Methyl orange

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Structural formula
Structural formula of methyl orange
Surname Methyl orange
other names
  • Sodium 4 - {[4- (dimethylamino) phenyl] diazenyl} benzenesulfonate ( IUPAC )
  • CI Acid Orange 52
  • CI 13025
  • Helianthin
  • Orange III
  • Golden orange
  • Tropaeolin D
Molecular formula C 14 H 14 N 3 NaO 3 S
Brief description

orange solid with a faint odor

External identifiers / databases
CAS number
  • 547-58-0
  • 502-02-3 (free acid)
EC number 208-925-3
ECHA InfoCard 100.008.115
PubChem 23673835
ChemSpider 16736152
Wikidata Q409196
Molar mass 327.34 g mol −1
Physical state



1.28 g cm −3

Melting point

> 300 ° C


poor in water (approx. 5 g l −1 at 20 ° C)

safety instructions
GHS labeling of hazardous substances
06 - Toxic or very toxic


H and P phrases H: 301
P: 301-330-331-310
Toxicological data

60 mg kg −1 ( LD 50ratoral )

As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Methyl orange is an azo dye from the application group of acid dyes . It is present as the sodium salt of 4- [4- (dimethylamino) phenylazo] benzenesulfonic acid and is mostly used as a pH indicator .


Sulphanilic acid is diazotized with nitrous acid :

Synthesis of methyl orange: diazotization

The diazonium salt is then coupled with N , N -dimethylaniline ( green ), converted into the sodium salt with sodium hydroxide solution and precipitated :

Synthesis of methyl orange: coupling


Methyl orange forms orange crystals.

Powdered methyl orange.

In the strongly acidic pH range, the azo group is protonated and a mesomeric quinoid system is obtained . When base is added, the group deprotonates in the pH range 3.0 to 4.4 and the color changes from red to yellow-orange.

Methyl orange: mesomeric boundary structures at pH <3 and color change in the pH range 3.0-4.4
Color changes from red to yellow-orange with increasing pH.

Web links

Commons : Methyl Orange  - Collection of images, videos, and audio files

Individual evidence

  1. a b c d e Entry on sodium 4- (4-dimethylaminophenylazo) benzenesulfonate in the GESTIS substance database of the IFA , accessed on January 10, 2017(JavaScript required) .
  2. Applicher: Safety data sheet methyl orange (CI 13025) ( Memento of the original from September 16, 2016 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. As of August 5, 2016, accessed on September 12, 2016 @1@ 2Template: Webachiv / IABot /
  3. Data sheet methyl orange (PDF) from Merck , accessed December 9, 2010.
  4. ^ Author collective: Organikum . 21st edition. Wiley-VCH, 2001, ISBN 3-527-29985-8 , p. 645.
  5. Indicator Reagents . In: Ullmann's Encyclopedia of Industrial Chemistry (engl.).