• Lichao Liu

[Chemistry] What is naphthol? -2

Coupling component

A naphthol or brenthol, which develops the hue on reaction with diazotised base.


Naphthols are insoluble in water and have no affinity for cellulosics. NaOH converts these into sodium salt of naphthols, called naphtholates, which are soluble and show affinity for cellulose. When used in excess, causes precipitation of naphthols (Shenai, 1987). Addition of excess NaOH produces disodium salts and the naphthol starts precipitating.


Is is an exhausting agent and imparts substantivity to naphthols. It is safe to use Glauber’s salt (Na2SO4, 10H2O) as presence of magnesium and calcium salts in impure NaCl mostly used in industries may cause precipitation of naphthols in bath.


Exposure of naphtholates or treated fabric to acid fume or atmospheric CO2 causes separation of free naphthols from their sodium salts. Naphthol solutions – to be stored – require stabilisation, which is done by adding HCHO in bath to form methylol compounds (Shenai, 1987). HCHO must be applied at room temperature, otherwise at higher temperature, methylol compounds are combined with a fresh naphthol molecule forming a methylene bridge with blocking of coupling position. Thus extent of coupling is reduced developing lighter shades.

Reaction of naphthol with formaldehyde

Naphthols possessing good stability to air, e.g., AS-OL, AS-ITR, AS-BS, AS-BG, AS-BI, AS-LB etc., do not require addition of HCHO. Some naphthols, e.g., brenthols lose their coupling power if HCHO is added, e.g., AS-G, AS-LG, AS-L3G, AS-L4G, AS-IRG, AS-AG (acetoacetyl arylamides). Solubilised aromatic naphthols are practically stabilised with HCHO if so required. Naphtholated goods dried after padding or to be developed without any such delay do not require stabilisation with HCHO.

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