Azoic dyes are insoluble azo dyes produced in situ on the fiber. In this dyeing process two components are necessary. The first component is Naphthol and the second is Fast base.
The first component which is nearly colorless or faintly yellow or brown in color has to be dissolved in water, containing NaOH and applied to cotton as if it were a direct dye but at room temperature and not at the boil.
The second component has to be subjected to a section called diazotization and then applied to cotton which has already been treated with a first component. These two compounds react (couple) with each other almost immediately, producing usually a bright colored pigment, which is insoluble in water. Since an azo group (—N=N—) is formed during the coupling of the two components, so these dyes are called ‘Azoic dyes’. They have excellent washing fastness.