Methyl red

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A reddish-violet powder that was formerly used as an acid-base indicator. Methyl red is a monoazo dye that was discovered by E. Rupp and R. Loose in 1908. When mixed as a 0.1% alcoholic solution, methyl red has a red color below pH 4.4 and yellow above pH 6.2. It has been used for titrating ammonia and weak organic bases, but it is not suitable for titrating acids. Currently, methyl red is rarely used because it is easily reduced to a colorless form.

Chemical structure

Methyl red.jpg

Synonyms and Related Terms

Acid Red 2; CI 13020; 2-[[(4-dimethylamino)phenyl]-azo]benzoic acid; 4'-dimethylaminoazobenzene-2-carboxylic acid; rojo de metilo (Esp.); rouge méthyle (Fr.); vermelho de metilo (Port.)


  • Toxic by ingestion and inhalation.
  • Contact may cause irritation
  • Potential carcinogen.
  • Flammable. Flash point = 12 C
  • Fisher Scientific: MSDS

Physical and Chemical Properties

Soluble in ethanol and acetic acid. Insoluble in water.

Composition (CH3)2NC6H4NNC6H4COOH
CAS 493-52-7
Melting Point 181-182 C
Molecular Weight mol. wt. = 269.29

Resources and Citations

  • G.S.Brady, Materials Handbook, McGraw-Hill Book Co., New York, 1971 Comment: p. 184
  • Richard S. Lewis, Hawley's Condensed Chemical Dictionary, Van Nostrand Reinhold, New York, 10th ed., 1993
  • Random House, Webster's Encyclopedic Unabridged Dictionary of the English Language, Grammercy Book, New York, 1997
  • The Merck Index, Martha Windholz (ed.), Merck Research Labs, Rahway NJ, 10th edition, 1983 Comment: entry 6199
  • S.R.Trotman, E.R. Trotman, Textile Analysis, J.B. Lippincott Company, Philadelphia, 1932
  • Colour Index International online at