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) wherek2andk3 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)]2-, respectively,K2is the protonation equilibrium constant of [H Fe(CN)6]3-, andKeq is the equilibrium constant for tribromide ion formation. The values of k2 and k3 were evaluated as (1.7 ± 0.1)× 105and (3.3 ±0.4)×103dm 3mol-1s-1, respectively. A mechanism consistent with the rate law is postulated in which the first step is formation of the Br2&z.rad;- intermediate.