The kinetics of gold(III) extraction by pyridine-2-azo‑p‑dimethylaniline in water and in micellar systems

Abstract

Abstract The kinetics of reaction between AuCl 4 − and the azo-dye pyridine-2-azo- p -dimethylaniline (PADA) have been investigated in water and in the presence of DTAC micelles, using classical spectrophotometry and the stopped-flow technique. PADA reacts with different chloro/hydroxo gold(III) complexes, in turn formed as the pH and Cl − concentration were changed, according to a network of parallel paths. In aqueous solution, at low pH values, a fast step is observed which is ascribed to the ligand induced expulsion of a labile water molecule from the reacting species Au(H 2 O)Cl 3 which forms at low pH values. At higher values of pH, the reaction is much slower because, in the key step, PADA has to replace the more inert Cl − ions in the gold coordination shell. In the presence of DTAC a remarkable catalytic effect is observed, owing to the absorption and attraction of the reactants on the micelle surface. Moreover, DTAC favors the formation of aquochloro aurates, thus inducing a change in the gold(III) speciation compared to that in water. The analysis of the data suggests that the aquo species Au(H 2 O)Cl 3 and Au(H 2 O) 2 Cl 2 + play a major role in the reaction mechanism.