Sulfur Dye

any of the organic dyes that are high-molecular-weight compounds containing the heterocycles

and the groups SH and S_n, where n ≥ 2 (the precise chemical structure has not been established). Sulfur dyes are amorphous substances, insoluble in water and most organic solvents; when acted upon by Na₂ S, they form water-soluble leuco compounds according to the scheme

After imparting color, these compounds are oxidized by oxygen in the air and are reconverted on the fiber to insoluble sulfur dyes.

Sulfur dyes are obtained by a prolonged heating of such organic compounds as aromatic amino, nitro, amino-hydroxy, and nitro-hydroxy compounds and heterocyclic azines with sulfur or a polysulfide of sodium (Na₂ S_n’n = 2–9).

Sulfur dyes are usually pale. The black, blue, brown, and green dyes have the greatest importance; the yellow and orange dyes have only limited use. Red sulfur dyes are unknown. While the black dyes are fast, the other sulfur dyes impart colors of lesser permanence. Sulfur dyes are inexpensive and convenient. They are widely used for dyeing cotton fabric; in the textile industry, however, they have been partially replaced by vat dyes and reaction dyes, which offer brighter and more permanent colors.