Methyl orange

Methyl orange
IUPAC name 4-dimethylaminoazobenzene-4'-sulfonic acid sodium salt
Other names p-dimethylamino-azobenzenesulfonic acid
Identifiers
CAS number	547-58-0 Y
ChemSpider	16736152 Y
Jmol-3D images	Image 1
SMILES [Na+].CN(C)c2ccc(/N=N/c1ccc(cc1)S([O-])(=O)=O)cc2
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
Properties
Molecular formula	C14H14N3NaO3S
Molar mass	327.33 g/mol
Density	1.28 g/cm3, solid
Melting point	>300 °C (Not precisely defined.)
Boiling point	Decomposes.
Solubility in water	Soluble in hot water
Y orange (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Methyl orange is a pH indicator frequently used in titrations.

It is often used in titrations because of its clear and distinct colour change. Because it changes colour at the pH of a mid-strength 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 has mutagenic properties^[1].

Indicator colours

Methyl orange (pH indicator)
below pH 3.1		above pH 4.4
3.1	↔	4.4

Methyl orange in xylene cyanol solution (pH indicator)
below pH 3.2		above pH 4.2
3.2	↔	4.2

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.

In an acid it is reddish and in alkali it is yellow. Methyl orange has a pKa of 3.47 in water at 25 degrees Celsius.^[2]

Other Indicators

• 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.
• A number of other common indicators are tabulated in the article on pH indicators.

See also

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

References

1. ^ 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

• Informative page on different titration indicators, including Methyl Orange