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Methyl orange

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Methyl orange
Names
IUPAC name
Sodium 4-[(4-dimethylamino)phenyldiazenyl]benzenesulfonate
Other names
Sodium 4-[(4-dimethylamino)phenylazo]benzenesulfonate
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.008.115 Edit this at Wikidata
  • InChI=1S/C14H15N3O3S.Na/c1-17(2)13-7-3-11(4-8-13)15-16-12-5-9-14(10-6-12)21(18,19)20;/h3-10H,1-2H3,(H,18,19,20);/q;+1/p-1 checkY
    Key: STZCRXQWRGQSJD-UHFFFAOYSA-M checkY
  • InChI=1/C14H15N3O3S.Na/c1-17(2)13-7-3-11(4-8-13)15-16-12-5-9-14(10-6-12)21(18,19)20;/h3-10H,1-2H3,(H,18,19,20);/q;+1/p-1
    Key: STZCRXQWRGQSJD-REWHXWOFAG
  • [Na+].CN(C)c2ccc(/N=N/c1ccc(cc1)S([O-])(=O)=O)cc2
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
Occupational safety and health (OHS/OSH):
Main hazards
 Toxic (T)
GHS labelling:
GHS06: Toxic
Danger
H301
P308, P310
NFPA 704 (fire diamond)
[1]
NFPA 704 four-colored diamondHealth 1: Exposure would cause irritation but only minor residual injury. E.g. turpentineFlammability 0: Will not burn. E.g. waterInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
1
0
0
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).
checkY verify (what is checkY☒N ?)

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. ^ a b c 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.
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