Pyrazolone azo dyes

The situation in the pyrazolone-azo dye series is more complex. In contrast to former assumptions the dyes are inaccurately described as azo compounds and in fact exist exclusively in a tautomeric phenylhydrazone form. The simple pyrazolone dye (116 R=H) absorbs at 398 nm in ethanol (67BCJ1239). In this instance the pyrazolone heterocyclic ring... 

The pyrazolone azo dyes constitute an important group which arise from Ziegler s discovery in 1884 that a yellow dye, known as Tartrazine, was obtained by heating phenylhydrazine-/>-sulphonic acid with dihydroxy tartaric add ... 

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