Methyl Orange - A pH Indicator

An indicator is used for the detection of endpoints in titrations. Methyl orange is one of the most popular indicators used for titration. Methyl orange is a weak base and is yellow in colour in the unionised form. When the medium turns acidic, it turns pinkish red in colour. It is a pH indicator used for titration in labs and industries because it gives clear and distinct colour variations at different pH levels.

What is Methyl Orange?

An azo dye is prepared from benzene, sulphanilic acid, and dimethylaniline. It is used in titration that gives different colours at different pH values. It is an indicator that tells whether a substance is acidic or basic. It is abbreviated as MeO. It gives a red colour with acids and yellow with the basic medium. Despite not having a full spectrum of colour change, it gives sharp end points.

When acidity decreases in a solution, methyl orange moves from red to orange and finally to yellow, the opposite occurs with the solution when acidity increases. The pH range of methyl orange is 3.1-4.4 and is mostly used for acidic solutions.

Structure of Methyl Orange

The structure of the methyl orange is shown below:

Methyl Orange Structure

General Properties of Methyl Orange

• IUPAC Name - Sodium 4-{(4-(dimethylamino)phenyl)diazenyl)}benzene-1-sulphonate

• Molecular Formula - C₁₄H₁₄N₃NaO₃S

• Density - 1.28 g/cm³

• Appearance - Orange or yellow solid

• Molecular Weight - 327.33g/mol

• Melting Point - 300°C approx.

General Preparation of Methyl Orange

A diazonium coupling reaction is used to prepare MeO. Firstly, sulphanilic acid and sodium nitrate react in presence of HCl. Then, the prepared diazotized sulfanilic acid is treated with N, N-dimethyl aniline to get the desired product that is methyl orange.

Preparation of Methyl Orange

It gets protonated at the central nitrogen atom from acid which gets reduced. It changes colour from orange to red.

Methyl Orange in Acid

Methyl Orange in Base

In the basic medium, the central nitrogen atoms again form double bonds and turn yellow from orange.

pH Range of Methyl Orange

• At pH 4.3, a transition occurs in Methyl Orange.

• It shows red colour – Acidic Medium, below pH 3.1.

• Orange colour – weak Acid, above 4.4.

• Yellow Colour – Basic medium, basic solution.

Difference between Phenolphthalein and Methyl Orange

Methyl orange in acidic conditions turns red whereas phenolphthalein remains colourless. In alkaline conditions, methyl orange turns yellow while phenolphthalein turns pink. Phenolphthalein has a pH range of 8.2-10.0, whereas methyl orange has a range of 3.1-4.4. The pKa of phenolphthalein is 9.4, whereas the pKa of methyl orange is 3.5.

Applications of Methyl Orange

• It is used as an indicator for titration.

• It gives colour variance at a different range of pH.

• It is a water-soluble azo dye.

Conclusion

There are many natural as well as synthetic acid-base indicators found around us. Natural indicators are the indicators that are found naturally and can help in determining the acidity or basicity of a substance. They can be found even at home like turmeric, red cabbage, china rose, onion, etc. Methyl orange and phenolphthalein are synthetic indicators. Synthetic indicators are prepared in the laboratory.

These indicators are used for titration purposes and to test the pH of different compounds. They are used in industries as well as in laboratories. Methyl orange is a water-soluble azo dye with the molecular formula C₁₄H₁₄N₃NaO₃S which is used for titration for testing pH values. It gives red colour in acidic medium and yellow in alkaline solution. It has a pH range of 3.1-4.4. Being useful in titration, it has some harmful effects also.