Acid dye

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Acid red 88 is an acid dye used to produce red woolen yarns.

An acid dye is a dye that is typically applied to a textile at low pH. They are mainly used to dye wool, not cotton fabrics.[1] Some acid dyes are used as food colorants,[2][3] and some can also be used to stain organelles in the medical field.


Acid dyes are generally divided into three classes which depend on fastness requirements, migration ability and dyeing pH. The classes generally depend on the type of fiber to be colored as well as the process used.[4]

Acid dyes affix to fibers by hydrogen bonding, Van der Waals forces[5] and ionic bonding. Whilst this experiment shows the dissolution of acid dyes in water, many choose to activate dyes in acid dye-baths instead. According to the Brønsted–Lowry acid–base theory, an acid is a molecule or ion capable of donating a proton, and this is determined by the acid dissociation constant. Compared to most acids, water has a much higher pKa value, meaning that it dissociates to give H+ with more difficulty. In this context, if an acid is used instead of water, then the hydrogen ion (H+) is more easily able to dissociate in order to react with the aniline dye anion, allowing the dye to dissolve.

Animal protein fibers and the synthetic fiber nylon contain many cationic sites that bind anionic dye. The strength (fastness) of this bond reflects the strength of this ionic interaction.

Some dyes are mutagenic and carcinogenic.



In the laboratory, home, or art studio, the acid used in the dye-bath 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.[6] They are effective in dyeing the synthetic fiber nylon, but of minimum interest in dyeing any other synthetic fibers.

Lee's stain used in gallbladder cells.
PTAH stain used in Human squamous epithelial cells.


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.[7] Regarding cellular structures, acid dyes will stain acidophillic structures that have a net positive charge due to the fact that they have a negatively charged chromophore. Acidophillic structures include the cytoplasm, collagen and mitochondria. The two have an affinity for each other due to the conflicting charges.[8][9] Examples of acid dyes used in medicine include:[10]

Food Industry[edit]

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.[11] 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.[12]


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.[13]
  • 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[edit]

There are three classifications of acidic dyes, which are all classified by their dyeing behaviour. This includes their wet fastness, migration ability, and dyeing pH:[1]

  • Leveling acid dyes: These dyes have the lowest molecular weight of all three, and as a result will migrate more rapidly before fixation occurs. Leveling acid dyes have simple and small molecules, and will exhibit low wet fastness due to their high mobility. Thus, they are not normally suited for use as apparel fabric. They require an acidic dyebath to be applied, often using sulfuric acid and sodium sulfate mixtures (pH2-4),[6] together with leveling agents such as ethoxylated fatty amines.
  • Milling dyes: These dyes have a higher molecule weight than Levelling dyes due to their larger molecules, meaning that they will migrate slower in comparison to Levelling dyes. Their low mobility causes them to exhibit a higher wet fastness, which is useful for dyeing wool materials. Milling acid dyes are sometimes called 'Neutral acid dyes' as they don't require an acidic dyebath, and will commonly be applied using Acetic acid (pH4-7).[6]
  • Metal complex acid dyes: These dyes are composed of large acid dye molecules complexed with a metal ion, which will usually be chromium or cobalt. Metal complex acid dyes have the highest molecular weight of all the dyes, giving them low mobility and the highest wet fastness. Once fixed to the fiber, they will not migrate. Due to this, they are commonly used on nylon and synthetic poly-amide fibers. These dyes are very economical, however they will produce duller shades compared to the other classes. Metal complex acid dyes take a larger range of pH in the dyebath (pH2-7).[6]


  1. ^ a b Booth, Gerald (2000). "Dyes, General Survey". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a09_073. ISBN 3527306730.
  2. ^ Trowbridge Filippone, Peggy. "Food Color Additives". Retrieved September 8, 2016.
  3. ^ Klaus Hunger, ed. (2003), Industrial Dyes: Chemistry, Properties, Applications (in German), Weinheim: WILEY-VCH Verlag, pp. 276ff, ISBN 978-3-662-01950-4
  4. ^ "Mechanism of Dyeing with Acid Dyes". Textile Learner. Mazharul Islam Kiron. Retrieved 2012-01-08.
  5. ^ Clark, Jim (2012). "Intermolecular bonding - van der Waals forces". Retrieved 15 June 2014.
  6. ^ a b c d "How Acid Dye Works". Retrieved October 21, 2019.
  7. ^ Bruckner, Monica Z. "Basic Cellular Staining". Retrieved December 12, 2013.
  8. ^ "Staining and Commonly Used Stains". Histology Learning system. Boston University. Retrieved 2019-11-05.
  9. ^ Gokhale, S (2008). Pharmaceutical Biology. Mahrashtra, India: Pragati Books Pvt. Ltd.
  10. ^ "Staining and Commonly Used Stains". Histology Learning system. Boston University. Retrieved 2019-11-05.
  11. ^ Frazier, R.A (2007). CAPILLARY ELECTROPHORESIS | Food Additives. Elsevier Ltd.
  12. ^ Vega, M (2000). Encyclopedia of Separation Science. Elsevier Ltd.
  13. ^ Hunger, Klaus; Mischke, Peter; Rieper, Wolfgang; Raue, Roderich; Kunde, Klaus; Engel, Aloys (2005). "Azo Dyes". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a03_245. ISBN 3527306730.