In staining during microscopic examination for diagnosis or research, acid dyes are used to color basic tissue proteins. In contrast, basic dyes are used to stain cell nuclei and some other acidic components of tissues.
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 proteinfibers and syntheticnylon 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.
The chemistry of acid dyes is complex and diverse. Most acid dyes are related in basic structure to the following:
Anthraquinone type: Many acid dyes are synthesized from chemical intermediates that form anthraquinone-like structures as their final state. Many blue dyes have this structure as their basic shape. The structure predominates in the leveling class of acid dye.
Azo dyes: The structure of azo dyes contains the azo group (R-N=N−R. Most azo dyes are not acid dyes, but many acid dyes are azo dyes. Many acid dyes of the azo type are red in color.
Triarylmethane dye: These predominate in the milling class of dye. There are many yellow and green dyes commercially applied to fibers that are related to triphenylmethane.
Leveling acid dyes: These dyes are of relatively low molecular weight, thus they migrate rapidly before fixation. Associated with their high mobility, leveling dyes exhibit low wet fastness therefore normally not suited for apparel fabric. These dyes are applied at low pH, often using sulphuric acid and sodium sulphate mixtures.
Milling dyes: These dyes are of relatively high molecular weight, thus they migrate more slowly than leveling dyes. Associated with their low mobility, milling dyes exhibit better wet fastness than leveling dyes. These dyes are applied at higher pH, often using acetic acid.
Metal complex acid dyes: More recent chemistry combined transition metals with dye precursors to produce metal complex acid dyes with the highest light fastness and wet fastness. These dyes are also very economical. They produce, however, duller shades.