Kinetics and Mechanism of the Oxidation of HSO3- by O2. 1. The Uncatalyzed Reaction

Abstract

The reaction between S(1V) and 0 2 in aqueous solution is of importance because of its involvement in flue gas desulfurization processes and in acid rain formation. Despite considerable research, a complete mechanism for the reaction has not been established. In this work a detailed study of the uncatalyzed reaction in the region of pH 4.5 has been carried out. The overall rate law was found to be -d[02]/dt = kobs [HS03-]2[H+]-2[02]o with kobs of 3.6 x lo6 M s-' at 25 "C and ionic strength 0.05 M. The yield of the intermediate S20?-formed duringthe reaction has been determined over the pH range 4.1-6.3. By comparison with the yield of S~0 7~-produced in the reaction of HSO3-with HS05-, it is concluded that S~0 7~-formed in the HS03--02 reaction comes from the reaction between HSO5-and HSO3-, and therefore HSO5-must be an intermediate in the HSO3--O2 reaction in this pH region. From the above yields it is concluded that 60% of the reaction proceeds through the intermediate HS05-. A detailed mechanism consistent with the overall rate law of the reaction has been proposed. A key feature is the proposal that the chain initiation occurs through the reaction of HSO5-with HSO3-to form S O iand Sod-. In order to study the individual rate laws for the initiation, propagation and termination reactions, a relaxation technique was applied. In these experiments, the reaction at a steady state rate was perturbed by suddenly introducing a change in the concentration of HSO3-or H+, and the relaxation of the reaction to the new steady state was recorded. Treatment of these experimental data not only confirmed the HSO3-dependence of the rates of initiation, propagation, and termination in the proposed mechanism but also gave the pH dependence of these reactions. A study of the remarkable inhibitory effect of methanol produced strong support for the proposed initiation reaction. The effect of the introduction of chain initiator S20g2-was also found to be consistent with the mechanism. Relative values of rate constants of reactions in the proposed mechanism have been evaluated from the present experimental data. Evaluation of absolute values of these rate constants requires one additional piece of experimental information, which, depending on the choice, yields a range of values. An arbitrary choice is used to give rough values of the rate constants for initiation, Propagation and termination and typical values for the steady state concentrations of chain carriers independent of the bisulfite concentration.

Bleaching and photobleaching of Orange II within seconds by the oxone/Co2+ reagent in Fenton-like processes

“…1) by adding different concentrations of Co 2+ -ions. The Orange II photobleaching traces present a trend similar to the trend observed in processes involving a radical chain branching mechanism [22,23]. Fig.…”

“…Oxone has been successfully used in dark and photochemical reactions [12] like: olefin epoxydations [13,14], chemiluminescence detection of oxidative processes [15] and diverse sulfate oxidation reactions [16]. Catalytic decomposition of oxone (PMS) in the presence of some semiconductors WO 3 , Bi 2 O 3 and Fe 2 O 3 [17,18] and metallic-ions like Mn 2+ [19], Cu 2+ [20] and Co 2+ [21,22] has also been recently reported.…”

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

“…Gupta et al [37] reported that radical mechanism operate in those reaction in which the inhibition parameters lies 10 3 -10 4 In the present study the value of inhibition parameter for uncatalysed and Ag(I) catalysed autoxidation of S(IV) by anilineare found to be in the range 10 3 -10 4 These values are in the same range as reported by Gupta et al [37] This is strongly support the radical mechanism in the present case too based on the observed results. The free radical mechanism is very complex, the present work close to the experimental condition of Connick et al [38]. …”

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

“…2), the value of 3.2 9 10 3 L mol -1 s -1 for k Fe at 30°C was found. Ethanol is a well-known inhibitor for sulfur(IV) autoxidation reactions catalyzed by iron(III) and other metal ions [11,[24][25][26][27]. In this study, the presence of ethanol led to the introduction of an induction period and decrease in the reaction rate, so much so that at 8.5 9 10 -3 mol L -1 ethanol, the reaction was completely seized ( [ethanol] was linear (Fig.…”