What is "acid dye"

n. dye in which the chromophore is part of a negative ion

An acid dye is a dye which is a salt of a sulfuric, carboxylic or phenolic organic acid. The salts are often sodium or ammonium salts. Acid dyes are typically soluble in water and possesses affinity for amphoteric fibers while lacking direct dyes' affinity for cellulose fibers. When dyeing, ionic bonding with fiber cationic sites accounts for fixation of colored anions in the dyed material. Acids are added to dyeing baths to increase the number of protonated amino-groups in fibers.

Some acid dyes are used as food colorants.