Oxidation of luminol by chlorine dioxide. Formation of 5-aminophthalazine-1,4-dione

Abstract

The pH-dependent reaction rate between luminol and chlorine dioxide has been determined. The net oxidation reaction proceeds in two steps. The first of these is the formation of a luminol–ClO₂ adduct. This adduct, which is unreactive towards oxygen, reacts rapidly with itself or with a second ClO₂ molecule. The final product has a broad absorbance between 450 and 650 nm. An identical absorbance is produced when luminol is oxidized by HOCl. The species displaying this absorbance is very reactive towards OH^– and undissociated luminol. In the presence of hydrogen peroxide and at pH > 7 the absorbance disappears. At the same time an intense chemiluminescence is observed, the decay of which is quantitatively in agreement with the decay of the 450–650 nm absorbance. From these findings it is concluded that the oxidation product of luminol with ClO₂ is 5-aminophthalazine-1,4-dione.