Methyl orange

From Wikipedia, the free encyclopedia
Jump to: navigation, search
Methyl orange
Methyl-orange-2D-skeletal.png
Methyl-orange-3D-vdW.png
Methyl-orange-sample.jpg
Identifiers
CAS number 547-58-0 YesY
ChemSpider 16736152 YesY
Jmol-3D images Image 1
Properties
Molecular 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[1]
Hazards
GHS pictograms The skull-and-crossbones pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)
GHS signal word Danger
GHS hazard statements H301
GHS precautionary statements P308, P310
EU classification Toxic T
R-phrases R25
S-phrases S37, S45
Main hazards Toxic (T)
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 1: Exposure would cause irritation but only minor residual injury. E.g., turpentine Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
LD50 60 mg/kg (rat, oral)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 YesY (verify) (what is: YesY/N?)
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.

Indicator colours[edit]

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 pKa of 3.47 in water at 25 degrees Celsius.[2]

Other indicators[edit]

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[edit]

Methyl orange has mutagenic properties.[1] Direct contact should be avoided.

See also[edit]

References[edit]

  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. 

External links[edit]