Nitrous acid

Nitrous acid (molecular formula HNO2) is a weak and monobasic acid known only in solution and in the form of nitrite (NO−2) salts. Nitrous acid is used to make diazonium salts from amines. The resulting diazonium salts are reagents in azo coupling reactions to give azo dyes. Nitrous acid (molecular formula HNO2) is a weak and monobasic acid known only in solution and in the form of nitrite (NO−2) salts. Nitrous acid is used to make diazonium salts from amines. The resulting diazonium salts are reagents in azo coupling reactions to give azo dyes. In the gas phase, the planar nitrous acid molecule can adopt both a cis and a trans form. The trans form predominates at room temperature, and IR measurements indicate it is more stable by around 2.3 kJ mol−1. Nitrous acid is usually generated by acidification of aqueous solutions of sodium nitrite with a mineral acid. The acidification is usually conducted at ice temperatures, and the HNO2 is consumed in situ. Free nitrous acid is unstable and decomposes rapidly. Nitrous acid can also be produced by dissolving dinitrogen trioxide in water according to the equation Gaseous nitrous acid, which is rarely encountered, decomposes into nitrogen dioxide, nitric oxide, and water: Nitrogen dioxide disproportionates into nitric acid and nitrous acid in aqueous solution: