A kinetic and mechanistic study of oxidation of arsenic(III) by hexacyanoferrate(III) in aqueous ethanoic acid

The kinetics of oxidation of As^IIIby Fe(CN)₆^3– has been studied spectrophotometrically in 60% AcOH–H₂O containing 4.0moldm^–3HCl. The oxidation is made possible by the difference in redox potentials. The reaction is first order each in [Fe(CN)₆^3–] and [As^III]. Amongst the initially added products, Fe(CN)₆^4– retards the reaction and As^Vdoes not. Increasing the acid concentration at constant chloride concentration accelerates the reaction. At constant acidity increasing chloride concentration increases the reaction rate, which reaches a maximum and then decreases. H₂Fe(CN)₆^–, is the active species of Fe(CN)₆^3–, while AsCl₅^2– in an ascending portion and AsCl₂⁺ in a descending portion are considered to be the active species of As^III. A suitable reaction mechanism is proposed and the reaction constants of the different steps involved have been evaluated.