Kinetics and mechanism of the reaction between 1,2-diaminopropanetetra-acetatoferrate(III) and cyanide ion

Abstract

The present study examines the kinetics and mechanism of the system [FePDTA(OH)] 2− + 5CN − ⇌ [Fe(CN) 5OH] 3− + PDTA 4− at pH= 11.0±0.02, I= 0.25 M and temperature = 25 ± 0.1 °C. The reaction has been studied spectrophotometrically at 395 nm (λ max of [Fe(CN) 5OH] 3−). The data show that the reaction has three distinguishable stages; the first stage is formation of [Fe(CN) 5OH] 3−, the second is conversion of [Fe(CN) 5OH)] 3− to [Fe(CN) 6] 3− and last is reduction of [Fe(CN) 6] 3− to [Fe(CN) 6] 3− by the released ligand, viz., PDTA. The first reaction shows variable order dependence on cyanide concentration, one at high cyanide concentration and two at low cyanide concentration. The second reaction exhibits first order dependence on the concentration of [Fe(CN) 5OH] 3− as well as cyanide. The reverse reaction between [Fe(CN) 5OH] 3− and PDTA is first order in [Fe(CN) 5OH] 3− and PDTA, and inverse first order in cyanide. On the basis of forward and reverse rate studies, a five-step mechanism has been proposed for the first reaction.