Acid dyes are generally divided into three classes which depend on fastness requirements, level dyeing properties and economy.
The classes overlap and generally depend on type of fiber to be colored as well as the process used.
Acid dyes affix to fibers by hydrogen bonding, Van der Waals forces and ionic bonding. They are normally sold as the Sodium salt, therefore they are in solution anionic. Animal protein fibers and synthetic nylon fibers contain many cationic sites. Therefore, there is an attraction of anionic dye molecule to a cationic site on the fiber. The strength (fastness) of this bond is related to the tendency of the dye to remain dissolved in water over fixation to the fiber.