Publication costs assisted by Brookhaven National LabcratoryThe reduction of nitro blue tetrazolium (NBT2+) by COf and Of has been studied by the stopped-flow method and the pulse-radiolysis technique. Both reductants form the tetrazolinyl radical, and the rate constants ares-1 and h N B p + + 02-= (5.88 f 0.12) X lo4 M-I s-'. In the absence of other reactants, the tetrazolinyl radicals (NBT',) disappear by a pH-dependent second-order disproportionation reaction to produce monoformazan (MF') and nitro blue tetrazolium (NBT2+) ions. The tetrazolinyl radical (NBT+.) has an absorption maximum at 405 nm with a pH-independent molar extinction coefficient ~4 0 5~~= 15 000 f 350 M-' cm-l. Monoformazan (MF+) has an absorption maximum at 530 nm; its molar extinction coefficient varies from E~~~~~ = 25 400 f 1200 M-' cm-l at pH 9.5-11.0 to €53onm = 12 800 f 640 M-l cm-' at pH 5.7-6.7. Mechanisms for the overall reaction are discussed.

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