REACTION-MECHANISM OF THE IRON(III)-CATALYZED AUTOXIDATION OF BISULFITE IN AQUEOUS-SOLUTION - STEADY-STATE DESCRIPTION FOR BENZENE AS RADICAL SCAVENGER

The Fe(III)-catalyzed oxidation of bisulfite in aqueous solution was s tudied at pH approximate to 3 in the presence of benzene by monitoring the rise of hydrogen ion activity and phenol product concentration as a function of time, keeping [HSO3(-) ]/[Fe-tot]much greater than 1, A fter 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 e nd of the reaction, when most of the S(IV) was consumed, Fe(II) was re oxidized toward Fe(III). In two sets of experiments either the concent ration of benzene or that of [Fe(III)], was varied in ranges of 0.3 -2 .2 mmol dm(-3) and 8.5-51 mu mol dm(-3), respectively, while keeping t he other parameters constant ([HSO3-], = 2 mmol dm (-3)). The data wer e analyzed by means of a steady stare expression developed for the tim e period of nearly constant [Fe(II)]/[Fe(III)] ratio on the basis of a radical-driven chain oxidation mechanism presented previously. The da ta display the behavior predicted by the steady state equation; this a llowed the determination of ratios of rate coefficients for reaction p airs: SO4- reacting with HSO3- and benzene, k(11)/k(12)=0.31+/-0.04; S O5- reacting with Fe2+ and HSO3- k(3)/k(10) = 54.1+/-1.7; HSO5- reacti ng with Fe2+ and HSO3- k(4)/k(16) = 2.2+/-0.2; and the branching ratio for the reaction of SO5- with HSO3- leading to (a) SO3- and (b) SO4- radicals, k(10b)/k(10)less than or equal to 0.04. The experimental res ults largely support the proposed reaction mechanism.