The nitrous acid is a weak, inorganic acid, whose chemical formula is HNOtwo. It is found mainly in aqueous solution with a pale blue color. It is very unstable, and rapidly decomposes into nitric oxide, NO, and nitric acid, HNO3.
It is usually found in aqueous solution in the form of nitrites. It also comes naturally from the atmosphere as a result of the reaction of nitric oxide with water. There, specifically in the troposphere, nitrous acid intervenes in the regulation of ozone concentration.
An HNO solution is shown in the image abovetwo where the characteristic pale blue color of this acid is appreciated. It is synthesized by the dissolution of nitrogen trioxide, NtwoOR3, in water. Likewise, it is the product of the acidification of sodium nitrite solutions at low temperatures..
The HNOtwo It has little commercial use, being used in the form of nitrite in the preservation of meat. On the other hand, it is used in the production of azo dyes.
It is used, in conjunction with sodium thiosulfate, in the treatment of patients with sodium cyanide poisoning. But, it is a mutagenic agent, and it is thought that it can cause substitutions in the bases of DNA chains, through an oxidative deamination of cytosine and adenine..
Nitrous acid has a dual behavior, since it can behave as an oxidative agent or as a reducing agent; that is, it can be reduced to NO or Ntwo, u oxidize to HNO3.
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The upper image shows the molecular structure of nitrous acid using a spheres and rods model. The nitrogen atom (blue sphere) is located in the center of the structure, forming a double bond (N = O) and a single bond (N-O) with the oxygen atoms (red spheres).
Note that the hydrogen atom (white sphere) is bonded to one of the oxygens and not directly to nitrogen. So, knowing this, the structural formula of HNOtwo is [HO-N = O] or [NO (OH)], and there is no such H-N bond (as the chemical formula may suggest).
The molecules in the image correspond to those of a gas phase; in water they are surrounded by water molecules, which can accept the hydrogen ion (weakly) to form NO ionstwo- and H3OR+.
Their structures can take two forms: cis or trans, called geometric isomers. In the cis isomer, the H atom is eclipsed by the neighboring oxygen atom; while in the trans isomer, both are in anti or opposite positions.
In the cis isomer, the formation of an intramolecular hydrogen bridge (OH-NO) is likely, which may disturb the intermolecular ones (ONOH-ONOH)..
-Nitrous acid
-Dioxonitric acid (III)
-Nitrosyl hydroxide
-Hydroxide oxidonitrogen (IUPAC Systematic Name)
Pale blue liquid, corresponding to nitrite solution.
47.013 g / mol.
It is a weak acid. Its pKa is 3.35 at 25 ºC.
It is only known in solution. Therefore, its melting point cannot be calculated, nor can its crystals be isolated..
As it does not exist pure but in water, the measurements of this property are not precise. On the one hand, it depends on the concentration of HNOtwo, and on the other hand, its heating produces its decomposition. That is why an exact boiling point is not reported..
Forms water soluble nitrites with Li+, Na+, K+, ACtwo+, Mrtwo+, Batwo+. But, it does not form salts with polyvalent cations, such as: Al3+ and / or Betwo+ (due to its high charge density). It is capable of forming stable esters with alcohols.
It is flammable by chemical reactions. May explode on contact with phosphorous trichloride.
It is a very unstable compound, and in aqueous solution it decomposes into nitric oxide and nitric acid:
2 HNOtwo => NOtwo + NO + HtwoOR
4 HNOtwo => 2 HNO3 + NtwoO + HtwoOR
Nitrous acid in aqueous solution occurs in the form of nitrite ions, NOtwo-, which undergo various reduction reactions.
Reacts with I ions- and Faithtwo+, in the form of potassium nitrite, to form nitric oxide:
2 KNOtwo + KI + HtwoSW4 => Itwo + 2 NO + 2 HtwoO + KtwoSWtwo
Potassium nitrite in the presence of tin ions is reduced to form nitrous oxide:
KNOtwo + 6 HCl + 2 SnCltwo => 2 SnCl4 + NtwoO + 3 HtwoO + 2 KCl
Potassium nitrite is reduced by Zn in an alkaline medium, forming ammonia:
5 htwoO + KNOtwo + 3 Zn => NH3 + KOH + 3 Zn (OH)two
In addition to being a reducing agent, nitrous acid can intervene in oxidation processes. For example: it oxidizes hydrogen sulfide, turning into nitric oxide or ammonia, depending on the acidity of the medium in which the reaction occurs.
