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Methyl red
Methyl red
Methyl red
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Methyl red
Skeletal formula of methyl red
Skeletal formula of methyl red
Ball-and-stick model of methyl red
Ball-and-stick model of methyl red
Names
Preferred IUPAC name
2-{[4-(Dimethylamino)phenyl]diazenyl}
benzoic acid
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.007.070 Edit this at Wikidata
EC Number
  • 207-776-1
KEGG
RTECS number
  • DG8960000
UNII
  • 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+ ☒N
    Key: CEQFOVLGLXCDCX-WUKNDPDISA-N ☒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
  • CN(C)c2ccc(/N=N/c1ccccc1C(O)=O)cc2
Properties
C15H15N3O2
Molar mass 269.304 g·mol−1
Density 0.791 g/cm3
Melting point 179–182 °C (354–360 °F; 452–455 K)[1]
Solubility soluble in ethanol[1]
Acidity (pKa) 5.1
UV-vis (λmax) 410 nm (yellow form)[1]
Hazards
GHS labelling:
GHS08: Health hazardGHS09: Environmental hazard
Warning
H351, H411
P201, P202, P273, P281, P308+P313, P391, P405, P501
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g. chloroformFlammability 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g. canola oilInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
2
1
0
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)
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 pKa 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 pKa), right: alkaline

Preparation

[edit]

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

Properties

[edit]

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

[edit]
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

[edit]

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 then mixed.

Expected results

[edit]

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

[edit]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Methyl red

View on Grokipedia
from Grokipedia
Methyl red is a synthetic and widely used in and for detecting pH changes in the range of 4.4 (red) to 6.2 (), characterized by its C₁₅H₁₅N₃O₂ and molecular weight of 269.3 g/mol. Appearing as dark red or violet crystals, methyl red has a of 179–182 °C and is practically insoluble in water but soluble in ethanol and acetic acid. Its structure features a central azo group (-N=N-) linking a moiety and a dimethylaminophenyl group, enabling its color transition based on in acidic conditions. In chemical applications, it serves as an indicator for acid-base titrations, such as those involving or weak organic bases, and in the determination of by measuring dye bleaching at 515 nm. In , methyl red is a key in the methyl red test for identifying capable of mixed acid , where it turns reddish in acidic environments produced by glucose metabolism and remains yellow in neutral or alkaline conditions. Beyond sensing, it finds use in , such as in methyl red-doped nematic liquid crystals, and as a model in photocatalytic degradation studies. Safety considerations include its classification as a suspected (IARC Group 3) and potential toxicity via ingestion or inhalation, necessitating careful handling.

Chemical Identity

Molecular Structure

Methyl red is an organic with the molecular formula C15H15N3O2C_{15}H_{15}N_3O_2. It consists of a central diazenyl group (-N=N-) that bridges two aromatic rings: a 2-carboxyphenyl moiety derived from and a 4-(dimethylamino)phenyl group. The azo bond is in the E (trans) configuration, as indicated by the IUPAC name 2-[(1E)-2-[4-(dimethylamino)phenyl]diazen-1-yl], which provides stability to the planar conjugated system essential for its chromophoric properties. The key functional groups in methyl red include a (-COOH) attached to the ortho position of one ring, a tertiary in the form of a dimethylamino group (-N(CH₃)₂) para to the azo linkage on the other ring, and the azo group (-N=N-) itself, which facilitates extended π-conjugation across the molecule. This arrangement of functional groups contributes to the molecule's reactivity and electronic delocalization, with the azo bridge acting as the primary . Methyl red exhibits azo-hydrazone tautomerism, a proton transfer equilibrium between the azo form (Ar-N=N-Ar') and the form (Ar-NH-N=Ar''), where the hydrogen from the or shifts to form a C=N bond within the ring system. This tautomerism alters the electronic distribution and conjugation length, influencing the compound's color through changes in absorption characteristics, though the azo form predominates under neutral conditions.

Nomenclature and Identifiers

Methyl red, a synthetic , is systematically named 2-[[4-(dimethylamino)phenyl]diazenyl]benzoic acid as its , reflecting its structure as a of with an azo linkage to a dimethylaminophenyl group. This adheres to IUPAC recommendations for azo compounds, emphasizing the diazenyl at the ortho position of the . Commonly referred to in chemical literature and commerce as Methyl Red or C.I. Acid Red 2, these synonyms highlight its historical use as a and its designation in the Colour Index system. Methyl red is classified as an under the Colour Index, assigned the number C.I. 13020, which standardizes its identification in the and dye industries. The following table summarizes key identifiers for methyl red:
Identifier TypeValue
CAS Registry Number
PubChem CID10303
InChI (Standard)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+
InChIKeyCEQFOVLGLXCDCX-UHFFFAOYSA-N
These identifiers facilitate precise referencing in databases and regulatory contexts.

