Nitrous Acid | Uses, Structure, Preparation, Properties |

Nitrous Acid.

Nitrous Acid.

Nitrous acid does not exist free, but is known only in solution.

Method Of Preparation:

  1. “Nitrous acid” can be prepared by the action of water on N2O3.

  2. When barium nitrite is treated with calculated quantity of dil. H2SO4, then HNO2 is produced. BaSO4 is precipitated down. Solution of nitrous acid is filtered. Since HNO2 is very unstable so the reaction is carried out at the temperature of freezing mixture.

Nitrous Acid Uses.

  1. Nitrous acid is used in organic chemistry for the preparation of diazo compounds. These compounds are extremely important in organic compound synthesis and preparation of dyes.
  2. It is used as oxidizing and reducing agent in analytical chemistry. 

Nitrous Acid Structure:

Following possible structures has been proposed for HNO2

The structure proposed from the IR study of Nitrous Acid, is as below:

Nitrous Acid Structure.

Physical Properties Of Nitrous Acid.

  1. The aqueous solution of Nitrous acid is pale blue. This blue colour is due to the presence of N2O3.
  2. It is a weak Acid.
  3. It is unstable compound and is decomposed even in cold solution.

Chemical Properties of Nitrous Acid.


It decomposes in cold solution and undergoes auto-oxidation

At higher temperature, it decomposes into NO and NO2

As a reducing agent.

Nitrous Acid can be oxidized into HNO3 by certain other oxidizing agents. Those oxidizing agents provide nascent oxygen to HNO2 to make it HNO3.
Following are some important reactions, in which certain strong oxidizing agents are reduced and they can oxidize HNO2 to HNO3.

Reduction of acidified KMnO4:

Reduction of Br2 and l2:

Reduction of acidified K2Cr2O7:

Reaction with alkalies:

HNO2 reacts with alkalies to form nitrites. The nitrites are mostly more stable than HNO2.

Reaction with urea:

It reacts with urea and decomposes it to give CO2 and N2

Reaction with aniline:

It reacts with aniline hydrochloride to give diazonium compounds. This process is called diazotization.

Safety Hazards or Health Effects.

Nitrous acid is a strong oxidizer, and is destroyed when combined with phosphorus trichloride (PCL3). It is not toxic enough, however, it can cause respiratory disease and cause some symptoms of irritation in asthma patients.

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