Congo red

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A brownish red synthetic Disazo dye. Congo red was first made in 1884 by P. Böttiger. It was used as a direct textile dye for Cotton. Congo red is now used as an acid-base indicator. It is prepared as a 0.1% aqueous solution that is red above pH 5 and blue below pH 3. Congo red is only used for the detection of inorganic acids since it is not sensitive to the presence of organic acids.

Chemical structure

Congo red.jpg

Synonyms and Related Terms

Direct Red 28; CI 22120; Kongorot (Deut.); Congo blue; Cotton red; sodium diphenyl-bis-alpha-naphthylamine sulfonate;


  • Contact may cause irritation and/or allergic reaction.
  • Fisher Scientific: MSDS

Physical and Chemical Properties

  • Soluble in water to for a yellowish red solution.
  • pH aq. solution = 8 - 9.5
  • Insoluble in ether. Slightly soluble in acetone.
Composition C32H22O6N6S2Na2
CAS 573-58-0
Molecular Weight mol. wt. = 696.67

Resources and Citations

  • Richard S. Lewis, Hawley's Condensed Chemical Dictionary, Van Nostrand Reinhold, New York, 10th ed., 1993
  • S.R.Trotman, E.R. Trotman, Textile Analysis, J.B. Lippincott Company, Philadelphia, 1932
  • The Merck Index, Martha Windholz (ed.), Merck Research Labs, Rahway NJ, 10th edition, 1983 Comment: entry 2562
  • A.Scharff, 'Synthetic dyestuffs for textiles and their fastness to washing', ICOM-CC Preprints Lyon, Getty Conservation Institute, Los Angeles, 1999
  • Colour Index International online at
  • Sigma Dyes, Stains and Natural Pigments, Infrared Library, Nicolet, 1991-1995 Comment: OMNIC: formula= C32H22O6N6S24, CAS= 573-58-0
  • Website:

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