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(II) 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 dm^–3, oxygen 0.029–0.22 mmol dm^–3 and cobalt 0.20–25.3 µmol dm^–3. 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 3.4–22 and cobalt 0.01–0.05 µmol dm^–3.

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: –d[SO^2–₃]//_dt=k[SO^2–₃]^1.5[Co²⁺]^0.5[O₂]⁰//_{1+k′/[Co²⁺]}. The activation energy in the low-concentration range was found to be 183 kJ mol^–1. Added hydroquinone inhibited the reaction strongly and a chain mechanism is proposed involving reaction via a series of cobalt complex intermediates.