Methyl red

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Structural formula
Structure of methyl red
General
Surname Methyl red
other names
  • 2 - {[4- (Dimethylamino) phenyl] diazenyl} benzoic acid ( IUPAC )
  • 4-dimethylaminoazobenzene-2'-carboxylic acid
  • CI Acid Red 2
  • CI 13020
Molecular formula C 15 H 15 N 3 O 2
Brief description

red-purple solid with a faint odor

External identifiers / databases
CAS number
  • 493-52-7
  • 845-10-3 (sodium salt)
EC number 207-776-1
ECHA InfoCard 100.007.070
PubChem 10303
ChemSpider 21493709
Wikidata Q412276
properties
Molar mass 269.31 g mol −1
Physical state

firmly

Melting point

178-182 ° C

solubility
  • bad in water
  • 2.5 g l −1 in ethanol
safety instructions
GHS labeling of hazardous substances
no GHS pictograms
H and P phrases H: no H-phrases
P: no P-phrases
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Methyl red , also 4'-dimethylamino-azobenzene-2-carboxylic acid , is a water-insoluble dye from the group of azo dyes . The water-soluble sodium salt is an acid-base indicator .

properties

Methyl red is a red-purple solid with a faint odor.

Methyl red sodium

The color of aqueous methyl red solutions changes to red at a pH value of less than 4.4 and to yellow from 6.2. The solution is orange between pH 4.4 and 6.2.

The color transition from methyl red at different pH values. Left: acidic, middle: transition area, right: basic
At pH values ​​below 4.4, methyl red is present as a protonated form and is red (structure on the right), but turns yellow after deprotonation.

synthesis

The synthesis proceeds via an azo coupling . To do this, the amino group of the anthranilic acid is first converted into a diazonium group with the help of sodium nitrite and hydrochloric acid and this is reacted with N , N -dimethylaniline . This reaction must take place in the cold because of the instability of the diazo compound.

Representation of methyl red by azo coupling

use

In analytical chemistry , methyl red is an important indicator for the titration of strong acids with weak bases .

In microbiology , methyl red is used to stain bacteria for microscopic examinations that produce organic acids during the fermentation of glucose .

Individual evidence

  1. ↑ a b c d e f data sheet methyl red (CI 13020) (PDF) from Merck , accessed on May 9, 2017.
  2. ↑ External identifiers or database links for methyl red sodium salt : CAS number: 845-10-3, EC number: 212-682-9, ECHA InfoCard: 100.011.530 , GESTIS substance database : 107499 , PubChem : 4465632 , ChemSpider : 92048 , Wikidata : Q27255991 .
  3. ↑ Martha Windholz: The Merck index: an encyclopedia of chemicals and drugs . Ed .: Merck. 9th edition. Rahway NJ 1976, ISBN 0-911910-26-3 , pp. 798 .
  4. ^ Organikum authors' collective , 22nd edition, Wiley-VCH, Weinheim 2004, p. 645.

Web links