Methyl red

Methyl red
Names
	Preferred IUPAC name 2-{[4-(Dimethylamino)phenyl]diazenyl}; benzoic acid
Identifiers
CAS Number	493-52-7 (free base) ; 63451-28-5 (HCl salt) ; 845-10-3 (sodium salt) ;
3D model (JSmol)	Interactive image;
ChEBI	CHEBI:49770 ;
ChemSpider	9881 ;
ECHA InfoCard	100.007.070
EC Number	207-776-1;
KEGG	C19459 ;
PubChem CID	10303;
RTECS number	DG8960000;
UNII	69083AX1ZX (free base) ; RFA0W63690 (HCl salt) ; 311GZ0QHWN (sodium salt) ;
CompTox Dashboard (EPA)	DTXSID90859408 DTXSID1042154, DTXSID90859408 ;
	InChI InChI=1S/C15H15N3O2/c1-18(2)12-9-7-11(8-10-12)16-17-14-6-4-3-5-13(14)15(19)20/h3-10H,1-2H3,(H,19,20)/b17-16+ Key: CEQFOVLGLXCDCX-WUKNDPDISA-N ; InChI=1/C15H15N3O2/c1-18(2)12-9-7-11(8-10-12)16-17-14-6-4-3-5-13(14)15(19)20/h3-10H,1-2H3,(H,19,20)/b17-16+ Key: CEQFOVLGLXCDCX-WUKNDPDIBD;
	SMILES CN(C)c2ccc(/N=N/c1ccccc1C(O)=O)cc2;
Properties
Chemical formula	C15H15N3O2
Molar mass	269.304 g·mol−1
Density	0.791 g/cm3
Melting point	179–182 °C (354–360 °F; 452–455 K)
Solubility	soluble in ethanol
Acidity (pKa)	5.1
UV-vis (λmax)	410 nm (yellow form)
Hazards
	GHS labelling:
Pictograms
Signal word	Warning
Hazard statements	H351, H411
Precautionary statements	P201, P202, P273, P281, P308+P313, P391, P405, P501
NFPA 704 (fire diamond)	2 1 0
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). verify (what is ?) Infobox references

Methyl Red (pH indicator)
below pH 4.4		above pH 6.2
4.4	⇌	6.2

Methyl red (2-(N,N-dimethyl-4-aminophenyl) azobenzenecarboxylic acid), also called C.I. Acid Red 2, is an indicator dye that turns red in acidic solutions. It is an azo dye, and is a dark red crystalline powder. Methyl red is a pH indicator; it is red in pH under 4.4, yellow in pH over 6.2, and orange in between, with a pK_a of 5.1.^[2] Murexide and methyl red are investigated as promising enhancers of sonochemical destruction of chlorinated hydrocarbon pollutants. Methyl red is classed by the IARC in group 3 - unclassified as to carcinogenic potential in humans.

Color transition of methyl red solution under different acid–base conditions. Left: acidic, middle: about pH 5.1 (the pK_a), right: alkaline

Preparation

As an azo dye, methyl red may be prepared by diazotization of anthranilic acid, followed by reaction with dimethylaniline:^[3]

Properties

The color of methyl red is pH dependent, because protonation causes it to adopt a hydrazone/quinone structure.

Methyl Red has a special use in histopathology for showing acidic nature of tissue and presence of organisms with acidic natured cell walls.

Methyl Red is detectably fluorescent in 1:1 water:methanol (pH 7.0), with an emission maximum at 375 nm (UVA) upon excitation with 310 nm light (UVB).^[4]

Methyl red test

In microbiology, methyl red is used in the methyl red test (MR test), used to identify bacteria producing stable acids by mechanisms of mixed acid fermentation of glucose (cf. Voges–Proskauer test).

The MR test, the "M" portion of the four IMViC tests, is used to identify enteric bacteria based on their pattern of glucose metabolism. All enterics initially produce pyruvic acid from glucose metabolism. Some enterics subsequently use the mixed acid pathway to metabolize pyruvic acid to other acids, such as lactic, acetic, and formic acids. These bacteria are called methyl-red positive and include Escherichia coli and Proteus vulgaris. Other enterics subsequently use the butylene glycol pathway to metabolize pyruvic acid to neutral end products. These bacteria are called methyl-red-negative and include Serratia marcescens and Enterobacter aerogenes.

Process

A tube filled with a glucose phosphate broth is inoculated with a sterile transfer loop. The tube is incubated at 35 °C (95 °F) for 2–5 days. After incubation, 2.5 ml of the medium are transferred to another tube. Five drops of the pH indicator methyl red is added to this tube. The tube is gently rolled between the palms to disperse the methyl red.

Expected results

Enterics that subsequently metabolize pyruvic acid to other acids lower the pH of the medium to 4.2. At this pH, methyl red turns red, a positive test. Enterics that subsequently metabolize pyruvic acid to neutral end products lower the pH of the medium to only 6.0. At this pH, methyl red is yellow, a negative test.

References

^ ^a ^b ^c Sigma-Aldrich Co., Methyl red Archived 2015-12-09 at the Wayback Machine. Retrieved on 2014-06-02.
^ IB chemistry Higher Level: "IB Chemistry higher level notes: Indicators". Archived from the original on 2009-09-22. Retrieved 2009-11-29.
^ H. T. Clarke & W. R. Kirner (1941). "Methyl Red". Organic Syntheses; Collected Volumes, vol. 1, p. 374.
^ Kumar Das, Diganta; Goswami, Priyanka; Barman, Champa; Das, Biva (2012-12-30). "Methyl Red: A Fluorescent Sensor for Hg2+ over Na+, K+, Ca2+, Mg2+, Zn2+, and Cd2+". Environmental Engineering Research. Korean Society of Environmental Engineering: 75–78. doi:10.4491/eer.2012.17.s1.s75. ISSN 1226-1025.

"Microbiology, A Photographic Atlas for the Laboratory", Alexander, Street, Pearson Education, 2001.

External links

Nile Chemicals -- Methyl Red A site showing some extra information on methyl red.
Synthesis of methyl red

[sigma-1] Sigma-Aldrich Co., Methyl red Archived 2015-12-09 at the Wayback Machine. Retrieved on 2014-06-02.

[2] IB chemistry Higher Level: "IB Chemistry higher level notes: Indicators". Archived from the original on 2009-09-22. Retrieved 2009-11-29.

[3] H. T. Clarke & W. R. Kirner (1941). "Methyl Red". Organic Syntheses; Collected Volumes, vol. 1, p. 374.

[Kumar_Das_Goswami_Barman_Das_pp._75–78-4] Kumar Das, Diganta; Goswami, Priyanka; Barman, Champa; Das, Biva (2012-12-30). "Methyl Red: A Fluorescent Sensor for Hg2+ over Na+, K+, Ca2+, Mg2+, Zn2+, and Cd2+". Environmental Engineering Research. Korean Society of Environmental Engineering: 75–78. doi:10.4491/eer.2012.17.s1.s75. ISSN 1226-1025.

[1]

[2]

[3]

[4]