Kinetics and mechanism of the multi-step oxidation of ethylenediaminetetraacetate by [Ag(OH)4]– in alkaline media

Abstract

Alkaline solutions of [Ag(OH)₄]^– oxidise ethylenediaminetetraacetate (edta) in a multistep sequence which produces iminodiacetate (ida), glycinate and glycolate ions, and eventually ammonia, formaldehyde, carbon dioxide and water. The reaction shows changing stoichiometry and product distribution. At [edta]₀/[Ag(OH)₄^–]₀= 1:1, glycinate and ida are produced in similar amounts with detectable ammonia and formaldehyde. When [edta]₀/[Ag(OH)₄^–]₀= 200:1, very little glycinate is produced with no ammonia or formaldehyde detected. The kinetics of the oxidation was studied by the stopped-flow method at high excesses of edta and 25°C and I= 1.2 mol dm^–3. The overall rate is as in equation (i), where the coefficients, obtained by non-linear least-square analysis, are A=k_obs=(A+B[OH^–])[edta^4–]//_{[OH^–]+C[edta^4–]}(i) 4.38 ± 0.57 s^–1, B= 34.8 ± 1.1 dm³ mol^–1 s^–1 and C= 26.3 ± 2.1. A multi-step electron-transfer mechanism is proposed involving initial C–N bond cleavage at the ethylenediamine group.