Nitrous acid

Ball-and-stick Stick Space-filling

Drag to rotate; pinch or Ctrl + scroll to zoom. Arrow keys rotate, + − zoom, R resets.

4 atoms, 3 bonds.

IUPAC name: nitrous acid
Molar mass: 47.0130(32) g/mol
Atoms: 4
SMILES: N(=O)O
InChIKey: IOVCWXUNBOPUCH-UHFFFAOYSA-N
PubChem CID: 24529

Made of

Structural formula of nitrous acid. Every atom in the molecule is drawn. It shows which atoms are joined to which, not the shape. The model above shows the shape.

Nitrous acid has no pure form that could be poured into a bottle. It is a weak acid known only in solution. The Hazardous Substances Data Bank (HSDB) describes that solution as pale blue, and Haz-Map, a US occupational health database, as light blue. Even dissolved, the compound does not last. In water it breaks down quickly, giving nitric oxide and nitric acid. Its absence from commerce comes down to that instability, on HSDB’s account.

Each molecule joins one hydrogen, one nitrogen and two oxygen atoms, and a mole of them weighs 47.013 g. Its salts are the nitrites, and with alcohols it forms stable esters, compounds that join an acid to an alcohol. It forms no salts with aluminium or beryllium, which HSDB describes as weak cations carrying more than one charge. Toward other substances it can act either as an oxidising agent or as a reducing agent.

Structure and bonding

Nitrous acid is a short open chain of four atoms, H–O–N=O. The hydrogen sits on one oxygen, the hydroxyl oxygen, which a single bond joins to nitrogen; nitrogen holds the second oxygen by a double bond. Labels prints H, O, N and O along the chain in that order. With two inner atoms, each bonded to two neighbours, the molecule has two centres and no single shape word. In PubChem’s computed model the O–N=O angle is about 111° and the H–O–N angle about 102°.

Turned edge-on, the model shows all four atoms lying in one plane. The Stick view drops the spheres, and the doubled N=O stick stands out against the two single bonds. In the Space-filling view the atoms overlap into one compact cluster, the hydrogen a small cap on its oxygen.

The model draws the neutral molecule, with no charge on any atom. In water, though, some molecules give up the hydrogen on the oxygen as a hydrogen ion, H⁺, and become nitrite ions, NO₂⁻, a form the viewer does not draw. The model also shows a molecule standing alone, whereas the compound is known only in solution.

Where it turns up

Fuel burned at high temperature is one source. Chem1 traces large amounts of nitrogen oxides to combustion in petrol and jet engines and in the turbines of fossil-fuelled power stations. Haz-Map lists nitrous acid among the products of combustion, and it also forms when nitrogen dioxide reacts with water.

The lungs may be another setting. In laboratory models of lung lipids, HSDB reports, nitrogen dioxide reacted with unsaturated fatty acids, chains that carry carbon–carbon double bonds, by two routes. At high concentrations the gas mostly added across the double bond. At low ones it removed a hydrogen from the neighbouring carbon, and that route gave nitrous acid rather than a product with a nitro group, NO₂, bonded to carbon. The nitrite ion from this route would be expected to spread through the whole body, while nitrated lipids would stay confined to the lungs.

Why it matters

Because it decomposes, nitrous acid is made at the point of use. HSDB names sodium nitrite, acted on by a strong acid, as industry’s main source of it for organic synthesis. It serves in diazotisation, its reaction with aromatic amines: relatives of ammonia built on rings like benzene’s. Haz-Map also lists it as a source of nitric oxide.

Living cells suffer from it too. HSDB classes nitrous acid as a mutagen, an agent that alters DNA. In double-stranded DNA it can tie the two strands together, mostly at CpG steps, where a cytosine is followed by a guanine. Two guanines on opposite strands then share a single nitrogen-containing link. In five strains of the bacterium Escherichia coli it cut survival and markedly reduced the uptake of leucine and glucose. HSDB puts the deaths down to action on many targets at once.

Properties

Properties of nitrous acid Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 24529.
Property	Value	Unit	Source
Molecular formula (Hill order)	HNO₂
Molar mass	47.0130(32) g/mol	g/mol	IUPAC/CIAAW 2024
Hydrogen atoms	1
Nitrogen atoms	1
Oxygen atoms	2
Bonds	3
Single bonds	2
Double bonds	1
PubChem molecular weight	47.014 g/mol	g/mol	PubChem

Safety

Nitrous acid is corrosive and toxic, and its classification notified to the European Chemicals Agency carries the signal word Danger. Every notification says it causes severe skin burns and eye damage, and half add that it is fatal if swallowed and very toxic to aquatic life. Haz-Map rates it a strong oxidiser, highly toxic when swallowed or breathed in, and HSDB warns that heating it to decomposition gives off highly toxic nitrogen-oxide fumes. The PubChem Laboratory Chemical Safety Summary for nitrous acid is the sheet to consult.

Questions

What is the difference between nitrous acid and nitric acid?

Nitric acid carries one more oxygen atom: HNO₃, 63.012 g/mol, against nitrous acid's HNO₂, 47.013 g/mol. Their names come from their anions. Of two related oxygen-containing anions, the one with fewer oxygens ends in -ite, so nitrite is NO₂⁻ and nitrate NO₃⁻; in the acid name, Chem1 notes, -ite turns into -ous and -ate into -ic. The nitric acid page compares the two as acids.

Is nitrous acid the same as nitrous oxide?

No, the shared word joins two separate compounds. Nitrous acid, HNO₂, holds one hydrogen, one nitrogen and two oxygen atoms, and a mole of it weighs 47.013 g. Nitrous oxide, N₂O, holds two nitrogen atoms and one oxygen with no hydrogen at all, at 44.013 g/mol. Nitrous acid also has no existence as a pure substance, only dissolved in water.

Is nitrous acid a strong or weak acid?

Weak. The Hazardous Substances Data Bank puts its pK, the figure chemists use to rate an acid's strength, at 3.35 at 25 °C. In water a share of the HNO₂ molecules lose the hydrogen on their oxygen as a hydrogen ion, H⁺, and become nitrite ions, NO₂⁻, while the rest stay whole. With lithium, sodium, potassium and several other metals it gives stable nitrite salts, HSDB notes.

Made of the same elements

HNO₃ Nitric acid 5 atoms, 4 bonds.