Definition: Acid dyes are a type of water-soluble dyes with acid groups on the structure. They are dyed in an acid medium. Most acid dyes contain sodium sulfonate, soluble in water, bright in color and complete in chromatography. Acid dyes are water-soluble anionic dyes. They are mainly used for dyeing wool, silk and nylon. They generally have no coloring power for cellulose fibers.
Compared with direct dyes, acid dyes have a simple structure and lack long conjugated double bonds and coplanar structure, so they lack directness to cellulose fibers and cannot be used for dyeing cellulose fibers. Different types of acid dyes have different dyeing properties due to their different molecular structures. The dyeing methods used are also different. The fastness to sunlight and wet treatment of acid dyes vary greatly with the type of dye.