| IUPAC name
| Systematic IUPAC name
| Other names
|Molar mass||47.013 g/mol|
|Appearance||Pale blueish solution|
|Density||1-1.1±0.1 g/cm3 (1.1 for maximum possible concentration)|
|Melting point||Only known in aqueous solution|
|Boiling point||Only known in aqueous solution|
|Solubility||Reacts with bases, hydrazoic acid|
|Safety data sheet||Guidechem|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Nitrous acid is a monobasic weak acid, which can only exist in dilute aqueous solution. Salts of this acid (nitrites) are stable and if stored properly, they have infinite shelf life. It has the chemical formula HNO2.
Free nitrous acid is unstable. It decomposes to release nitrogen oxides:
- 2 HNO2 → NO2 + NO + H2O
Nitrous acid can only exist as solution and as such, all of its properties result from the solution, rather than pure compound.
Nitrous acid is not sold by chemical suppliers due to its instability and must be made in situ.
Nitrous acid can be prepared by carefully adding a cold and dilute solution of a strong acid to a nitrite salt. If the acid solution is too concentrated or the addition is too sudden, some of the nitrous acid will rapidly decompose.
- 3 HX + 3 MNO2 → 3 MX + HNO2 + NO + NO2 + H2O
Dissolving dinitrogen trioxide in cold water will result in a solution of pure nitrous acid:
- N2O3 + H2O → 2 HNO2
- Make nitrite salts
- Destroy azides
- Make diazonium salts
Nitrous acid is corrosive and it tends to give off nitrogen oxides, which are irritant and toxic.
Nitrous acid cannot be stored, due to its disproportionation into nitrogen oxides and nitric acid. It is made in situ and is used immediately.
Nitrous acid can be neutralized with any base. Sodium percarbonate can also be used, as it will oxidize the nitrous acid to nitric acid and the sodium carbonate will neutralize the said acid to the relatively harmless sodium nitrate.