{{Distinguish|nitric acid}} {{Redirect|Hono|the place in Sweden|Hönö}} {{Chembox | Verifiedfields = changed | verifiedrevid = 462262187 | ImageFile = Nitrous acid acsv.svg | ImageClass = skin-invert | ImageName = Nitrous acid | IUPACName = Nitrous acid<ref>{{Cite web |title=Nitrous Acid |url=https://pubchem.ncbi.nlm.nih.gov/compound/24529#section=IUPAC-Name&fullscreen=true}}</ref> |Section1={{Chembox Identifiers | CASNo = 7782-77-6 | CASNo_Ref = {{cascite|correct|CAS}} | UNII_Ref = {{fdacite|correct|FDA}} | UNII = T2I5UM75DN | PubChem = 24529 | ChemSpiderID = 22936 | ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}} | EINECS = 231-963-7 | KEGG_Ref = {{keggcite|changed|kegg}} | KEGG = C00088 | MeSHName = Nitrous+acid | ChEBI_Ref = {{ebicite|correct|EBI}} | ChEBI = 25567 | SMILES = O=NO | ChEMBL_Ref = {{ebicite|correct|EBI}} | ChEMBL = 1161681 | StdInChI_Ref = {{stdinchicite|correct|chemspider}} | StdInChI = 1S/HNO2/c2-1-3/h(H,2,3) | StdInChIKey_Ref = {{stdinchicite|correct|chemspider}} | StdInChIKey = IOVCWXUNBOPUCH-UHFFFAOYSA-N | Gmelin = 983 | 3DMet = B00022}} |Section2={{Chembox Properties | Formula = HNO<sub>2</sub> | Appearance = Pale blue solution | MolarMass = 47.013 g/mol | Density = Approx. 1 g/ml | MeltingPt = Only known in solution or as gas | ConjugateBase = Nitrite | pKa = 3.15<ref name=P82db>{{cite book|title=Ionisation Constants of Inorganic Acids and Bases in Aqueous Solution|editor-first=D.&nbsp;D.|editor-last=Perrin|edition=2nd|series=IUPAC Chemical Data|issue=29|publisher=Pergamon|location=Oxford|year=1982|publication-date=1984|orig-date=1969|lccn=82-16524|isbn=0-08-029214-3|at=Entry&nbsp;156}}</ref> }} |Section7={{Chembox Hazards | GHS_ref = <ref>{{cite web |title=Nitrous Acid |url=https://pubchem.ncbi.nlm.nih.gov/compound/24529#section=Safety-and-Hazards |website=pubchem.ncbi.nlm.nih.gov |access-date=20 December 2025 |language=en}}</ref> | GHSPictograms = {{GHS05}}{{GHS06}}{{GHS09}} | GHSSignalWord = Danger | HPhrases = {{HPhrases|H300|H314|H400}} | PPhrases = {{PPhrases|P260|P264|P264+P265|P270|P273|P280|P301+P316|P301+P330+P331|P302+P361+P354|P304+P340|P305+P354+P338|P316|P317|P321|P330|P363|P391|P405|P501}} | NFPA-H = 4 | NFPA-F = 0 | NFPA-R = 2 | NFPA-S = OX | FlashPt = Non-flammable }} |Section8={{Chembox Related | OtherAnions = Nitric acid | OtherCations = Sodium nitrite<br/>Potassium nitrite<br/>Ammonium nitrite | OtherCompounds = Dinitrogen trioxide }} }}

'''Nitrous acid''' (molecular formula {{chem|H|N|O|2}}) is a weak and monoprotic acid known only in solution, in the gas phase, and in the form of nitrite ({{chem|NO|-|2}}) salts.<ref name=G&E/> It was discovered by Carl Wilhelm Scheele, who called it "phlogisticated acid of niter". Nitrous acid is used to make diazonium salts from amines. The resulting diazonium salts are reagents in azo coupling reactions to give azo dyes.

