|CAS number||, (HCl salt), (sodium salt)|
|Jmol-3D images||Image 1|
|Molar mass||269.30 g mol−1|
|Melting point||179–182 °C (354–360 °F; 452–455 K) |
|Solubility||soluble in ethanol|
|R-phrases||R20, R21, R22, R36/37/38, R40|
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
|(what is: / ?)|
|Methyl Red (pH indicator)|
|below pH 4.4||above pH 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 pKa of 5.1. 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.
Methyl red test
The methyl red test is the "M" portion of the four IMViC tests used to characterize enteric bacteria. The methyl red test is used to identify enteric bacteria based on their pattern of glucose metabolism. All enterics initially produce pyruvic acid from glucose metabolism. Some enteric 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.
An isolate is inoculated into a tube with a sterile transfer loop. The tube is incubated at 35°C for 2–5 days. After incubation, 2.5ml of the medium is 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 of the hands to disperse the methyl red.
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 red color represents 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 yellow color represents a negative test.
- "Microbiology, A Photographic Atlas for the Laboratory", Alexander, Street, Pearson Education, 2001.
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