Methyl orange

Methyl orange
Names
	Preferred IUPAC name Sodium 4-{[4-(dimethylamino)phenyl]diazenyl}benzene-1-sulfonate
	Other names Sodium 4-[(4-dimethylamino)phenylazo]benzenesulfonate
Identifiers
CAS Number	547-58-0 ;
3D model (JSmol)	Interactive image;
ChemSpider	16736152 ;
ECHA InfoCard	100.008.115
EC Number	208-925-3;
PubChem CID	23673835;
UNII	6B4TC34456;
UN number	3143
CompTox Dashboard (EPA)	DTXSID60883437 ;
	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 Key: STZCRXQWRGQSJD-UHFFFAOYSA-M ; 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
Appearance	Orange or yellow solid
Density	1.28 g/cm3
Melting point	> 300 °C (572 °F; 573 K) (not precisely defined)
Boiling point	Decomposes
Solubility in water	5 g/L (20 °C)
Solubility in diethyl ether	Insoluble
Hazards
Main hazards	Toxic (T)
GHS pictograms
GHS Signal word	Danger
GHS hazard statements	H301
GHS precautionary statements	P308, P310
NFPA 704 (fire diamond)	0 1 0
	Lethal dose or concentration (LD, LC):
LD50 (median dose)	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).

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 pK_a 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. In a solution becoming less acidic, methyl orange changes from red to orange and, finally, to yellow—with the reverse process occurring in a solution of increasing acidity.

Indicator colors

Methyl orange solutions

In a solution that decreases in acidity, methyl orange moves from the colour red to orange and finally to yellow with the opposite occurring for a solution increasing in acidity.

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 pK_a of 3.47 in water at 25 °C (77 °F).^[3]

Other indicators

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.^[2]

References

^ ^a ^b Haynes, William M., ed. (2016). CRC Handbook of Chemistry and Physics (97th ed.). CRC Press. p. 3.384. ISBN 9781498754293.
^ ^a ^b ^c MSDS Archived 2014-05-12 at the Wayback Machine 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.

External links

Informative page on different titration indicators, including methyl orange

[crc-1] Haynes, William M., ed. (2016). CRC Handbook of Chemistry and Physics (97th ed.). CRC Press. p. 3.384. ISBN 9781498754293.

[sciencelab-2] MSDS Archived 2014-05-12 at the Wayback Machine from ScienceLab.com, Inc. Retrieved 2011-09-24

[3] 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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