fibers. Histology 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. Regarding cellular structures, acid dyes will stain acidophilic structures that have a net positive charge due to the fact that they have a negatively charged chromophore. Acidophilic structures include the cytoplasm, collagen and mitochondria. The two have an affinity for each other due to the conflicting charges. Examples of acid dyes used in medicine include: Lee's stain (stains reddish-pink). Phosphotungstic Acid Hematoxylin (PTAH) stain (stains blue). Eosin stain (stains pinkish-orange). Food Industry Acid dyes can also be used as food colouring, helping to increase the attractiveness of certain foods, and thus becoming more appealing to customers. Some examples include erythrosine, tartrazine, sunset yellow and allura red, to name a few, many of which are azo dyes. These dyes can be used in frosting, cookies, bread, condiments or drinks. In order to prevent health hazards, a dye must be approved for consumption before it can be marked as edible. Some separation methods that can be used to identify unapproved dyes include the solid phase extraction process, the overpressured thin layer chromatography process, and the use of reversed-phase plates. Structures 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. Classes of acid dyes Acidic dyes can be classified according to their dyeing behavior. This includes their wet fastness, migration ability, and dyeing pH: Leveling acid dyes: These dyes have relatively low molecular weights. Consequently, they migrate more readily before fixation and they exhibit low wet fastness. They are not normally suited for use as apparel fabric. They require an acidic dye bath, often