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

In ozone reactions in aqueous solutions, •OH and O2•- 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 O2•- 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 CO2 plus 4 nitrite ions (38%). TNM reacts with O3 (k = 10 M-1 s-1), yielding 4 mol of nitrate (plus CO2) and 4 mol of O3 are consumed in this reaction. NF- reacts with O3 (k = 1.4 × 104 M-1 s-1) by O-transfer. The resulting products, (NO2)3CO- and (NO2)2CO, 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 NO3- yield has to be measured as well. For the determination of hydroperoxides, Fe2+-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 + (HOCH2CH2)2S] = 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 H2O2 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 dm3 mg-1 s-1). Some examples of •OH and O2•- formation in ozone reactions are given.