Acid dyeing

Acid dyes are anionic dyes with sulfonic or carboxyl groups, negatively charged in aqueous solution. Most belong to the azo class but there are also anthraquinone, triphenylmethane and xanthene derivatives. They are applied at pH 2 to 6 — in this environment the amino groups of wool, silk and polyamide protonate to positive ammonium groups and attract the anionic dye through ion exchange.

Three classes exist: levelling dyes (small molecules, good levelling but weaker wet fastness), milling dyes (medium molecules, medium levelling and wet fastness) and chrome or mordant dyes (large molecules, very high wet fastness via hydrophobic adsorption). For dark wool shades with high fastness, metal complex dyes (1:1 and 1:2) are also used.

Well-known representatives are C.I. Acid Black 1 (blue-black shades with very good light fastness) and C.I. Acid Yellow 36. For merino wool, cashmere and fine silks acid dyeing remains indispensable; polyamide sports socks and leggings are also predominantly dyed with acid dyes.