{{short description|Chemical indicator that turns red in acidic solutions}} {{Chembox | Verifiedfields = changed | Watchedfields = changed | verifiedrevid = 433999215 | Name = Methyl red | ImageFile1 = Methyl red.svg{{!}}class=skin-invert | ImageSize1 = | ImageName1 = Skeletal formula of methyl red | ImageFile2 = Methyl-red-from-xtal-3D-balls.png{{!}}class=bg-transparent | ImageSize2 = | ImageName2 = Ball-and-stick model of methyl red | ImageFile3 = Crystals of Methyl red sodium salt.jpg | ImageSize3 = | PIN = 2-<nowiki/>{[4-(Dimethylamino)phenyl]diazenyl}<br />benzoic acid |Section1={{Chembox Identifiers | CASNo_Ref = {{cascite|correct|CAS}} | CASNo = 493-52-7 | CASNo_Comment = (free base) | CASNo1_Ref = {{cascite|correct|CAS}} | CASNo1 = 63451-28-5 | CASNo1_Comment = (HCl salt) | CASNo2_Ref = {{cascite|correct|CAS}} | CASNo2 = 845-10-3 | CASNo2_Comment = (sodium salt) | UNII_Ref = {{fdacite|correct|FDA}} | UNII = 69083AX1ZX | UNII_Comment = (free base) | UNII1_Ref = {{fdacite|correct|FDA}} | UNII1 = RFA0W63690 | UNII1_Comment = (HCl salt) | UNII2 = 311GZ0QHWN | UNII2_Comment = (sodium salt) | UNII2_Ref = {{fdacite|correct|FDA}} | ChEBI_Ref = {{ebicite|correct|EBI}} | ChEBI = 49770 | EINECS = 207-776-1 | KEGG_Ref = {{keggcite|correct|kegg}} | KEGG = C19459 | SMILES = CN(C)c2ccc(/N=N/c1ccccc1C(O)=O)cc2 | PubChem = 10303 | RTECS = DG8960000 | ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}} | ChemSpiderID = 9881 | 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+ | InChIKey = CEQFOVLGLXCDCX-WUKNDPDIBD | StdInChI_Ref = {{stdinchicite|changed|chemspider}} | StdInChI = 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+ | StdInChIKey_Ref = {{stdinchicite|changed|chemspider}} | StdInChIKey = CEQFOVLGLXCDCX-WUKNDPDISA-N }} |Section2={{Chembox Properties | C=15 | H=15 | N=3 | O=2 | Density = 0.791 g/cm<sup>3</sup> | MeltingPtC = 179-182 | MeltingPt_ref = <ref name=sigma /> | BoilingPt = | LambdaMax = 410 nm (yellow form)<ref name=sigma>Sigma-Aldrich Co., [https://www.sigmaaldrich.com/catalog/product/sial/250198 Methyl red] {{Webarchive|url=https://web.archive.org/web/20151209184611/http://www.sigmaaldrich.com/catalog/product/sial/250198 |date=2015-12-09 }}. Retrieved on 2014-06-02.</ref> | SolubleOther = soluble in ethanol<ref name=sigma /> | pKa = 5.1 }} |Section7={{Chembox Hazards | GHSPictograms = {{GHS08}}{{GHS09}} | GHSSignalWord = Warning | HPhrases = {{H-phrases|351|411}} | PPhrases = {{P-phrases|201|202|273|281|308+313|391|405|501}} | NFPA-H = 2 | NFPA-F = 1 | NFPA-R = 0 }} }}
{{pH_indicator_template|indicator_name=Methyl Red|low_pH=4.4|high_pH=6.2|low_pH_color=red|high_pH_color=yellow|low_pH_text=white}} '''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 p''K''<sub>a</sub> of 5.1.<ref>IB chemistry Higher Level: {{cite web |url=https://ibchem.com/IB/ibnotes/full/aab_htm/18.6.htm |title=IB Chemistry higher level notes: Indicators |access-date=2009-11-29 |url-status=dead |archive-url=https://web.archive.org/web/20090922042431/https://ibchem.com/IB/ibnotes/full/aab_htm/18.6.htm |archive-date=2009-09-22 }}</ref> 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. thumb|none|450px|Color transition of methyl red solution under different acid–base conditions. Left: acidic, middle: about pH 5.1 (the p''K''<sub>a</sub>), right: alkaline
==Preparation== As an azo dye, methyl red may be prepared by diazotization of anthranilic acid, followed by reaction with dimethylaniline:<ref>{{OrgSynth | author = H. T. Clarke |author2=W. R. Kirner |name-list-style=amp | title = Methyl Red | collvol = 1 | collvolpages = 374 | prep = cv1p0374 | year = 1941}}</ref> :558x558px
==Properties== The color of methyl red is pH dependent, because protonation causes it to adopt a hydrazone/quinone structure. :543x543px
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).<ref name="Kumar Das Goswami Barman Das pp. 75–78">{{cite journal | last1=Kumar Das | first1=Diganta | last2=Goswami | first2=Priyanka | last3=Barman | first3=Champa | last4=Das | first4=Biva | title=Methyl Red: A Fluorescent Sensor for Hg2+ over Na+, K+, Ca2+, Mg2+, Zn2+, and Cd2+ | journal=Environmental Engineering Research | publisher=Korean Society of Environmental Engineering | date=2012-12-30 | issn=1226-1025 | doi=10.4491/eer.2012.17.s1.s75 | pages=75–78}}</ref>
==Methyl red test== [[File:Methylrot Probe methyl red test.jpg|thumb|left|Methyl red test: ''Escherichia coli'' (left side) showing a 'positive' result, and ''Enterobacter cloacae'' (right side) showing a 'negative' result]] 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 {{convert|35|C}} 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 then mixed.
===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.
==See also== *Chrysophenin *Methyl *Universal Indicator *Tashiro's indicator *pH indicators *Methyl yellow *Methyl orange *Methyl violet
==References== {{reflist}} *"Microbiology, A Photographic Atlas for the Laboratory", Alexander, Street, Pearson Education, 2001.
== External links == {{Commons category|Methyl red}} * [https://www.nilechemicals.com/methylred.htm Nile Chemicals -- Methyl Red] A site showing some extra information on methyl red. * [https://www.orgsyn.org/demo.aspx?prep=CV1P0374 Synthesis of methyl red] {{Clinical microbiology techniques}} Category:IARC Group 3 carcinogens Category:PH indicators Category:Azo dyes Category:Anthranilic acids Category:Dimethylamino compounds