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 (JSmol)
ECHA InfoCard 100.008.115
EC Number 208-925-3
UN number 3143
Molar mass 327.33 g·mol−1
Appearance Orange solid
Density 1.28 g/cm3
Melting point > 300 °C (572 °F; 573 K) (not precisely defined)
Boiling point Decomposes
0.5 g/100 mL (20 °C)
Solubility in diethyl ether Insoluble[1]
Main hazards Toxic (T)
GHS pictograms GHS06: Toxic
GHS signal word Danger
P308, P310
NFPA 704
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 titration because of its clear and distinct color variance at different pH values. Methyl orange shows red color in acidic medium and yellow color in basic medium. Because it changes color at the pH of a mid strength acid, it is usually used in titration for acids. Unlike a universal indicator, methyl orange does not have a full spectrum of color change, but it has a sharp end point.

Indicator colors[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]

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]