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Acid Dyes

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By Janet Bohdanowicz, Liz ClampAcid Dyes

An acid dye is a dye in which the coloring component is in the anion •They are often applied from an acidic solution in order to intensify the staining. In the laboratory, the home or art studio, the acid used in the dyebath is often vinegar (acetic acid) or citric acid. •The uptake rate of the dye is controlled with the use of sodium chloride. •In textiles, acid dyes are effective on protein fibers, i.e. animal hair fibers like wool, alpaca and mohair. •They are also effective on silk. They are effective in dyeing the synthetic fiber nylon but of minimal interest in dyeing any other synthetic fibers. •In staining for microscopic examination for diagnosis or research acid dyes are used to color basic tissue proteins in contrast to basic dyes, which are used to stain cell nuclei and some other acidic components of tissues •Acid dyes are thought to fix 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 desire/ chemistry of the dye to remain dissolved in water over fixation to the fiber. Equalising/levelling acid dyes: Highest level dyeing properties. Quite combinable in trichromatic shades. Relatively small molecule therefore high migration before fixation. Low wet fastness therefore normally not suited for apparel fabric. •Milling acid dyes: Medium to high wet fastness. Some milling dyes have poor light fastness in pale shades. Generally not combinable. Used as self shades only •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...

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