A term coined by Michaelis to designate the chlorides of certain quaternary bases derived from γ,γ′-dipyridyl. Presently used as dichlorides, dibromides and diiodides. Prepn: Michaelis, Biochem. Z. 250, 564 (1932); Michaelis, Hill, J. Am. Chem. Soc. 55, 1481 (1933); J. Gen. Physiol. 16, 859-873 (1933). Viologens are useful as oxidation-reduction indicators because their potential range is very negative. In contrast to other redox indicators, the color is exhibited by the reduced form, whereas usually the oxidized form is the colored one and secondly, the redox potential of these substances is independent of pH. Review of electrochemistry: C. L. Bird, A. T. Kuhn, Chem. Soc. Rev. 10, 49-82 (1981).