Methyl orange
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| Names | |
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| IUPAC name
Sodium 4-[(4-dimethylamino)phenyldiazenyl]benzenesulfonate
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| Other names
Sodium 4-[(4-dimethylamino)phenylazo]benzenesulfonate
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| Identifiers | |
3D model (JSmol)
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| ChemSpider | |
| ECHA InfoCard | 100.008.115 |
CompTox Dashboard (EPA)
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| Properties | |
| C14H14N3NaO3S | |
| Molar mass | 327.33 g/mol |
| Density | 1.28 g/cm3, solid |
| Melting point | >300 °C (Not precisely defined.) |
| Boiling point | Decomposes. |
| Soluble in hot water | |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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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
References
- ^ MSDS from ScienceLab.com, Inc. Retrieved 2011-09-24
- ^ 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.