Kinetics of the dissociation reaction of the cadmium (II)-N-hydroxyethylethylenediamine-triacetate complex in polarography

Abstract

The polarographic behaviour of the Cd(II) complex with N-hydroxyethylethylenediamine-triacetic acid (EDTA·OH, H 3X) has been studied in an acetate buffer medium of pH 3–4. Kinetic current, which is controlled by the rate of dissociation of the complex, decreases with the increase of pH and EDTA·OH concentration. It was found that the pH dependence is due to the fact that EDTA·OH is present mostly as H 2X −, but enters the complex as X 3−. The formation constant of the complex was calculated from the shifts of the half-wave potentials of the kinetic wave with the result of log K CdX = 13·6 ± 0·1 (at 25°C, μ = 0·1). The mechanism of the dissociation reaction at the surface of the dropping mercury electrode has been elucidated to be ▪ where ⇌ represents the very fast mobile equilibrium, ▪ the rate determining step, ⥮ el. and ↓ el. reversible and irreversible electron transfer processes, respectively. The rate constant of dissociation of the complex has been evaluated to be k d = 30·5 ± 0·8 sec −1 (at 25°C, μ = 0·1).