A weak acid, HNO2, known only in solution and in the gas phase. It is prepared by the action of acids upon nitrites, preferably using a combination that removes the salt as an insoluble precipitate (e.g. Ba(NO2)2 and H2SO4). The solutions are unstable and decompose on heating to give nitric acid and nitrogen monoxide. Nitrous acid can function both as an oxidizing agent (forms NO) with I − and Fe 2+, or as a reducing agent (forms NO3−) with, for example, Cu 2+; the latter is most common. It is widely used (prepared in situ) for the preparation of diazonium compounds in organic chemistry. The full systematic name is dioxonitric(III) acid.