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

The kinetics of reaction between AuCl4−and the azo-dye pyridine-2-azo-p-dimethylaniline (PADA) havebeen investigated in water and in the presence of DTAC micelles, using classical spectrophotometryand the stopped-flow technique. PADA reacts with different chloro/hydroxo gold(III) complexes, in turnformed as the pH and Cl−concentration were changed, according to a network of parallel paths. In aqueoussolution, at low pH values, a fast step is observed which is ascribed to the ligand induced expulsion ofa labile water molecule from the reacting species Au(H2O)Cl3which forms at low pH values. At highervalues of pH, the reaction is much slower because, in the key step, PADA has to replace the more inertCl−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 theformation of aquochloro aurates, thus inducing a change in the gold(III) speciation compared to that inwater. The analysis of the data suggests that the aquo species Au(H2O)Cl3and Au(H2O)2Cl2+play a majorrole in the reaction mechanism.