Nitrous acid and nitric acids are two types of acids containing nitrogen, among which nitrous acid is weaker than nitric acid. Nitrous acid with a pK_a value of 3.37 ionizes in water to give a nitrite ion and the hydronium ion.

The nitrous acid is unstable. Hence, it is formed in situ from a solution of sodium nitrite and cold aqueous acids such as hydrochloric or sulfuric acid. In an acidic solution, the –OH group of nitrous acid undergoes protonation to give oxonium ion, followed by water loss to form nitrosonium ion. Nitrosonium ion is a highly reactive electrophile that undergoes a reaction with the nucleophilic nitrogen atom of an amine.

Nitrous acid can react with different classes of amines, such as primary, secondary, and tertiary aliphatic or aromatic amines, to give different products. For example, nitrous acid reacts with primary amines to form a diazonium salt. The reaction is called the diazotization of amines. Nitrous acid reacts with secondary amines to yield N-nitrosamine. With tertiary aliphatic amines, nitrous acid reacts to generate water-soluble salts and N-nitrosoammonium compounds. In contrast, tertiary arylamines undergo electrophilic aromatic substitution to form aromatic C-nitroso compounds.