Gamma radiolysis of Cu(II) iminodiacetates in aqueous solution

Abstract

Neutral aqueous solutions of 1:1 and 1:2 copper(II) iminodiacetates (Cu^II-IDA and Cu^II(IDA)₂) were irradiated with ⁶⁰Co-gamma rays both in the presence and in the absence of various scavengers. The reducing radicals, e.g., e⁻ _aq, H, CO⁻ ₂ and CH₂OH were found to reduce the metal ion in both 1:1 and 1:2 complexes producing corresponding Cu(I) species which were found to be easily oxidizable by oxygen. If oxygen is present in the system, the superoxy radicals are initially formed which have been found to attack the metal chelates to produce Cu(I) species which is, however, found to be remarkably stable. This extraordinary stability has been ascribed as due to formation of Cu(II)-superoxy complexes. The reaction of the hydroxyl radical with the complexes leads to the degradation of the ligand yielding glycine, formaldehyde and glyoxalic acid. It is argued that two types of carbonyl compounds could arise from two different modes ofOH attack; while the formation of CH₂O was held due to OH attacking the metal centre, the formation of CHO.COOH was assumed to arise from the fraction of OH attacking the ligand. The rate constants for the reactions of OH with Cu^II-IDA and Cu^II(IDA)₂, leading to the formation of formaldehyde were determined through competition kinetic studies as (9.5 ± 0.5) × 10⁸ and (6.5 ± 0.5) × 10⁸ M⁻¹ s⁻¹ respectively. The rate constant for the reaction of OH with Cu^II(IDA)₂, where glyoxalic acid is formed was found to be (4.0 ± 0.2) × 10⁸ M⁻¹ s⁻¹