METHYL RED: CHARACTERISTICS, PREPARATION AND APPLICATIONS - BIOLOGY - 2025

The methyl red is a compound which functions as a pH indicator. Its chemical formula is C ₁₅ H ₁₅ N ₃ O _2, and the scientific name is dimethylamino-4-phenylazo-2 benzoic acid. The primary use of this pH indicator is limited to revealing carbohydrate bacterial fermentation reactions, specifically in the methyl red test.

In this type of test, methyl red detects changes in the pH of the medium. The methyl red pH indicator is itself acidic, and its color is red. This pH indicator will remain red below pH 4.2, while above 6.3 it will turn yellow, while in the intermediate range it produces various shades of orange.

Representative image of the methyl red indicator at acid pH and alkaline pH. Source: Pixabay.com/ ESMokrossA

In this sense, if the bacteria ferment the carbohydrate, it will form mixed acids that will acidify the culture medium, making them visible thanks to the pH indicator.

Methyl red also has other applications, such as in the spectrophotometric determination of bromine, being very useful to control the concentration of this halogen. It should be noted that bromine is present in residual waste generated by laboratories that manufacture chemical substances for pharmaceutical use.

On the other hand, this pH indicator is not toxic to humans, unlike other indicators and colorants found on the market.

Research is underway involving the methyl red indicator as a potential bioremediator in the removal of chlorinated hydrocarbons in nature. In addition, it has been used as a model to evaluate the action of silica nanoparticles that can serve to degrade residues containing azo dyes.

characteristics

Methyl red or dimethylamino-4-phenylazo-2 benzoic acid, also called 4-dimethylaminoazobenzene 2-carboxylic acid, results from the coupling between anthranilic acid and N, N-dimethylaniline.

It has a molecular mass of 269.116427 g / mol. Its range of turn is between 4.2-6.3. In this range its hue is orange. The absorption λmax is at 410 nm.

As its main characteristic, it has the ability to change its structure when it accepts protons or when it gives up protons. This structural change causes it to vary in color. This is a common feature with other pH indicators.

However, unlike others, it has the property of detecting acids at much lower pH. Therefore, it detects strong acids.

On the other hand, unlike other pH indicators and colorants, methyl red is not known to have toxicity, or any carcinogenic power. Therefore, it is classified within group 3 by the International Agency for Research on Cancer (IARC).

Preparation

Methyl red is prepared as follows: 0.1 gram of methyl red is weighed out and dissolved in 1500 ml of methanol.

The methyl red used as developer for the test of the same name (methyl red test) is prepared as follows:

0.1 g of methyl red is weighed into 300 ml of 95 ° ethyl alcohol. Subsequently, 200 ml of distilled water are added to the previous preparation.

It is recommended that the prepared solution be stored in a refrigerator, and if possible in aliquots at -20 ° C, the better. In this form it is stable for up to a month.

Applications

In the methyl red test

There is a laboratory test called methyl red. It is actually the modified Clark and Lubs medium, called Methyl Red / Voges-Proskauer (RM / VP). This liquid culture medium has nutritional elements (polypeptones), a pH buffer system and a glucose load.

The methyl red test measures the ability of microorganisms to produce acids via the mixed acids. The reaction is revealed by adding a few drops of the methyl red pH indicator.

If the indicator remains red when adding the drops and mixing, the test is positive. This means that the formation of acids occurred via the mixed acids. If, on the other hand, the color fades and remains the same color as the medium, the test is negative, indicating that other compounds were produced that alkalize the medium.

Acid-base indicator

Methyl red is useful as an acid-base indicator in chemical analysis. For example, it is used in the determination of proteins by the micro Kjeidahl method. In this technique, a mixed pH indicator prepared with 0.1% methyl red is used together with 0.2% bromocresol green in 95% alcohol. This mixture is used in the titration step.

Spectrophotometric determination of bromine

Methyl red is used in the spectrophotometric determination of bromine, especially in chemical bioactivity laboratories, where chemical substances for pharmaceutical use are produced that involve bromine as the main chemical element.

An example is the synthesis of 2-bromo-5 (-2-bromo-2-nitrovinyl) -furan. During the synthesis process toxic substances are produced.

These companies must regulate the amount of toxic substances in the waste they generate to ensure that said concentration is within the permissible limits. Gaytán et al. Describe a technique to detect low concentrations of bromine in residual waste, using methyl red.

The authors describe that the technique obtained excellent results under the following working conditions: pH = 2.5, time of 20 min and the addition of 3 mL of Na ₂ S ₂ O ₃.

Use of methyl red in scientific research

Methyl red has been used in various investigations, in order to find a possible solution to decontaminate water sources that receive the toxic waste generated by the textile industries, which use various azo dyes.

In this sense, Mahmoud in 2009 studied the photocatalytic degradation of the methyl red dye. They discovered that silica nanoparticles (SiO2 NP) are active in the photocatalytic degradation of this dye.

On the other hand, this pH indicator is being investigated as a potentially useful substance in the destruction of chlorinated hydrocarbons by electrochemical agents. This is extremely important, since chlorinated hydrocarbons are toxic compounds that damage the environment.

References

1. Gaytán E, Hernández B, Rodríguez, Negrín Z, Milián D. Spectrophotometric determination of bromine with methyl red. Cuban Journal of Chemistry, 2005; 17 (1): 54-60.
2. "Methyl red." Wikipedia, The Free Encyclopedia. 3 Oct 2018, 07:51 UTC. 17 May 2019, 17:38. en.wikipedia.
3. Himedia Laboratories. Methyl Red Indicator. Available at: himedialabs.com
4. Wikipedia contributors. "Methyl red." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 2 Jul. 2018. Web. May 17 2019.
5. Mahmoud MA, Poncheri A., Badr Y., Abd El Wahed MG Photocatalytic degradation of methyl red dye. S. Afr. j. sci. 2009; 105 (7-8): 299-303. Available from:.scielo.
6. Sandoval D. Kinetic study of bromination of methyl Red. 2004. Thesis to qualify for the degree of Bachelor of Chemistry. National Autonomous University of Nicaragua. Available at: riul.unanleon.edu