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Congo Red

Congo Red

Class: Azo
Ionisation: Acid

Chemical structure of Congo Red

Common Name: Congo red
Suggested Name: Congo red
Other Names:

Direct red Cotton red

C.I. Number: 22120
C.I. Name: Direct red 28
Color: Red
Solubility Aqueous: Readily
Solubility Ethanol: Readily
Absorption Maximum: 497 (Conn, Aldrich), 488 (Merck), 486 (Gurr)
Empirical Formula: C32H22N6O6S2Na2
Formula Weight: 696.7


Congo red is used to stain amyloid and is considered the reference standard for the purpose. It is believed that its hydrogen bonds to the beta-pleated sheet structure of amyloid with the dye molecules aligned in a very regular manner. This forms a "pseudocrystal", which results in a characteristic, green birefringence of the stained amyloid.

Compare this dye to other direct cotton dyes for amyloid.


  1. R. D. Lillie.
    Conn’s Biological Stains
    Williams & Wilkins, Baltimore, MD., U.S.A.
  2. Edward Gurr, 1971
    Synthetic dyes in biology, medicine and chemistry
    Academic Press, London, England.
  3. Aldrich chemical catalogue, 1992
    Aldrich Chemical Company, Milwaukee, WI, USA.
  4. Susan Budavari, Editor,
    The Merck Index, Ed. 12
    Merck & Co., Inc., Whitehouse Station, NJ, USA