Kinetics and Mechanism of the Permanganate Ion Oxidation of Sulfite in Alkaline Solutions. The Nature of Short-Lived Intermediates

Article abstract of DOI:10.1021/ja00300a023

The oxidation of sulfite by permanganate ion has been studied by the stopped-flow technique combined with rapid scan spectrophotometry.The overall reaction involves manganate(VI) as an intermediate, whose fate depends on the pH.The formation and disappearance of manganate(VI) represent two distinct phases: (1) reduction of permanganate to manganate(VI) via outersphere electron transfer; (2) slower disproportionation of manganate(VI) to permanganate and a soluble manganese(VI) product.The direct oxidation of sulfite by manganate(VI) is too slow to compete with the permanganate route.Manganate(V) has been detected for the first time by rapid scan spectrophotometry as an intermediate of manganate(VI) disproportionation.The first phase obeys overall second-order kinetics, the rate constant being pH-independent between 13 and 9.5.Disproportionation is second order in manganate, and the observed rate constant increases with decreasing pH.