Kinetics and mechanism of the cobalt-catalysed reaction of oxygen and sulphite at very low concentrations

Abstract

The reaction between oxygen and sulphite in the presence of cobalt(1r) as catalyst has been studied at the low concentrations which are of interest in relation to water treatment processes employed in the ' waterflood ' secondary oil-recovery technique. A continuous-flow apparatus was used to study reaction times of 0.1-1.7 s using iodine titration of the sulphite to follow the reaction, and initial concentrations of sulphite 0.35-1.13 mmol dmP3, oxygen 0.029-0.22 mmol dm-3 and cobalt 0.20-25.3 pmol drnp3. A static method was used to measure sulphite as a function of time at lower concentrations, using the fuschin-formaldehyde method. Reaction times were 2-60 min, with initial concentrations of sulphite 10-62, oxygen 3.422 and cobalt 0.01-0.05 pmol dmP3.In both concentration ranges the reaction showed an induction period during which little or no reaction occurred, followed by an almost linear decay in sulphite concentration. The orders of reaction with respect to sulphite and oxygen were 1.5 and 0 over the whole concentration range studied. However, the order with respect to cobalt increased from 0.5 to 1.5 on moving to the lower concentrations, and a general rate equation is developed to express the experimental rate behaviour :The activation energy in the low-concentration range was found to be 183 kJ mol-l. Added hydroquinone inhibited the reaction strongly and a chain mechanism is proposed involving reaction via a series of cobalt complex intermediates.

Kinetics of Catalytic Oxidation of Sulfite in Diluted Aqueous Solutions

“…These simplistic models do not account for an induction period, which was confirmed by experiments [6,17].…”

“…Contrary to these widely accepted reaction orders, under homogeneous conditions the following values are consistently reported [5,6]: zero order in oxygen, 1.5 order in sulfite, and 0.5 order in the catalyst. Differences in the mechanism between the two experimental setups are underlined by the activation energies reported.…”

“…The sulfite oxidation kinetics was studied by a steady-state rapid mixing method [5,6]. A scheme of the apparatus is presented in Fig.…”

“…Sulfite oxidation kinetic data at stoichiometric excess of sulfite under homogeneous conditions have been reported by several groups [5,6]. Probably due to poor reproducibility, Chen and Barron [5] do not provide the value of the kinetic constant.…”

Lesões em DNA induzidas pela autoxidação de S(IV) na presença de íons metálicos de transição

“…Portanto, o radical 10 não deveria ter caráter catiônico em DNA a pH neutro, não sendo esperado que se hidrate, produzindo finalmente 8-oxodA (15). O radical 10 reage com agentes redutores para regenerar adenina 85 .…”