Rouge de méthyle

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. As an azo dye, methyl red may be prepared by diazotization of anthranilic acid, followed by reaction with dimethylaniline: Methyl red displays pH dependent photochromism, with protonation causing 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). 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. A tube filled with a glucose phosphate broth is inoculated with a sterile transfer loop.