ABSTRACT
This chapter considers dyes containing single azo groups. Certain azo dyes in this present chapter contain chromophores in addition to their azo group, for example, alcian yellow also contains thiazole chromophores, and Janus green is an azo–azine dye. Azo dyes used as biological stains exist predominantly as their trans isomers, and are shown as such below. Most basic azo dyes carry quaternary nitrogen substituents, and so are cationic under most staining conditions. Such cationic charges are usually delocalized, but the Kekule structures used below do not show this, only depicting extreme electron distributions. A related issue is the acid strength of the hydroxy group in hydroxyazo dyes. As a necessary preliminary the azo-hydrazone tautomerism in such compounds is illustrated. Ionization also strongly influences dye hydrophilicity/lipophilicity, and hence staining patterns. This is so even with nominally nonionic dyes such as those used for partition-based lipid staining, as was suggested by Michaelis as long ago as 1901.
