Kinetics of iron(III)-catalyzed autoxidation of sulfur(IV) in acetate buffered medium

Abstract

The kinetics of the environmentally important oxidation of sulfur(IV) by oxygen in acetate buffered medium in the presence of Fe(III) and the pH range 5.27-5.70 has been studied. The results were in agreement with the rate law:The role of iron(III) appears to be that of production of SO 3 -radicals in Fe(III)SO 3 2-complex by an internal 1-equivalent redox reaction. Subsequently, a radical mechanism involving oxysulfur radicals, viz., SO 3 -, SO 4 -, and SO 5 -operates. Addition of ethanol leads to the introduction of an induction period and decrease in reaction rate, most likely due to scavenging of SO 4 -radicals. The value of apparent energy of activation is 45.4 kJ mol -1 .

Kinetics of the uninhibited and ethanol-inhibited CoO, Co2O3 and Ni2O3 catalyzed autoxidation of sulfur(IV) in alkaline medium

“…The experimental procedure was exactly the same as described earlier [22,23] and is briefly given here. All chemicals used were of reagent grade and their solutions were prepared in double distilled water.…”

“…For this purpose, 10 cm 3 of buffer made from Na 2 HPO 4 (0.067 mol L -1 ) and KH 2 PO 4 (0.067 mol L -1 ) for alkaline medium, and 10 cm 3 of buffer made from CH 3 COONa (1.0 mol L -1 ) and CH 3 COOH (1.0 mol L -1 ) for acidic medium were used (total volume 100 cm 3 ) for obtaining the desired pH. The kinetics were followed by withdrawing the aliquot samples periodically and titrating the unreacted S(IV) iodometrically in slightly acidic medium as described earlier [23]. The reproducibility of the replicate measurements was generally better than ±10%.…”

“…Had the reaction occurred through a radical mechanism involving oxysulfur radicals, the reaction rate would be first order in [S(IV)] because the radical mechanisms proposed in several uncatalyzed [1] and Fe(III)-catalyzed studies [8,17,23] require a first order dependence in [S(IV)]. This rules out the radical mechanism.…”

“…The inhibition is caused mainly through scavenging of SO 4 -by inhibitors such as ethanol and benzene [17]. In this regard, the inhibition parameter, C, provides some insight about the mechanism [10,[14][15][16][17][18]23]. In alkaline medium, C has a high value (Table 1) for both CoO and Co 2 O 3 catalyses.…”

The Inhibitive Action of Aniline on the Autoxidation of Sodium Sulfite in Acidic Medium

“…Both non radical and radical mechanism have been proposed. The atmospheric oxidation of SO 2 by O 2 in aqueous media has been the subject of numerous studies, and the subject matter of several reviews, monographs and papers, notably by Kuo et al [1][2][3][4][5][6][7][8][9][10][11]. It is interesting to point out that reaction is strongly inhibited by alcohol, benzene, and other compounds.…”