==Structure== In the gas phase, the planar nitrous acid molecule can adopt both a ''syn'' and an ''anti'' form. The ''anti'' form predominates at room temperature, and IR measurements indicate it is more stable by around 2.3&nbsp;kJ/mol.<ref name="G&E">{{Greenwood&Earnshaw}} p. 462.</ref>

<gallery widths="180px" heights="120px"> Image:Trans-nitrous-acid-2D-dimensions.png | Dimensions of the ''anti'' form<br />(from the microwave spectrum) Image:Trans-nitrous-acid-3D-balls.png | Model of the ''anti'' form Image:Cis-nitrous-acid-3D-balls.png | ''syn'' form </gallery>

==Decomposition and preparation== {{See also|Dinitrogen trioxide}}

Free, gaseous nitrous acid is unstable, rapidly disproportionating to nitric oxides: :2 HNO<sub>2</sub> → NO<sub>2</sub> + NO + H<sub>2</sub>O In aqueous solution, the nitrous acid also disproportionates, for a net reaction producing nitric oxide and nitric acid:<ref name="Nitros">{{cite book|title=Nitrosation|first=D.&nbsp;L.&nbsp;H.|last=Williams|publisher=Cambridge University|location=Cambridge, UK|year=1988|isbn=0-521-26796-X|url=https://archive.org/details/nitrosation0000will|url-access=registration}}</ref>{{rp|1}}<ref>{{Cite journal |last1=Kameoka |first1=Yohji |last2=Pigford |first2=Robert |date=February 1977 |title=Absorption of Nitrogen Dioxide into Water, Sulfuric Acid, Sodium Hydroxide, and Alkaline Sodium Sulfite Aqueous |journal=Ind. Eng. Chem. Fundamen. |volume=16 |issue=1 |pages=163–169 |doi=10.1021/i160061a031}}</ref> :3&nbsp;HNO<sub>2</sub>&nbsp;→ 2&nbsp;NO&nbsp;+ HNO<sub>3</sub>&nbsp;+ {{H2O-nl}}

Consequently applications of nitrous acid usually begin with mineral acid acidification of sodium nitrite. The acidification is usually conducted at ice temperatures, and the HNO<sub>2</sub> consumed ''in situ''.<ref>{{Cite journal |last1=Petit |first1=Y. |last2=Larchevêque |first2=M. |year=1998 |title=Ethyl Glycidate from (S)-Serine: Ethyl (R)-(+)-2,3-Epoxypropanoate |journal=Org. Synth. |volume=75 |page=37 |doi=10.15227/orgsyn.075.0037 |doi-access=free}}</ref><ref>{{Cite journal |last1=Smith |first1=Adam P. |last2=Savage |first2=Scott A. |last3=Love |first3=J. Christopher |last4=Fraser |first4=Cassandra L. |year=2002 |title=Synthesis of 4-, 5-, and 6-methyl-2,2'-bipyridine by a Negishi Cross-coupling Strategy: 5-methyl-2,2'-bipyridine |journal=Org. Synth. |volume=78 |page=51 |doi=10.15227/orgsyn.078.0051 |doi-access=free}}</ref>

Nitrous acid equilibrates with dinitrogen trioxide in water, so that concentrated solutions are visibly blue:<ref name="Nitros" />{{rp|2}} : N<sub>2</sub>O<sub>3</sub> + H<sub>2</sub>O {{eqm}} 2 HNO<sub>2</sub> Addition of dinitrogen trioxide to water is thus another preparatory technique.

==Chemical applications== Nitrous acid is the main chemophore in the Liebermann reagent, used to spot-test for alkaloids.

At high acidities ({{Math|''p''H &ll; 2}}), nitrous acid is protonated to give water and nitrosonium cations.<ref name="Nitros" />{{Rp|page=2}}

===Reduction=== With I<sup>−</sup> and Fe<sup>2+</sup> ions, NO is formed:<ref name="InorgChem">{{Cite book |last1=Housecroft |first1=Catherine E. |title=Inorganic Chemistry, 3rd Edition |last2=Sharpe |first2=Alan G. |publisher=Pearson |year=2008 |isbn=978-0-13-175553-6 |page=449 |chapter=Chapter 15: The group 15 elements}}</ref>

