NITROUS ACID

Chemical Properties

Nitrous acid,HN02, is the aqueous solution of nitrogen trioxide. It is a moderately strong and rapid oxidizing agent used for diazotization.

Uses

Nitrous acid is a diazotizing agent. The acid diazotizes primary aromatic amines to diazo derivatives in manufacturing azo dyes.

Reactions

Nitrous acid is unstable. It decomposes to form nitric acid and nitric oxide:
3HNO2 → NO3¯ + H3O+ + 2NO
Strong oxidizing agents, such as permanganate, readily oxidize nitrous acid to nitric acid.
Nitrous acid is an effective oxidizing agent. It oxidizes hydrogen sulfide to sulfur forming either nitric oxide or ammonia, depending on the acidity of the solution:
2HNO2 + H2S → S + 2NO + 2H2O
HNO2 + 3H2S → 3S + NH3 + 2H2O
In acid medium it oxidizes iodide ion to iodine:
HNO2 + I¯ + 6H+ → 3I2 + NH3 + 2H2O

Description

Nitrous acid (molecular formula?HNO2) is a weak and monobasic acid known only in solution and in the form of nitrite salts. Nitrous acid rapidly decomposes into nitrogen oxide, nitric oxide and water when in solution:
2HNO2 ? NO2 + NO+H2O
It can also decompose into nitric acid and nitrous oxide and water.
4HNO2 ? 2HNO3 +N2O +H2O
Nitric acid (HNO3), also known as “aqua fortis” and “spirit of nitre”, is a highly corrosive and toxic strong acid that can cause severe burns. It is colorless when pure and a slight yellow when “glacial”. Older samples tend to acquire a yellow cast due to the accumulation of various oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as “fuming nitric acid”.

Chemical Properties

A weak acid occurring only in the form of a light-blue solution.

Physical properties

Pale blue solution; stable only in solution; weak acid, Ka 4.5x10^-4.

Uses

Formation of diazotizing compounds by reaction with primary aromatic amines, source of nitric oxide.

Uses

Nitrous acid is a nitrogen oxoacid. It is a conjugate acid of a nitrite. It (as sodium nitrite) is used as part of an intravenous mixture with sodium thiosulfate to treat cyanide poisoning. There is also research to investigate its applicability towards treatments for heart attacks, brain aneurysms, pulmonary hypertension in infants, and Pseudomonas aeruginosa infections.

Preparation

Nitrous acid may be obtained in solution by adding a strong acid to nitrite; e.g., adding hydrochloric acid to sodium nitrite solution:
H⁺ + NO₂ ^ˉ → HNO₂.

Definition

A weak acid known only in solution, obtained by acidifying a solution of a nitrite. It readily decomposes on warming or shaking to nitrogen monoxide and nitric acid. The use of nitrous acid is very important in the dyestuffs industry in the diazo reaction: nitrous acid is liberated by acidifying a solution of a nitrite (usually sodium nitrite) in the presence of the compound to be diazotized. Nitrous acid and the nitrites are normally reducing agents but in certain circumstances they can behave as oxidizing agents, e.g. with sulfur dioxide and hydrogen sulfide.

Definition

nitrous acid: A weak acid, HNO₂,known only in solution and in thegas phase.It is prepared by the actionof acids upon nitrites, preferablyusing a combination that removesthe salt as an insoluble precipitate(e.g.Ba(NO₂)₂ and H₂SO₄). The solutionsare unstable and decompose on heating to give nitric acid and nitrogenmonoxide.Nitrous acid can functionboth as an oxidizing agent(forms NO) with I– and Fe2+, or as areducing agent (forms NO₃^-) with,forexample, Cu²⁺; the latter is mostcommon.It is widely used (preparedin situ) for the preparation of diazoniumcompounds in organic chemistry.The full systematic name isdioxonitric(III) acid.

Hazard

Rapidly forms nitric oxide and nitric acid in water; [Merck Index] A strong oxidizer; Causes burns; Highly toxic by ingestion and inhalation.

Safety Profile

Mutation data reported. Flammable by chemical reaction; a powerful oxidizer. Explodes on contact with phosphorus trichloride. Reacts violently with PH3 and Pcb. Reactions with l-amino- 5-nitrophenol, ammonium decahydroborate(2-), hydrazine (product is hydrogen azide) may give explosive products. Incompatible with anilines (e.g., 4- bromoahe , 2-chloroaniline, 3- chloroaniline, 2-nitroadine, 3-nitroaniline, 4-nitroaniline, aniline ), semicarbazone, silver nitrate. When heated to decomposition it emits hghly toxic fumes of NOx. See also NITRIC OXIDE.

Raw materials

Preparation Products