Methyl orange

Methyl orange
Names
IUPAC name
Sodium 4-[(4-dimethylamino)phenyldiazenyl]benzenesulfonate
Other names
Sodium 4-[(4-dimethylamino)phenylazo]benzenesulfonate
Identifiers
547-58-0 YesY
ChemSpider 16736152 YesY
Jmol 3D model Interactive image
Properties
C14H14N3NaO3S
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]
Hazards
Main hazards Toxic (T)
GHS pictograms
GHS signal word Danger
H301
P308, P310
T
R-phrases R25
S-phrases S37, S45
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 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 orange colour in acidic medium and yellow colour in basic medium.

Indicator colours

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 colour 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 pHa of 3.47 in water at 25 °C (77 °F).[2]

Other indicators

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 to green as the solution becomes more basic.

Safety

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

See also

References

  1. 1 2 3 MSDS from ScienceLab.com, 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

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