A Study on the Preparation of Nitrous Acid

Nitrous acid, along with nitric oxide, is produced from nitrites and has antimicrobial activity.  Nitrous acid is weak; the conjugate acid of nitrite. It is a very unstable compound and mainly can be found in a solution state owing to its instability.  Nitrous acid is also called nitrogen oxoacid and has a molecular formula of HNO₂.

The commonly accepted IUPAC name for this structure/compound is nitrous acid.  The structure of nitrous acid is also found in other chemical compounds; some examples of these are-

• Rolitetracycline nitrate

• Amine nitrate

• Tetrahydrozoline nitrate

• Naphazoline nitrate

Some properties of nitrous acid

Since it is readily decomposable, nitrous acid is rarely found in a gaseous state. Whenever present in the gas phase, nitrous acid can adapt both syn or anti conformations. Anti conformation being more stable is abundant.

It is a pale blue colour solution. The molecular mass of nitrous acid is recorded to be 47.014, and the density is 1gm/ml. _pK_a of the nitrous acid is 3.15. it is a monoprotic acid.

Uses of nitrous acid

1. Nitrous acid is the core ingredient of a chemical reagent used in spot tests for the detection of alkaloids. The reagent is known as the Liebermann reagent.

2. Sodium nitrite and resulting nitrous acid after decomposition come under the list of essential medicines given by the World Health Organization (WHO). It is given in combination with sodium thiosulphate in the treatment of cyanide poisoning.

3. Apart from that, nitrous acid and its derivatives are very important from a biological or physiological perspective. Some studies show that nitrous acid (30 per cent) works for blood vessel dilation, inflammation, and antimicrobial component.

Preparation of nitrous acid

Owing to the instability of nitrous acid, it can be stored as nitric acid. Nitrous acid thus has to be prepared for any experiment at the instant of its application.

Nitrous acid is prepared by the acidification method. The reaction of sodium nitrite solution and mineral acid generates nitrous acid. Step by step procedure for laboratory synthesis of nitrous acid is as follows:

1. If sodium nitrite is not available, the preparation could start with sodium nitrate solution. Heat the solution of sodium nitrate so that it starts decomposing. The product of sodium nitrate decomposition is nothing but sodium nitrite. Sodium nitrite is a yellow colour powder and will precipitate as the reaction proceeds.

2. The second step is the collection of precipitates formed from sodium nitrate. This sodium nitrate powder is then added to the water to get an aqueous solution of sodium nitrite.

3. In the third step, you will use a second reagent that is a mineral acid, like hydrochloric acid, in dilute form (dil HCl). Ice cold dilute HCl is added into the above aqueous solution of sodium nitrite. The reaction will generate nitrous acid and sodium chloride. Nitrous acid is very dynamic and remains stable under low-temperature conditions only. Hence it is important to maintain a low temperature during the entire reaction and even after that.

An alternative process for the generation of nitrous acid is from dinitrogen trioxide. Dinitrogen trioxide in an aqueous solution readily reacts with water to form nitrous acid. But as mentioned above, maintaining low-temperature conditions is the key point in this method as well.

Despite its role in pharmacology, nitrous acid is not, rather cannot be produced on a commercial scale. Hence these two methods for the preparation of nitrous acid are of importance.

Conclusion

Nitrous acid is a highly unstable compound but of major importance as a biologically active molecule. The readily degradable nature of nitrous acid also puts restrictions on efforts for its generation on a large-scale basis for commercial use. Nevertheless, sodium nitrite can be prepared and stored very easily. This sodium nitrite is used for the transient production of nitrous acid, as mentioned above in detail. Also, the key factor in the successful preparation of nitrous acid, which could be stably present in an aqueous solution at least for some time, is managing cold temperature. The low temperature should be maintained during the reaction as well as afterwards for transient storage.