The Reduction Potential of Benzyl Viologen: An Important Reference Compound for Oxidant/Radical Redox Couples

Bipyridinium compounds such as the viologens (V2+) have been used extensively as redox indicators, but radical dimerization leads to reduction potentials of the oxidant/radical one-electron couple, E(V2+/V2+) varying with radical concentration: a particular problem with benzyl viologen. Pulse radiolysis has been used to characterize the one-electron transfer equilibria between methyl (MV2+) or benzyl (BV2+) viologen and a 2-nitroimidazole (misonidazole), and between methyl and benzyl viologens directly. Total radical concentrations were typically approximately 0.4 mumol dm-3 to minimize dimerization. Based on E(MV2+/MV2+) = -450 mV, a value of E(BV2+/BV2+) = -374 mV is now recommended. The implications for other 'reference' redox couples are discussed.