Congo red

Description

A brownish red water-soluble synthetic disazo dye. Congo red was first made in 1884 by P. Böttiger and was used as a direct textile dye for cotton and rayon. Congo red is now used as an acid-base indicator where it is prepared as a 0.1% aqueous solution that is red above pH 5.2 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

Synonyms and Related Terms

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

Risks

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

Physical and Chemical Properties

• Soluble in water forming 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 www.colour-index.org
• Sigma Dyes, Stains and Natural Pigments, Infrared Library, Nicolet, 1991-1995 Comment: OMNIC: formula= C32H22O6N6S24, CAS= 573-58-0
• Website: www.straw.com/sig/dyehist
• Wikipedia: Congo red Accessed April 2026