chemistry

Methyl red

Methyl red is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Methyl red rather than just read about it. In short: 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.

Key takeaways

Methyl red belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Methyl red to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Methyl red from memory before moving on to harder problems.

Reference excerpt

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.

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

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).

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 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

"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

Illustrations

Worked examples

Example 1 — a first encounter with Methyl red

Start with the simplest possible case. Write down what Methyl red claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Methyl red before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Methyl red ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Methyl red

In research: Methyl red appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Methyl red in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Methyl red is common in secondary-school and first-year university syllabi. It links to neighbouring topics Anthranilic acids, Azo dyes, Dimethylamino compounds, so understanding it makes those chapters shorter.
In everyday life: Look for Methyl red outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Methyl red in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Methyl red means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Methyl red out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Methyl red in simple terms?

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.

Why does Methyl red matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Methyl red?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Methyl red.