Reaction Mechanism of the Iron(III)‐Catalyzed Autoxidation of Bisulfite in Aqueous Solution: Steady State Description for Benzene as Radical Scavenger

Abstract

AbstractThe Fe(III)‐catalyzed oxidation of bisulfite in aqueous solution was studied at pH = 3 in the presence of benzene by monitoring the rise of hydrogen ion activity and phenol product concentration as a function of time, keeping [HS03−/[Fetot]>>l. After a brief initial period, during which product concentrations rose rapidly and Fe(III) was largely converted toward Fe(II), the reaction underwent a period of slower growth characterized by a quasistationary state and nearly constant [Fe(II)]/[Fe(III)] ratio until toward the end of the reaction, when most of the S(IV) was consumed, Fe(II) was reoxidized toward Fe(III). In two sets of experiments either the concentration of benzene or that of [Fe(III)]o was varied in ranges of 0.3–2.2 mmol dm−3 and 8.5–51 μmol dm−3, respectively, while keeping the other parameters constant ([HSO3−]o = 2 mmol dm−3). The data were analyzed by means of a steady state expression developed for the time period of nearly constant [Fe(II)]/[Fe(III)] ratio on the basis of a radical‐driven chain oxidation mechanism presented previously. The data display the behavior predicted by the steady state equation; this allowed the determination of ratios of rate coefficients for reaction pairs: SO4− reacting with HSO3− and benzene, k11/k12 = 0.31±0.04; S05− reacting with Fe2+ and HSO3−, k3/k10 = 54.1±1.7; HSO5− reacting with Fe2+ and HSO3−, k4/k16 = 2.2±0.2; and the branching ratio for the reaction of SO5− with HSO3− leading to (a) SO3− and (b) SO4− radicals, k10b/k10≤0.04. The experimental results largely support the proposed reaction mechanism.