Uranyl-catalyzed chemiluminescent reaction of U4+ oxidation by dioxygen in aqueous HClO4 solution

Abstract

Chemiluminescence (CL) accompanying the reaction of U4+ with O2 in 0.0004–0.1M HClO4 was studied. It was found that the electron-excited uranyl ion (UO22+)* is the CL emitter. The fact that the reaction rate and the CL yield increase as the solution acidity decreases was explained by different reactivities of the Uaq4+ aquation and the products of its stepwise hydrolysis, UOH3+ and U(OH)22+, toward O2. Based on the results of analysis of the chain-radical mechanism of the reaction between U4+ and O2, it was concluded that transfer of an electron from the UO2+ ion to the oxidizing agent (a ·OH radical) is the most plausible elementary step of the reaction of (UO22+)* formation. It was found that the reaction rate, as well as the CL yield, increase substantially in the presence of uranyl ion. Catalytic action of UO22+ was explained by the formation of a UO22+·UO2+ complex, which reduces the rate of the UO2+ disproportionation reaction (UO2+ is an intermediate of the reaction and is involved in chain propagation), and by regeneration of the active center, UO2+, in the reaction of UO22+ with U4+.