The effect of complexing agents on the DMF–chromium(VI) reaction. A kinetic study

Abstract

The kinetics of the reduction of chromium(VI) by dimethylformamide (DMF) in the presence of ethylenediaminetetraacetic acid (EDTA) and 2,2′-bipyridyl (bpy) was investigated in aqueous HClO4 acid solutions spectrophotometrically. The experimental findings, that the reaction has an induction period followed by autoacceleration, is explained. After the reduction, chromium(III) is present as the EDTA- and bpy-complexes, although such complexes form very slowly under the same experimental conditions from chromium(III) and the EDTA and bpy, respectively. Increases in the reaction rate with increasing [EDTA] were observed, while added bpy had negligible effect on the reaction rate. The reaction is first-order each in [CrVI] and [H+]. The first-order kinetics with respect to EDTA at low concentrations shift to zero-order at higher concentrations. The reaction is considered to proceed through the formation of a very stable chromium(VI)–DMF–EDTA complex. The suggested mechanism refers only to the induction period of the reaction. The net rate of oxidation of DMF, as measured by the consumption of chromium (VI), is given by: −d[CrVI]/dt = kK1Kb[H+][DMF]T[EDTA][CrVI]T/(1 + K1[EDTA])(1 + Kb[H+])