: 2 HNO<sub>2</sub> + 2 KI + 2 H<sub>2</sub>SO<sub>4</sub> → I<sub>2</sub> + 2 NO + 2 H<sub>2</sub>O + 2 K<sub>2</sub>SO<sub>4</sub> : 2 HNO<sub>2</sub> + 2 FeSO<sub>4</sub> + 2 H<sub>2</sub>SO<sub>4</sub> → Fe<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub> + 2 NO + 2 H<sub>2</sub>O + K<sub>2</sub>SO<sub>4</sub>

With Sn<sup>2+</sup> ions, N<sub>2</sub>O is formed:

: 2 HNO<sub>2</sub> + 4 HCl + 2 SnCl<sub>2</sub> → 2 SnCl<sub>4</sub> + N<sub>2</sub>O + 3 H<sub>2</sub>O

With SO<sub>2</sub> gas, NH<sub>2</sub>OH is formed:

: 2 KNO<sub>2</sub> + 6 H<sub>2</sub>O + 4 SO<sub>2</sub> → 3 H<sub>2</sub>SO<sub>4</sub> + K<sub>2</sub>SO<sub>4</sub> + 2 NH<sub>2</sub>OH

With Zn in alkali solution, NH<sub>3</sub> is formed:

: 5 H<sub>2</sub>O + KNO<sub>2</sub> + 3 Zn → NH<sub>3</sub> + KOH + 3 Zn(OH)<sub>2</sub>

With {{chem|N|2|H|5|+}}, both HN<sub>3</sub> and (subsequently) N<sub>2</sub> gas are formed:

: HNO<sub>2</sub> + [N<sub>2</sub>H<sub>5</sub>]<sup>+</sup> → HN<sub>3</sub> + H<sub>2</sub>O + H<sub>3</sub>O<sup>+</sup>

: HNO<sub>2</sub> + HN<sub>3</sub> → N<sub>2</sub>O + N<sub>2</sub> + H<sub>2</sub>O

Oxidation by nitrous acid has a kinetic control over thermodynamic control, this is best illustrated that dilute nitrous acid is able to oxidize I<sup>−</sup> to I<sub>2</sub>, but dilute nitric acid cannot.

: I<sub>2</sub> + 2 e<sup>−</sup> ⇌ 2 I<sup>−</sup> {{pad|3em}} ''E''<sup>o</sup> = +0.54&nbsp;V

: {{chem|NO|3|−}} + 3 H<sup>+</sup> + 2 e<sup>−</sup> ⇌ HNO<sub>2</sub> + H<sub>2</sub>O {{pad|3em}} ''E''<sup>o</sup> = +0.93&nbsp;V

: HNO<sub>2</sub> + H<sup>+</sup> + e<sup>−</sup> ⇌ NO + H<sub>2</sub>O {{pad|3em}} ''E''<sup>o</sup> = +0.98&nbsp;V

It can be seen that the values of ''E''{{su|b=cell|p=o}} for these reactions are similar, but nitric acid is a more powerful oxidizing agent. Based on the fact that dilute nitrous acid can oxidize iodide into iodine, it can be deduced that nitrous is a faster, rather than a more powerful, oxidizing agent than dilute nitric acid.<ref name="InorgChem" />

===Organic chemistry=== Nitrous acid is used to prepare diazonium salts: :HNO<sub>2</sub> + ArNH<sub>2</sub> + H<sup>+</sup> → {{chem|ArN|2|+}} + 2 H<sub>2</sub>O where Ar is an aryl group.

Such salts are widely used in organic synthesis, e.g., for the Sandmeyer reaction and in the preparation azo dyes, brightly colored compounds that are the basis of a qualitative test for anilines.<ref>{{Cite journal |last1=Clarke |first1=H. T. |last2=Kirner |first2=W. R. |date=1922 |title=Methyl Red |url=http://orgsyn.org/demo.aspx?prep=CV1P0374 |journal=Organic Syntheses |volume=2 |pages=47 |doi=10.15227/orgsyn.002.0047 |doi-access=free|url-access=subscription }}</ref> Nitrous acid is used to destroy toxic and potentially explosive sodium azide. For most purposes, nitrous acid is usually formed ''in situ'' by the action of mineral acid on sodium nitrite:<ref>{{Cite book |url=http://books.nap.edu/openbook.php?record_id=4911&page=165 |title=Prudent practices in the laboratory: handling and disposal of chemicals |publisher=National Academy Press |year=1995 |isbn=978-0-309-05229-0 |location=Washington, D.C. |doi=10.17226/4911}}</ref> It is mainly blue in colour

: NaNO<sub>2</sub> + HCl → HNO<sub>2</sub> + NaCl : 2 NaN<sub>3</sub> + 2 HNO<sub>2</sub> → 3 N<sub>2</sub> + 2 NO + 2 NaOH

Reaction with two α-hydrogen atoms in ketones creates oximes, which may be further oxidized to a carboxylic acid, or reduced to form amines. This process is used in the commercial production of adipic acid.