Physical and Chemical Properties

Physical Properties

Methyl red appears as a dark crystalline or violet crystals under standard conditions. Its is 269.30 g/mol. The compound has a of 179–182 °C, at which it decomposes. Methyl red is insoluble in but soluble in (approximately 1 g/L), acetic acid, , and . In the form, it exhibits a UV-vis absorption maximum at λ_max = 410 nm. Methyl red displays fluorescence with an emission maximum at 375 nm upon excitation at 310 nm when dissolved in a 1:1 water:methanol mixture at pH 7.0.
PropertyValueConditions/Notes
AppearanceDark red crystalline powder or violet crystalsSolid form
Molar mass269.30 g/mol-
Melting point179–182 °CDecomposes
Solubility in waterInsoluble-
Solubility in ethanol~1 g/LAt room temperature
UV-vis λ_max (yellow)410 nmIn basic methanol
Fluorescence emission375 nm (exc. 310 nm)1:1 water:methanol, pH 7.0

Chemical Properties

Methyl red, chemically known as 2-[(4-dimethylamino)phenyl]diazenylbenzoic acid, exhibits acid-base properties characteristic of its and azo functionalities. The group has a pKa of approximately 2.3, while the of the azo group occurs with an apparent pKa of approximately 5.0, enabling the color transition in the range of 4.4–6.2. In its protonated state (azo -NH=N+), the extended conjugation across the azo linkage and aromatic rings imparts a , while disrupts this conjugation, shifting the absorption to produce a yellow hue. This / equilibrium is central to its reactivity in aqueous environments. The compound demonstrates notable stability under standard conditions but is susceptible to certain degradative influences. It is easily reduced, particularly at the azo group, leading to cleavage and loss of color as the chromophore is destroyed. Methyl red is also sensitive to light, with exposure causing photodegradation that varies by pH, and it reacts with strong oxidizing agents, potentially resulting in oxidative breakdown of the azo linkage. These sensitivities necessitate careful handling and storage away from reducing agents, light, and oxidants to maintain its integrity. In terms of behavior, the azo (-N=N-) group in methyl red is readily reducible to corresponding amines, such as 4-(dimethylamino) and , under mild reducing conditions like those involving or biological enzymes. This reduction pathway is exploited in analytical and studies, highlighting the compound's vulnerability in reductive environments. Additionally, its is pH-dependent; while sparingly soluble in neutral , it shows enhanced solubility in basic media due to the formation of the anion, which increases hydrophilicity and ionic interactions.

Synthesis

Preparation Methods

Methyl red is primarily synthesized through a diazotization-coupling reaction involving (2-aminobenzoic acid) and N,N-dimethylaniline. In the diazotization step, is treated with in the presence of at low temperature to form the corresponding diazonium salt. This intermediate then undergoes with N,N-dimethylaniline in an alkaline medium to yield the final . The reaction can be represented in two stages: First, the diazotization: \ceC6H4(NH2)COOH+NaNO2+HCl−>[0−5∘C][C6H4(N2+)COOH]Cl−+NaCl+H2O\ce{C6H4(NH2)COOH + NaNO2 + HCl ->[0-5^\circ C] [C6H4(N2+)COOH]Cl^- + NaCl + H2O} Followed by : \ce[C6H4(N2+)COOH]Cl−+C6H5N(CH3)2−>[alkaline]C15H15N3O2+HCl\ce{[C6H4(N2+)COOH]Cl^- + C6H5N(CH3)2 ->[alkaline] C15H15N3O2 + HCl} The overall process is conducted under controlled conditions to prevent of the unstable diazonium salt, with diazotization typically performed at 0–5°C using ice baths. The coupling occurs in a buffered alkaline solution, often with , at temperatures below 7°C initially, followed by warming to 20–25°C. Laboratory procedures commonly achieve yields of 62–66% after purification by recrystallization from or . The crude product is filtered, washed with acetic acid and , and purified to obtain the red crystalline solid with a of 181–182°C. Industrial-scale synthesis follows similar steps but uses larger quantities and optimized recovery of excess by . Alternative methods include variations in the diazotization medium, such as using instead of for the formation of the diazonium salt, which can improve in certain setups. Modern approaches may incorporate catalytic systems for , though the classical nitrite-based route remains predominant due to its simplicity and efficiency.

