Nitrous Acid
Properties
| State | Liquid (exists only in cold dilute aqueous solution) |
| Color | Colorless to pale blue |
| Solubility | Miscible with water; decomposes in solution |
| Melting Point | Not applicable (solution only) |
| Boiling Point | Not applicable (decomposes) |
About Nitrous Acid
Nitrous acid is a weak, unstable inorganic acid with the formula HNO2 and a molar mass of 47.013 g/mol. Nitrogen sits in the +3 oxidation state here, which is why HNO2 can act as either oxidizer or reductant depending on what it's reacting with. You cannot bottle it — in solution it disproportionates: 3 HNO2 to HNO3 + 2 NO + H2O, with the brown NO2 vapor giving the reaction away above 5 °C. Despite that instability, nitrous acid is the workhorse reagent of diazotization. Treat a primary aromatic amine with HNO2 in cold dilute HCl and you get an aryl diazonium salt (ArN2+), the gateway to phenols (warm hydrolysis), aryl halides (Sandmeyer reaction with CuCl, CuBr, or KI), nitriles (CuCN), and the entire azo dye family that built the German chemical industry from BASF onward. The standard prep is in situ: NaNO2 + HCl yields HNO2 + NaCl, kept on ice at 0-5 °C so the diazonium product survives long enough to react. In molecular biology, HNO2 is a deaminating mutagen — it converts cytosine to uracil and adenine to hypoxanthine, generating point mutations during replication. In atmospheric chemistry, HONO photolysis is now recognized as a major daytime source of OH radicals over urban areas.
Where you'll encounter it
If you've ever run a Sandmeyer reaction in undergraduate organic lab, the orange-tinged solution sitting in your ice bath was nitrous acid generated in situ. You add solid sodium nitrite to a cold acidic solution of an aniline derivative, watch the diazonium form, then quench it with copper(I) chloride or potassium iodide to install a halide. Industrial dye chemists do the same thing at ton scale: every azo pigment in printer toner, leather dyes, and food colorants like Sunset Yellow FCF starts with a diazonium salt made from nitrous acid. In a microbiology lab, HNO2 still shows up as a chemical mutagen for generating point-mutation libraries in bacterial strains, and atmospheric chemists measure HONO with LOPAP or DOAS instruments to track urban OH-radical budgets at sunrise.
Common Uses
- Diazotization of aromatic amines en route to azo dyes and pigments
- Generating aryl diazonium salts for the Sandmeyer reaction sequence
- Reagent for nitrosation of secondary amines and active methylene compounds
- Chemical mutagen for generating point mutations in bacterial strain libraries
- Atmospheric-chemistry tracer for urban OH radical production studies
Safety Information
Corrosive and toxic. Decomposes in solution to release brown NO2 and colorless NO, both of which damage lung tissue at low ppm. GHS classifications: H290 (corrosive to metals), H314 (causes severe skin burns and eye damage), H331 (toxic if inhaled). Occupational exposure limits for both nitrogen oxides are correspondingly low. Never store HNO2 solutions — generate fresh from sodium nitrite and acid in a fume hood, on ice, and use immediately. Diazonium salts derived from HNO2 can detonate when dry, so keep them wet and never scale beyond what you can handle in glass.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.