The iron(III)-catalyzed oxidation of EDTA in aqueous solution

Abstract

At temperatures above 100 °C iron(III) oxidizes coordinated EDTA to ethylenediaminetriacetic acid in aqueous solution in the absence of molecular oxygen. The reaction proceeds with an activation energy of 28.6 kcal/mol, and its rate is directly proportional to the concentration of Fe(III) and inversely proportional to pH. At 125 °C, the halflife of Fe(III) in the presence of excess EDTA is about 3 h at pH 9.3, but increases to >70 h at pH 5.4. The reaction is stoichiometric and no other reaction products or by-products were detected by nmr, gc, and gc – mass spectroscopy. In the presence of oxygen iron catalyzes quantitative oxidation of ethylenediamine-N,N,N′,N′-tetraacetic acid (EDTA) to ethylenediaminetriacetic acid. The copper(II)–EDTA chelate undergoes a similar reaction but higher temperatures [Formula: see text] are required. Iron(III) also oxidizes nitrilotriacetic acid (NTA) to iminodiacetic acid (IDA) and glycine. The hydrolyzed species Fe(OH)EDTA is shown to be the reactive intermediate, and the well-known (Fe–EDTA)2O4− μ-oxo dimer is shown not to exist at elevated temperatures (above 100 °C). Probable mechanisms are proposed for these reactions and comparisons are made with earlier work.