HNO2

Nitrous acid, with the chemical formula HNO2, is a weak acid that exists primarily in aqueous solution. It is a monoprotic acid, meaning it can donate only one proton (H+) per molecule. Nitrous acid is unstable and readily decomposes, particularly in concentrated solutions or upon heating, into nitric acid (HNO3) and nitric oxide (NO). The decomposition reaction can be represented as 3 HNO2 → HNO3 + 2 NO + H2O.

This compound is often generated in situ for specific chemical reactions, as it is not typically isolated in a pure form. Its preparation commonly involves the reaction of a nitrite salt, such as sodium nitrite (NaNO2), with a strong acid, like sulfuric acid (H2SO4) or hydrochloric acid (HCl). For example, NaNO2 + HCl → HNO2 + NaCl.

Nitrous acid plays a role in various chemical processes, including diazotization reactions, which are important in the synthesis of dyes and pharmaceuticals. In biological systems, it can be formed from the metabolism of amino acids and can react with amines to form nitrosamines, some of which are known carcinogens. Due to its instability and potential hazards, handling of nitrous acid and its precursors requires caution.