An Acid Dye is a dye, in chemical regard a sodium (less oftenammonium) salt of a sulfonic, carboxylic or phenol organic acid. Acid dye is soluble in water and possesses the 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.