2 HNOtwo + HtwoS => S + 2 NO + 2 HtwoOR
HNOtwo + 3 HtwoS => S + NH3 + 2 htwoOR
Nitrous acid, in an acidic pH medium, can oxidize iodide ion to iodine.
HNOtwo + I- + 6 h+ => 3 Itwo + NH3 + 2 htwoOR
It can also act as a reducing agent by acting on Cutwo+, giving rise to nitric acid.
To the HNOtwo It can be given other names, which depend on the type of nomenclature. Nitrous acid corresponds to the traditional nomenclature; dioxonitric acid (III), to the stock nomenclature; and hydrogen dioxonitrate (III), to the systematic.
Nitrous acid can be synthesized by dissolving nitrogen trioxide in water:
NtwoOR3 + HtwoO => 2 HNOtwo
Another method of preparation consists of the reaction of sodium nitrite, NaNO3, with mineral acids; such as hydrochloric acid and hydrobromic acid. The reaction is carried out at a low temperature and the nitrous acid is consumed in situ..
Elder brother3 + H+ => HNOtwo + Na+
The H ion+ comes from either HCl or HBr.
Given its properties and chemical characteristics, there is little information about the direct toxic effects of HNO.two. Perhaps some harmful effects that are believed to be produced by this compound are actually caused by nitric acid, which can be produced by the breakdown of nitrous acid.
It is noted that HNOtwo may have harmful effects on the respiratory tract and be capable of producing irritating symptoms in asthmatic patients.
In the form of sodium nitrite, it is reduced by deoxyhemoglobin, producing nitric oxide. This is a powerful vasodilator that produces relaxation of vascular smooth muscles, estimating an LD50 dose of 35 mg / kg for oral consumption in humans.
Sodium nitrite toxicity manifests itself with cardiovascular collapse, followed by severe hypotension, due to the vasodilator action of nitric oxide, produced from nitrite..
Nitrogen dioxide, NOtwo, present in polluted air (smog), under certain conditions it can cause nitrous acid; which, in turn, can react with amines to form nitrosamines, a gamma of carcinogenic compounds.
A similar reaction occurs with cigarette smoke. Nitrosamine residues have been found adhering to the interior lining of smokers' vehicles.
Nitrous acid is used in industry in the production of diazonium salts, through its reaction with aromatic amines and phenols.
HNOtwo + ArNHtwo + H+ => ArN = NAr + HtwoOR
Diazonium salts are used in organic synthesis reactions; for example, in the Sandmeyer reaction. In this reaction, the substitution of an amino group (HtwoN-), in a primary aromatic amine, by the Cl groups-, Br- and CN-. To obtain these aromatic products, cuprous salts are required.
Diazonium salts can form bright azo compounds that are used as colorants and also serve as a qualitative test for the presence of aromatic amines..
Nitrous acid is used for the removal of sodium azide (NaN3), which is potentially dangerous due to its tendency to explode.
2 NaN3 + 2 HNOtwo => 3 Ntwo + 2 NO + 2 NaOH
Nitrous acid can react with ketone groups to form oximes. These can be oxidized to form carboxylic acids or reduced to form amines..
This process is used in the commercial preparation of adipic acid, the monomer used in the production of nylon. It is also involved in the production of polyurethane and its esters are plasticizers, mainly PVC.
Nitrous acid, in the form of sodium nitrite, is used in the treatment and preservation of meat; since it prevents bacterial growth and is capable of reacting with myoglobin, producing a dark red color that makes the meat more attractive for consumption.
This same salt is used, together with sodium thiosulfate, in the intravenous treatment of sodium cyanide poisoning..
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