Methyl orange

Methyl orange

Names
IUPAC name Sodium 4-[(4-dimethylamino)phenyldiazenyl]benzenesulfonate
Other names Sodium 4-[(4-dimethylamino)phenylazo]benzenesulfonate
Identifiers
CAS Number	547-58-0 Y
ChemSpider	16736152 Y
Jmol 3D model	Interactive image
InChI 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 Y Key: STZCRXQWRGQSJD-UHFFFAOYSA-M Y 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
SMILES [Na+].CN(C)c2ccc(/N=N/c1ccc(cc1)S([O-])(=O)=O)cc2
Properties
Chemical formula	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
Solubility in water	0.5 g/100 mL (20 °C) soluble in hot water
Solubility	insoluble in diethyl ether
Vapor pressure	{{{value}}}
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.

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 pH_a of 3.47 in water at 25 °C (77 °F).

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. Direct contact should be avoided.

See also

• pH indicator
• Methyl red
• Litmus
• Phenolphthalein
• Bromothymol blue
• Universal indicator

External links

• Informative page on different titration indicators, including Methyl Orange