Kinetics and mechanism of ligand substitution reaction of the Zn(II) chelate of 2-(5-bromo-2-pyridylazo)-5-(N-propyl-N-sulfopropylmino)phenol and ethylenedlamine-N,N,N’,N’-tetraacetic acid

　The kinetics and mechanism of ligand substitution reaction of zinc(II) chelate of 2-(5-bromo-2-pyridylazo)-5-(N-propyl-N-sulfopropylamino) phenol (Zn(II)-PAPS) and ethylenediamine-N,N,N’,N’-tetraacetic acid (EDTA) have been studied spectrophotometrically by a stopped-flow technique in the pH range of 6.8-8.8 at the ionic strength 0.1(NaClO₄) and 25.0℃. The reaction order with respect to Zn(II)-PAPS was of first order while that for EDTA varied from first to zero with increasing EDTA concentration. The exeperimental results indicate that the substitution reaction proceeds through two intermediates, the partially dissociated Zn(II)-PAPS chelate and the mixed ligand complex. At low EDTA concentration, the rate-determining step is the dissociation of PAPS from the mixed ligand intermediate, whereas at high EDTA concentration, the formation of the partially dissociated intermedite becomes the rate-determining step.