azo dye 
noun Chemistry
Any of a large class of synthetic dyes whose molecules contain two adjacent nitrogen atoms between carbon atoms.Urobilinogen reacts with a chromogen to form an azo dye which appears as various shades of pink or purple....
- Thus Paul Ehrlich's Salvarsan was an arsenic analogue of an azo dye, and the first sulfa, or wonder, drug, Prontosil, was the metabolic product of a red azo dye.
- Nitrite was determined spectrophotometrically upon diazotation and forming of azo dye.