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(HNO2) a monoprotic, unstable, and quite weak acid; it exists only in dilute cold water solutions. Its structural formula is HO—N=0. Its dissociation constant is 4.5 x 10-4 at 18°C. It forms along with nitric acid when N02 is dissolved in water:
2N02 + H20 = HN02 + HN03
Upon heating and the action of strong acids or oxidizers, nitrous acid dissociates with the formation of nitric oxide:
3HN02 = HNO3 + 2NO + H20
The compounds N20, NO, NH2OH, NH3, and others may be obtained by the reduction of nitrous acid. The salts of nitrous acid (nitrates) are obtained by the reduction of nitrates.
The most important property of nitrous acid is its ability to diazotize aromatic amines. For this reason, sodium nitrate (NaN02) is widely used in the production of azo dyes; free nitrous acid is produced upon the action of acids on this salt:
NaN02 + HCl = NaCl + HN02
Nitrous acid and its salts are poisonous if ingested. Poisoning of professionals is rare. Sodium nitrite is used in medicine in cases of cardiac angina and spasms of the vessels of the cerebrum.