Methyl Orange

What is Methyl Orange?

Methyl orange has the property to colour alkaline and neutral water yellow. The water turns red as soon as it becomes acidic. At pH 4.3, the transition occurs. The titration is performed with hydrochloric acid at a concentration of 0.1 mol/L if the solution is yellow.

Methyl Orange

Because of its simple and distinct colour variation at various pH values, methyl orange is a pH symbol commonly used in titration. In an acidic medium, methyl orange is red, and in a basic medium, it is yellow.

Methyl Orange is a weak acid that breaks down into orange neutral molecules when it comes into contact with water. The equilibrium is to the left in acidic conditions, and the concentration of neutral molecules is too poor to see the orange colour.

General Properties of Methyl Orange

Some general properties of methyl orange are tabulated below.

Chemical formula C14H14N3NaO3S
Molar mass 327.33 g·mol−1
Appearance Orange or yellow solid
Density 1.28 g/cm3

Methyl Orange Structure

The molecular formula for methyl orange is C14H14N3NaO3S. The structure is depicted below. 

Methyl Orange Structure

General Preparation of Methyl Orange

In alkaline solution its colour is yellow, but the colour changes to red on the addition of a mineral acid. This change of colour is not produced by carbonic or other feeble acids. Hence, this indicator may be used for the titration of the more powerful mineral acids in the presence of carbonic acid and the feebler organic acids. The presence of much water causes the red colour of a faintly acid solution of methyl orange to become yellow, probably due to hydrolytic dissociation.

The indicator solution generally used is prepared by dissolving 0.1gm in 100cc of distilled water. One drop of this solution is used for each 20cc of solution to be titrated. 

A somewhat more sensitive solution, but one in which it requires more experience to detect the colour change, is prepared by dissolving 0.02gm in 100cc of hot water, allowing the solution to cool, and filtering out any deposited m-sulfonic acid. This more dilute indicator is to be preferred for very exact titrations with very dilute acids and alkalis.

Preparation of Methyl Orange from Sulfanilic Acid

A diazonium coupling reaction was used to produce methyl orange from sulfanilic acid and N, N-dimethylaniline, which is a typical reaction for treating an aliphatic amine to yield a carbocation. A primary aliphatic amine reacts with nitrous acid to form an unstable diazonium salt that loses N2 to form a carbocation.

After that, the carbocation may lose a proton to form an alkene, react with a nucleophile, or rearrange. Dimethylaniline is the nucleophile in this case. The bulky dimethylamine substituent acts as a steric hindrance in the ortho position, causing attack in the para position. Since you’re making an azo dye, a Spectrophotometer would be able to tell you how pure your product is.

The chemical reactions for the preparation of methyl orange from sulfanilic acid is given below.

Preparation of Methyl Orange from Sulfanilic Acid

The mechanism of this reaction is given below.

Preparation of Methyl Orange Machanism

Difference between Phenolphthalein and Methyl Orange

Some main differences between methyl orange and phenolphthalein are given below.

  • In acid, the methyl orange indicator turns red, while phenolphthalein remains colourless.
  • In a basic solution, methyl orange turns yellow, while phenolphthalein turns pink.
  • Titration to a pH of 8.3 determines phenolphthalein alkalinity, which means the total hydroxide and half of the carbonate present. Titration to a pH of 5.1, 4.8, 4.5, or 3.7, depending on the amount of carbon dioxide present, determines total alkalinity.

Structure of Methyl Orange in Acidic and Basic Medium

In an acidic medium, methyl orange turns red, while in a basic medium, it turns yellow. It’s commonly used in acid titration because it changes colour at the pKa of a mid-strength acid. Methyl orange, unlike a universal predictor, does not have a complete spectrum of colour transition, but it does have a sharp end point.

Methyl Orange is a weak acid that breaks down into orange neutral molecules when it comes into contact with water. The equilibrium is to the left in acidic conditions, and the concentration of neutral molecules is too poor to see the orange colour.

The colour of the solution changes to red when methyl orange is added to dilute hydrochloric acid. Methyl orange is a popular pH indicator that is used in titration. The colour of the solution turns red when methyl orange is used as an indicator of acid. When methyl orange is mixed with a base, the result is yellow, or the colour changes to yellow. As a pH symbol, methyl orange is used.

Structure of Methyl Orange in Acidic and Basic Medium

Frequently Asked Questions – FAQs

Is methyl orange a weak acid or base?

Methyl Orange is a weak acid that breaks down into orange neutral molecules when it comes into contact with water. The equilibrium is to the left in acidic conditions, and the concentration of neutral molecules is too poor to see the orange colour.

Why is methyl orange a good indicator?

Because of its simple and distinct colour shift, methyl orange is a commonly used pH indicator in titrations. It’s commonly used in acid titrations because it changes colour at the pH of a midi-strength acid. In an acidic medium, methyl orange turns red, while in a basic medium, it turns yellow.

Why does methyl orange turn yellow in an alkaline solution?

The sodium salt is protonated to produce normal sulfonic acid as the solution is made more alkaline (SO3H). As this occurs, the chemical’s light absorbance properties change, and it becomes yellow.

Is methyl orange toxic?

If swallowed, it can be fatal. Nausea, vomiting, and diarrhoea can occur as a result of gastrointestinal irritation. Inhalation: Can irritate the respiratory tract. This substance’s toxicological properties have yet to be thoroughly studied.

What happens when methyl orange is added to vinegar?

As a solution becomes less acidic, methyl orange changes colour from red to orange to purple, and vice versa when the acidity of the solution increases. The entire colour shift takes place in an acidic setting.

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