Kinetic and Mechanistic Investigations on the Development of [Fe(CN)5OH]3– via the Interaction of [FeTMDTA(OH)]2– with CN– Ions

The mechanism and kinetics of [FeTMDTA(OH)]2–’s (TMDTA = trimethylenediaminetetraacetic acid) interaction with cyanide ions have been investigated using spectrophotometry at 393 nm (the λmax of [Fe(CN)5OH]3–) by monitoring a spike in absorbance. The specified conditions of reaction are; T = 298 K, ionic strength, I = 0.2 M (NaClO4), and pH 10.5. The findings indicate that the reaction comprises three distinct phases; the initial phase involves the generation of [Fe(CN)5OH]3–, the subsequent phase entails the transformation of [Fe(CN)5OH]3– into [Fe(CN)6]3–, and the final phase involves the interaction of [Fe(CN)6]3– with the ligand (TMDTA) released in the initial stage, resulting in the formation of [Fe(CN)6]4–. The interaction of [FeTMDTA(OH)]2– with CN– exhibits a variable order dependency on [CN–] during the initial stage, fluctuating between one to three at elevated and diminished [CN–]. The subsequent phase of the process exhibits a first-order reliance on both [CN–] and [Fe(CN)5OH3–]. The last stage of the reaction adheres to a general second-order kinetics, exhibiting first-order characteristics with respect to both [Fe(CN)5OH3–] and [TMDTA4–]. Studies have also examined the thermodynamically disadvantageous reversed reaction of [Fe(CN)5OH]3– with TMDTA4–, which is merely driven by a substantial excess of TMDTA4–. The reverse reaction demonstrates an inverse first-order reliance in [CN–] and first-order dependences in [TMDTA4–] and [Fe(CN)5OH3–]. The fourth step of the recommended mechanism, which is the rate-determining step, is further supported by the forward reaction rate’s dependency on ionic strength. The suggested mechanistic scheme is further substantiated by the activation parameters of both the inverse and forward reactions during the initial stage of the reaction.