Methyl orange

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Methyl orange
Preferred IUPAC name
Sodium 4-{[4-(dimethylamino)phenyl]diazenyl}benzene-1-sulfonate
Other names
Sodium 4-[(4-dimethylamino)phenylazo]benzenesulfonate
3D model (Jmol)
ECHA InfoCard 100.008.115
Molar mass 327.33 g·mol−1
Density 1.28 g/cm3, solid
Melting point >300 °C (572 °F; 573 K)
not precisely defined
Boiling point decomposes
0.5 g/100 mL (20 °C)
Solubility insoluble in diethyl ether[1]
Main hazards Toxic (T)
GHS pictograms The skull-and-crossbones pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)
GHS signal word Danger
P308, P310
Toxic T
R-phrases R25
S-phrases S37, S45
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 1: Exposure would cause irritation but only minor residual injury. E.g., turpentine Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
Lethal dose or concentration (LD, LC):
60 mg/kg (rat, oral)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Methyl orange is a pH indicator frequently used in titrations because of its clear and distinct colour change. Because it changes colour at the pH of a midstrength acid, it is usually used in titrations for acids. Unlike a universal indicator, methyl orange does not have a full spectrum of colour change, but has a sharper end point. Methyl orange shows red colour in acidic medium and yellow colour in basic medium.

Indicator colours[edit]

Methyl orange solutions

In a solution becoming less acidic, methyl orange moves from red to orange and finally to yellow with the reverse occurring for a solution increasing in acidity.The entire color change occurs in acidic conditions.

Methyl orange (pH indicator)
below pH 3.1 above pH 4.4
3.1 4.4

In an acid, it is reddish and in alkali, it is yellow. Methyl orange has a pKa of 3.47 in water at 25 °C (77 °F).[2]

Other indicators[edit]

Main article: pH indicator
Methyl orange in xylene cyanol solution (pH indicator)
below pH 3.2 above pH 4.2
3.2 4.2

Modified (or screened) methyl orange, an indicator consisting of a solution of methyl orange and xylene cyanol, changes from grey-violet to green as the solution becomes more basic.


Methyl orange has mutagenic properties.[1] Direct contact should be avoided.

See also[edit]


  1. ^ a b c MSDS from, Inc. Retrieved 2011-09-24
  2. ^ Sandberg, Richard G.; Henderson, Gary H.; White, Robert D.; Eyring, Edward M. (1972). "Kinetics of acid dissociation-ion recombination of aqueous methyl orange". The Journal of Physical Chemistry. 76 (26): 4023–4025. doi:10.1021/j100670a024. 

External links[edit]