Wavelength controlled production of SO₄^- radicals in the ultraviolet photolysis of ceric sulfate solutions

Abstract

Solutions of cerium(III)/(IV) and formic acid in 0.4 M sulfuric acid have been photolysed under 254 nm and 365 nm light. Marked differences in the reaction kinetics and quantum yields are observed at the two different wavelengths. At 365 nm, the reactions leading to cerium(IV) reduction are caused almost exclusively by the SO₄- radical. The ratio of rate constants, k(SO₄^- + Ce^III)/ k(SO₄^- + HCOOH), is 116 ± 11 and the quantum yield of sulfate radicals, ф(SO₄^-), is 0.023 ± 0.002. At 254 nm, the reactions leading to cerium(IV) reduction are caused mainly by the OH radical, but approximately 35% of the oxidizing radicals formed in the primary photochemical reaction are SO₄^-. Cerium(III) species, excited at 254 nm, transfer energy to cerium(IV) and this results in an additional yield of OH and SO₄^- radicals. Fluorescence measurements confirmed the efficiency of the energy transfer reaction. The ratio of rate constants, k(OH+Ce^III)/k(OH+HCOOH), is 2.22 ± 0.18 and ф(Ce^IV*) and ф(Cel^III*) giving oxidizing radicals are 0.116 ± 0.010 and 0.0083 ± 0.0008 respectively. Thus about 5 times more total oxidizing radicals are produced from excited cerium(IV) species at 254 nm than at 365 nm.