Monoazo Acid dyes:- (Part 2)
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Monoazo Acid dyes:-
Acid orange IV and Metanil Yellow are prepared by the diazotisation of sulphanilic acid (180) by the reverse method and of metanilic acid by direct method respectively and coupling with diphenylamine . Diphenylamine being very weakly basic and insoluble in water as well as dilute hydrochloric acid, the coupling is carried out with an emulsion of diphenylanine or in alcoholic solution or using pyridine. For example, in the preparation of Metanil Yellow, Metanilic acid is diazotised with sodium nitrite and hydrochloric acid solutions at 50C and after the diazotisation is completed the diazonium salt solution is made acidic and slowly added to the emulsion of diphenylamine made by intimately mixing a mixture of diphenylamine, water and small quantities of glue dispersing agent. The coupling is slow and takes very long time to complete. After the coupling is completed, salt solution in added and the mixture is stirred at room temperature for few hours when the dye precipitates out.
