Equilibrium and kinetic studies on the N-(2-hydroxyethyl)ethylenediamine-NN′N′-triacetatochromium(III) complex in aqueous perchlorate media

Abstract

Acid dissociation constants for the chromium(III) complex of N-(2-hydroxyethyl)ethylenediamine-NN′N′-triacetic acid (H₃TOH) have been determined by potentiometric titration. The three species involved are [Cr^III(TOH)-(H₂O)], [Cr^III(TOH)(OH)]^–, and [Cr^III(TO)(OH)]^2– and at 25 °C, I= 0·1M(KNO₃) : pk₁= 6·02 ± 0·05 and pk₂= 9·85 ± 0·10. When the pH is greater than ca. 13·1 de-chelation of the ligand from the metal ion occurs, and the kinetics of two consecutive slow steps, giving [Cr^III(TO)(OH)₂]^3– and [Cr^III(TO)(OH)₃]^4– respectively, have been studied by conventional spectrophotometry. There is no significant hydrogen-ion dependence and values of the rate constants at 25 °C, I= 1·0M(NaClO₄) are k₃= 1·7 × 10^–3 and k₄= 2·0 × 10^–4 s^–1. A slower concurrent process, k₃₄= 2·0 × 10^–5 s^–1 at 25 °C, has also been studied.