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)63–]/dt=(k3+k2K2/[H+])[Br2][Fe(CN)64–]/(1 +Keq[Br–])(ii) and [H2Fe(CN)6]2–, respectively, K2 is the protonation equilibrium constant of [H Fe(CN)6]3–, and Keq 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.