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Methyl orange

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Methyl orange
Names
Preferred IUPAC name
Sodium 4-{[4-(dimethylamino)phenyl]diazenyl}benzene-1-sulfonate
Other names
Sodium 4-[(4-dimethylamino)phenylazo]benzenesulfonate
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.008.115 Edit this at Wikidata
EC Number
  • 208-925-3
UNII
UN number 3143
  • 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
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]
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 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

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

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.

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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