Metal–aminopolycarboxylic acid complexes. Part V. Kinetic study of the dissociation of cadmium complexes of N-carboxymethyliminobis(ethylenenitrilo-N′N′-diacetic acid) by a polarographic method

Abstract

The polarographic behaviour of Cd²⁺ in the presence of excess of N-carboxymethyliminobis(ethylenenitrilo-N′N′-diacetic acid)(H₅ceda) has been studied over the pH range 2–7. Three different polarographic waves can be distinguished. The wave occurring at the most negative potentials and highest pH values is assigned to an irreversible reduction of [Cd(Hceda)]^2–[equation (i)], where k_c= 5·4 × 10⁴ l mol^–1s^–1 and k_–c= 3·6 s^–1. The wave occurring [Cd(Hceda)]^2– [graphic omitted] [Cd(ceda)]^3–+ H⁺(i) at intermediate potentials and pH values arises by irreversible reduction of [Cd(H₂ceda)]^– formed in reaction (ii)[Cd(H₂ ceda)]^– [graphic omitted] [Cd(Hceda)]₂+ H⁺(ii) where k_B=(1·1 ± 0·9)× 10⁴ l mol^–1 s^–1 and k_–B= 4 ± 3 s^–1. The reversible wave occurring at the most positive potentials and lowest pH values arises by reduction of aquated Cd²⁺ formed in reaction (iii), where Cd²⁺+ H₃ceda^2– [graphic omitted] [Cd(H₂ceda)]^–+ H⁺(iii)k_A=(2·5 ± 2·0)× 10⁵and K_–A=(2·0 ± 1·6)× 10⁸ l mol^–1 s^–1. The stability constant log K_Cd(ceda)= 18·8; this and the rate constants are conditional constants obtained at an ionic strength of 0·2M and at 25° C. The proton-assisted dissociation of the complex is interpreted as stepwise unwrapping from one end of the molecule with cadmium–nitrogen fissions corresponding to the rate-determining steps.