Determination of •OH, O2•-, and Hydroperoxide Yields in Ozone Reactions in Aqueous Solution

Abstract

In ozone reactions in aqueous solutions, • OH and O 2 •- are often generated as short-lived intermediates and hydroperoxides are formed as labile or stable final products. Tertiary butanol reacts with ozone only very slowly but readily with • OH. In the presence of dioxygen, formaldehyde is a prominent final product, 30 ± 4%, whose ready determination can be used as an assay for • OH. Although dimethyl sulfoxide reacts much more readily with ozone, its fast reaction with • OH which gives rise to methanesulfinic acid can also be applied for the determination of • OH, at least in fast ozone reactions. The formation of O 2 •- can be assayed with tetranitromethane (TNM), which yields nitroform anion (NF - ) at close to diffusion-controlled rates. TNM is stable in neutral and acid solution but hydrolyzes in basic solution ( k = 2.7 M -1 s -1 ), giving rise to NF - plus nitrate ion (62%) and CO 2 plus 4 nitrite ions (38%). TNM reacts with O 3 ( k = 10 M -1 s -1 ), yielding 4 mol of nitrate (plus CO 2 ) and 4 mol of O 3 are consumed in this reaction. NF - reacts with O 3 ( k = 1.4 × 10 4 M -1 s -1 ) by O-transfer. The resulting products, (NO 2 ) 3 CO - and (NO 2 ) 2 C O, rapidly hydrolyze ( k > 10 s -1 ), and most of the nitrite released is further oxidized by ozone to nitrate. In the case of slow ozone reactions, these reactions have to be taken into account; i.e. the NO 3 - yield has to be measured as well. For the determination of hydroperoxides, Fe 2+ -based assays are fraught with considerable potential errors. Reliable data may be obtained with molybdate-activated iodide. The kinetics of this reaction can also be used for the characterization of hydroperoxides. Reactive hydroperoxides undergo rapid O-transfer to sulfides, e.g., k (HC(O)OOH + (HOCH 2 CH 2 ) 2 S] = 220 M -1 s -1, and the corresponding reaction with methionine may be used for their quantification (detection of methionine sulfoxide by HPLC). Distinction of organic hydroperoxides and H 2 O 2 by elimination of the latter by reaction with catalase can often be used with advantage but fails with formic peracid, which reacts quite readily with catalase ( k = 1.3 × 10 -3 dm 3 mg -1 s -1 ). Some examples of • OH and O 2 •- formation in ozone reactions are given.