Historical Development

Methyl red, an , was first synthesized in by German chemists Emanuel Rupp and Richard Loose during the expansion of research that originated with Peter Griess's discovery of diazo salts in 1858. This synthesis involved the diazotization of and subsequent coupling with N,N-dimethylaniline, building on established methods for creating colored aromatic compounds used initially in textiles and later in analytical applications. The compound gained prominence as a in the early , with William Mansfield Clark and Herbert Thomas Lubs introducing its use in 1915 for detecting acid production in bacterial cultures, enabling differentiation of coliform organisms in microbiological assays. A seminal publication in 1921 by H. T. Clarke and Barney in Organic Syntheses provided a standardized procedure for its , emphasizing controlled diazotization in alcoholic solution to achieve high yields of the crystalline product, which solidified its role in laboratory practices. Commercial production of in was first documented in , coinciding with increasing demand for reliable indicators in chemical and biological testing. Over the subsequent decades, synthesis evolved from early empirical approaches reliant on trial-and-error coupling reactions to precise diazotization protocols that optimized temperature, , and reagent , reducing side products and improving scalability for industrial use. Key milestones included patents for variants in the early , though specific methyl red patents focused on purification enhancements rather than novel routes. In recent years, advancements in sustainable synthesis have targeted azo dyes broadly, with methods such as nitrate-based diazonium formation in organic solvents offering reduced and by-product-free processes compared to traditional routes; however, these have not yet been widely adapted for methyl red production.

Applications

As a

Methyl red functions as a , exhibiting a color transition from to across a specific range. Below 4.4, it appears ; between 4.4 and 6.2, it transitions through orange; and above 6.2, it is . The mechanism of this color change involves and of key functional groups, primarily the azo (-N=N-) linkage and the moiety, which modulate the molecule's electronic conjugation and absorption properties. In acidic conditions (protonated form), the azo group accepts a proton, extending the π-conjugation across the and shifting absorption to longer wavelengths (around 500-520 nm), resulting in the red color. In basic conditions (deprotonated form), loss of the proton disrupts this conjugation, moving absorption to shorter wavelengths (around 420-430 nm) and producing the color. In acid-base titrations, methyl red is employed to detect the endpoint, particularly in titrations of weak bases with strong acids (e.g., ), where the pH is approximately 5.3 and falls within its transition range. The indicator solution is typically prepared by dissolving 0.02% (w/v) methyl red in , which provides solubility and stability for addition to the titrand. One advantage of methyl red is its sharp color transition, facilitating clear visual detection of the endpoint without requiring precise . However, limitations include its narrow range, restricting applicability to titrations with equivalence points in the acidic to neutral region, and potential non-reversibility of the color change under rapid fluctuations or extreme conditions, which may lead to incomplete reversion upon adjustment.

In Microbiology

The methyl red (MR) test is a key biochemical in used to differentiate based on their glucose pathways, particularly within the Enterobacteriaceae family. As one of the four components of the test series—alongside , Voges-Proskauer, and citrate utilization—the MR test specifically detects the ability of to produce and maintain stable acidic end products from glucose through mixed acid . The biochemical basis of the MR test relies on the detection of significant acid production that lowers the of the culture medium to below 4.4. During mixed acid , certain , such as those in the group, convert glucose to stable acids like lactic, acetic, formic, and succinic acids, which do not volatilize and thus sustain a low environment. Methyl red, a -sensitive indicator, changes color in response to this acidification: it remains red at values of 4.4 or lower, indicating a positive result for mixed acid producers. In contrast, that perform , such as those in the group, produce neutral end products like and 2,3-, resulting in less acidification and a higher . To perform the MR test, a bacterial inoculum is added to , a glucose-phosphate medium that supports without buffering the excessively, and incubated aerobically at 37°C for 48 to 72 hours to allow sufficient acid accumulation. After incubation, approximately 2.5 mL of the culture is transferred to a clean tube, and 6-8 drops of methyl red reagent—prepared as a 0.02% solution of methyl red in 95% —are added directly to the sample. The color is observed immediately upon mixing, as the indicator reacts instantaneously to the prevailing without requiring further incubation. A positive MR test result is indicated by a persistent red color in the medium, signifying a below 4.4 and confirming mixed acid fermentation; examples include and species, which are commonly identified in clinical and environmental samples for their enteric pathogenic potential. A negative result appears as a or orange color, corresponding to a above 6.0 due to insufficient acid stability; representative organisms include and . This differentiation aids in and identification, particularly in distinguishing coliforms from non-coliforms in assessments and infection diagnostics.