Nitrous acid reacts rapidly with aliphatic alcohols to produce alkyl nitrites, which are potent vasodilators:

:(CH<sub>3</sub>)<sub>2</sub>CHCH<sub>2</sub>CH<sub>2</sub>OH + HNO<sub>2</sub> → (CH<sub>3</sub>)<sub>2</sub>CHCH<sub>2</sub>CH<sub>2</sub>ONO + H<sub>2</sub>O

The carcinogens called nitrosamines are produced, usually not intentionally, by the reaction of nitrous acid with secondary amines: :HNO<sub>2</sub> + R<sub>2</sub>NH → R<sub>2</sub>N-NO + H<sub>2</sub>O

==Atmosphere of the Earth== Nitrous acid is involved in the ozone budget of the lower atmosphere, the troposphere. The heterogeneous reaction of nitric oxide (NO) and water produces nitrous acid. When this reaction takes place on the surface of atmospheric aerosols, the product readily photolyses to hydroxyl radicals.<ref>{{Cite journal |last1=Spataro |first1=F |last2=Ianniello |first2=A |date=November 2014 |title=Sources of atmospheric nitrous acid: state of the science, current research needs, and future prospects |journal=Journal of the Air & Waste Management Association |volume=64 |issue=11 |pages=1232–1250 |doi=10.1080/10962247.2014.952846 |pmid=25509545 |doi-access=free|bibcode=2014JAWMA..64.1232S }}</ref><ref>{{Cite journal |last1=Anglada |first1=Josef M. |last2=Solé |first2=Albert |date=November 2017 |title=The Atmospheric Oxidation of HONO by OH, Cl, and ClO Radicals |journal=The Journal of Physical Chemistry A |volume=121 |issue=51 |pages=9698–9707 |bibcode=2017JPCA..121.9698A |doi=10.1021/acs.jpca.7b10715 |pmid=29182863}}</ref>

==DNA damage and mutation== Treatment of ''Escherichia coli'' cells with nitrous acid causes damage to the cell's DNA including deamination of cytosine to uracil, and these damages are subject to repair by specific enzymes.<ref>{{Cite journal |last1=Da Roza |first1=R. |last2=Friedberg |first2=E. C. |last3=Duncan |first3=B. K. |last4=Warner |first4=H. R. |date=1977-11-01 |title=Repair of nitrous acid damage to DNA in Escherichia coli |journal=Biochemistry |volume=16 |issue=22 |pages=4934–4939 |doi=10.1021/bi00641a030 |issn=0006-2960 |pmid=334252}}</ref> Also, nitrous acid causes base substitution mutations in organisms with double-stranded DNA.<ref>{{Cite journal |last1=Hartman |first1=Z. |last2=Henrikson |first2=E. N. |last3=Hartman |first3=P. E. |last4=Cebula |first4=T. A. |date=1994 |title=Molecular models that may account for nitrous acid mutagenesis in organisms containing double-stranded DNA |journal=Environmental and Molecular Mutagenesis |volume=24 |issue=3 |pages=168–175 |doi=10.1002/em.2850240305 |issn=0893-6692 |pmid=7957120|bibcode=1994EnvMM..24..168H }}</ref>

==See also== {{Commons category|Nitrous acid|lcfirst=yes}} * Demjanov rearrangement * Nitric acid (HNO<sub>3</sub>) * Nitrosyl-''O''-hydroxide * Tiffeneau-Demjanov rearrangement

==References== {{reflist}}

{{Nitrogen compounds}} {{Nitrites}} {{Authority control}}

Category:Nitrogen oxoacids Category:Nitrogen cycle Category:Oxidizing acids Category:Nitrogen(III) compounds