Systematic design of chemical oscillators. Part 56. Kinetics and mechanism of the oxidation of hexacyanoferrate(II) by aqueous bromine

Abstract

The kinetics and mechanism of the reaction between aqueous bromine and hexacyanoferrate(II) ion have been studied in the pH range 0.4–2.5. The stoicheiometry of the reaction is as in equation (i). Br₂(aq)+ 2[Fe(CN)₆]^4–→ 2Br^–+ 2[Fe(CN)₆]^3–(i) The predominant ferrocyanide species in this range are [HFe(CN)₆]^3– and [H₂Fe(CN)₆]^2–. The reaction is first order in the concentrations of both bromine and ferrocyanide. At 25.0 ± 0.1 °C and ionic strength 0.5 mol dm^–3(NaClO₄) the expression for the rate of appearance of [Fe(CN)₆]^3– is (ii) where k₂ and k₃ are the bimolecular rate constants for the reaction of bromine with [HFe(CN)₆]^3–½d[Fe(CN)₆^3–]/dt=(k₃+k₂K₂/[H⁺])[Br₂][Fe(CN)₆^4–]/(1 +K_eq[Br^–])(ii) and [H₂Fe(CN)₆]^2–, respectively, K₂ is the protonation equilibrium constant of [H Fe(CN)₆]^3–, and K_eq is the equilibrium constant for tribromide ion formation. The values of k₂ and k₃ were evaluated as (1.7 ± 0.1)× 10⁵ and (3.3 ± 0.4)× 10³ dm³ mol^–1 s^–1, respectively. A mechanism consistent with the rate law is postulated in which the first step is formation of the Br₂˙^– intermediate.