Other Uses

In , methyl red serves as a to highlight acidic tissues and microorganisms with acidic cell walls in fixed samples, aiding in the visualization of pH-related structural features during histological analysis. For instance, it has been applied to validate activity in dental tissues by indicating acidic environments post-dye application on sections. This utility stems from its color transition in response to local acidity, complementing standard histological techniques for targeted . In , methyl red is employed in testing through indicator-impregnated strips that change color based on sample acidity, typically in combination with bromthymol blue to cover a range of 5.0 to 8.5. These strips facilitate rapid, non-invasive assessment of urinary , which is crucial for diagnosing conditions like urinary tract infections or metabolic disorders. Emerging applications include methyl red as a component in optical sensors for environmental monitoring, particularly for assessing water acidity in aquatic systems. For example, fiber-optic pH sensors incorporating methyl red as the optical indicator enable real-time detection of pH variations in water samples, supporting pollution tracking and ecosystem health evaluation. Recent developments have also explored its integration into membrane-based systems for enhanced sensitivity in low-pH environmental assays. In photocatalysis, methyl red is studied as a model azo dye for evaluating the efficiency of semiconductor materials like ZnO in degrading textile effluents under solar irradiation, contributing to wastewater treatment advancements. Methyl red is also used in nonlinear optics, such as in methyl red-doped nematic liquid crystals for photoalignment applications. Industrially, methyl red, known as Acid Red 2, finds use in dyeing for imparting red hues to fabrics, leveraging its azo for strong coloration on natural and synthetic fibers. However, its application is limited by concerns, including potential carcinogenicity and environmental persistence, prompting shifts toward greener alternatives in modern processes. Despite these constraints, it remains relevant in niche sectors for acid techniques.

Safety and Environmental Considerations

Health Hazards

Methyl red has no harmonized classification under the (GHS) for carcinogenicity, though its structure raises general concerns based on limited from and mechanistic considerations, with comprehensive human data lacking. The International Agency for Research on Cancer (IARC) categorizes methyl red as Group 3, not classifiable as to its carcinogenicity to humans, due to inadequate from epidemiological and . Potential mutagenic effects have been noted in some classifications for related azo compounds, but specific data for methyl red are inconclusive. Acute toxicity of methyl red is low, with an oral LD50 greater than 2000 mg/kg in rats, suggesting it is not highly poisonous upon single exposure. Primary exposure routes include dermal contact, ocular exposure, inhalation of dust from its powdered form, and accidental ingestion. Skin and eye contact may cause irritation or severe damage, while inhalation can irritate the respiratory tract; chronic exposure might lead to organ effects, though evidence is limited to general azo dye concerns. Safe handling requires (PPE) such as gloves, safety goggles, and respirators in dusty environments to prevent absorption or . Avoid ingestion and ensure good ventilation; store in a cool, dry place away from incompatibles like strong oxidizers. measures include immediately flushing eyes with water for at least 15 minutes, washing skin with soap and water, seeking fresh air for exposure, and inducing vomiting or seeking medical attention if ingested—do not give anything by mouth to an unconscious person. Methyl red is registered under the European REACH regulation (EC) 1907/2006, with annual production/import volumes of 1-10 tonnes in the EEA, subjecting it to evaluation for safe use. No specific permissible exposure limit (PEL) has been established by the Occupational Safety and Health Administration (OSHA).

Environmental Impact

Methyl red, an azo dye, poses risks to aquatic ecosystems due to its ecotoxicity, classified under the Globally Harmonized System (GHS) as H411: toxic to aquatic life with long-lasting effects. Its bioaccumulation potential is low, with an octanol-water partition coefficient (log Kow) of approximately 3.8, suggesting limited partitioning into fatty tissues of organisms. As a persistent , methyl red degrades slowly in natural environments, particularly under aerobic conditions, but undergoes anaerobic reduction to form aromatic amines such as N,N-dimethyl-p-phenylenediamine, which exhibit higher than the original . This transformation exacerbates ecological harm by producing more bioavailable and mutagenic byproducts. Methyl red's environmental fate involves moderate solubility in (especially in alkaline conditions), facilitating its and detection in effluents from analyses and industries. Regulatory frameworks address these concerns through restrictions on azo dyes under EU REACH Annex XVII (entry 43), prohibiting those that cleave to carcinogenic aromatic amines in textiles and leather at levels above 30 mg/kg. In the United States, the EPA enforces effluent monitoring for synthetic dyes under the Clean Water Act, requiring treatment to limit discharges into surface waters. studies demonstrate 70-95% removal of methyl red in systems, influenced by microbial consortia and operational parameters like and oxygen levels. To mitigate environmental release, plants are recommended to employ biological processes such as or advanced microbial systems, which enhance dye decolorization and mineralization while minimizing toxic intermediates.

References

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