the stoichiometry of the complex was seen at pH < 5. Kinetic measurements were carried out using stopped-flow spectrophotometer in the presence of an excess concentration of U( VI) in the pH range 6.5 to 8. The dependence of the pseudo-first-order rate constant, k(obs), on the concentrations of U(VI), ligand and hydrogen ion showed that Br-PADAP reacts with UO2(OH)(+) to form an intermediate species ( equilibrium constant = 1.28 x 10(4) mol . dm(-3)) that then rearranges ( rate constant = 5.6 x 10(-2) s(-1)) to form the product species. UO2(OH)(+) is present in equilibrium with the unreactive species UO2(OH)(2), as well as with the unreactive sulfosalicylic acid complex.