Kinetic aspects of the reaction of aquaethylenediaminetetraacetatoferrate(III) with cyanide ion

Abstract

The kinetics and mechanism of reaction between ethylenediaminetetraacetatoferrate(III) and cyanide ion at pH= 10.5, 25 °C and I=0.25 M have been investigated spectrophotometrically at 395 nm. The reaction consists of three observable stages. The first involves the formation of [Fe(C-N)5(OH)]3−, the second is the conversion of [Fe(CN)5(OH)]3− into [Fe(CN)6]3− and the third is its reduction to [Fe(CN)6]4− by oxidation of EDTA. The first reaction shows a variable order dependence on the concentration of cyanide, ranging from one at high cyanide concentration to three at low cyanide concentration. The second reaction exhibits first order dependence on the concentration of [Fe(CN)5(OH)]3− as well as in cyanide; while the third reaction follows over-all second order kinetics, first order each in [Fe(CN)6]3− and EDTA released in the reaction. This reaction is highly dependent on hydroxide ion concentration. The reverse reaction between [Fe(CN)5(OH)]3− and EDTA shows an inverse first-order dependence on cyanide concentration along with first-order dependence each on [Fe(C-N)5(OH)]3− and EDTA concentrations. A five-step mechanism is proposed for the first reaction.