Methyl orange, sodium salt

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A pale, orange powder used as an acid-base indicator. Methyl orange is an azo dye that was discovered by several workers near the same time period: P. Griess in 1875; O.N. Witt in 1876; and Z. Roussin in 1876. As an indicator, methyl orange forms a red color below pH 3.1, turns orange above pH 4.4 and becomes yellow in alkaline solutions. It is used for titrating mineral acids. Methyl orange is also used for dyeing textiles.

Chemical structure

Methyl orange, sodium salt.jpg

Synonyms and Related Terms

4-[[(4-dimethylamino)phenyl]-azo]benzenesulfonic acid sodium salt; sodium p-dimethylaminoazobenzenesulfonate; helianthine B; Acid Orange 52; CI 13025; Orange III; Gold Orange; Topaeolin D


  • Toxic by ingestion.
  • Inhalation and skin contact may cause irritation.
  • ThermoFisher: SDS

Physical and Chemical Properties

Soluble in hot water. Insoluble in ethanol.

Composition (CH3)2NC6H4NNC6H4SO3Na
CAS 547-58-0
Density 1.00 g/ml
Molecular Weight mol. wt.=327.34

Resources and Citations

  • The Merck Index, Susan Budavari (ed.), Merck Research Labs, Whitehouse Station, NJ, 12th Edition, 1996 Comment: entry 6180
  • Colour Index International online at
  • Random House, Webster's Encyclopedic Unabridged Dictionary of the English Language, Grammercy Book, New York, 1997
  • Richard S. Lewis, Hawley's Condensed Chemical Dictionary, Van Nostrand Reinhold, New York, 10th ed., 1993

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