Nitrite
The nitrite ion has the chemical formula. Nitrite is widely used throughout chemical and pharmaceutical industries. The nitrite anion is a pervasive intermediate in the nitrogen cycle in nature. The name nitrite also refers to organic compounds having the –ONO group, which are esters of nitrous acid.
Production
is made industrially by passing a mixture of nitrogen oxides into aqueous sodium hydroxide or sodium carbonate solution:The product is purified by recrystallization. Alkali metal nitrites are thermally stable up to and beyond their melting point. Ammonium nitrite can be made from dinitrogen trioxide,, which is formally the anhydride of nitrous acid:
Structure
The nitrite ion has a symmetrical structure, with both N–O bonds having equal length and a bond angle of about 115°. In valence bond theory, it is described as a resonance hybrid with equal contributions from two canonical forms that are mirror images of each other. In molecular orbital theory, there is a sigma bond between each oxygen atom and the nitrogen atom, and a delocalized pi bond made from the p orbitals on nitrogen and oxygen atoms which is perpendicular to the plane of the molecule. The negative charge of the ion is equally distributed on the two oxygen atoms. Both nitrogen and oxygen atoms carry a lone pair of electrons. Therefore, the nitrite ion is a Lewis base.In the gas phase it exists predominantly as a trans-planar molecule.
Reactions
Acid-base properties
Nitrite is the conjugate base of the weak acid nitrous acid:Nitrous acid is also highly unstable, tending to disproportionate:
This reaction is slow at. Addition of acid to a solution of a nitrite in the presence of a reducing agent, such as iron, is a way to make nitric oxide in the laboratory.
Oxidation and reduction
The formal oxidation state of the nitrogen atom in nitrite is +3. This means that it can be either oxidized to oxidation states +4 and +5, or reduced to oxidation states as low as −3. Standard reduction potentials for reactions directly involving nitrous acid are shown in the table below:The data can be extended to include products in lower oxidation states. For example:
Oxidation reactions usually result in the formation of the nitrate ion, with nitrogen in oxidation state +5. For example, oxidation with permanganate ion can be used for quantitative analysis of nitrite :
The product of reduction reactions with nitrite ion are varied, depending on the reducing agent used and its strength. With sulfur dioxide, the products are NO and ; with tin the product is hyponitrous acid ; reduction all the way to ammonia occurs with hydrogen sulfide. With the hydrazinium cation the product of nitrite reduction is hydrazoic acid, an unstable and explosive compound:
which can also further react with nitrite:
This reaction is unusual in that it involves compounds with nitrogen in four different oxidation states.
Analysis of nitrite
Nitrite is detected and analyzed by the Griess Reaction, involving the formation of a deep red-colored azo dye upon treatment of a -containing sample with sulfanilic acid and naphthyl-1-amine in the presence of acid.Coordination complexes
Nitrite is an ambidentate ligand and can form a wide variety of coordination complexes by binding to metal ions in several ways. For example, the red nitrito pentaamminecobalt complex is metastable, isomerizing to the yellow nitro complex .Nitrite is processed by several enzymes, all of which utilize coordination complexes.
Hazardous reactions
When heated with cyanides or thiosulfates, nitrites violently explode.Biochemistry
In nitrification, ammonium is converted to nitrite. Important species include Nitrosomonas. Other bacterial species such as Nitrobacter, are responsible for the oxidation of the nitrite into nitrate.Nitrite can be reduced to nitric oxide or ammonia by many species of bacteria. Under hypoxic conditions, nitrite may release nitric oxide, which causes potent vasodilation. Several mechanisms for nitrite conversion to NO have been described, including enzymatic reduction by xanthine oxidoreductase, nitrite reductase, and NO synthase, as well as nonenzymatic acidic disproportionation reactions.
Uses
Chemical precursor
s and other colorants are prepared by the process called diazotization, which requires nitrite.Nitrite in food preservation and biochemistry
The addition of nitrites and nitrates to processed meats such as ham, bacon, and sausages speeds up the curing of meat and also imparts an attractive colour. Nitrite reacts with the meat's myoglobin by attaching to the heme iron atom, forming reddish-brown nitrosomyoglobin and the characteristic pink "fresh" color of nitrosohemochrome or nitrosyl-heme upon cooking.The academic and industrial consensus is that nitrites also reduces growth and toxin production of Clostridium botulinum.
On the other hand, a 2018 study by the British Meat Producers Association determined that legally permitted levels of nitrite do not affect the growth of C. botulinum.
Addition of ascorbic acid, erythorbic acid, or one of their salts enhance the binding of nitrite to the iron atom in myoglobin. These chemicals also reduce the formation of nitrosamine in the stomach, but only when the fat content of a meal is less than 10%, beyond which they instead increase the formation of nitrosamine.
In the U.S., meat cannot be labeled as "cured" without the addition of nitrite. In the US, nitrite has been formally used since 1925. According to scientists working for the industry group American Meat Institute, this use of nitrite started in the Middle Ages.
In some countries, cured-meat products are manufactured without nitrate or nitrite, and without nitrite from vegetable sources. Parma ham, produced without nitrite since 1993, was reported in 2018 to have caused no cases of botulism. This is because the interior of the muscle is sterile and the surface is exposed to oxygen. Other manufacture processes do not assure these conditions, and reduction of nitrite results in toxin production.
Historians and epidemiologists argue that the widespread use of nitrite in meat-curing is closely linked to the development of industrial meat-processing. French investigative journalist Guillaume Coudray asserts that the meat industry chooses to cure its meats with nitrite even though it is established that this chemical gives rise to cancer-causing nitroso-compounds. Some traditional and artisanal producers avoid nitrites. As many researchers nowadays try to point out the hazardous generation of nitrosamines as nitrites link with free peptides in the gastrointestinal system, the EU published a regulation which obliges the reduction of nitrites on meat curing from.
In mice, food rich in nitrites together with unsaturated fats can prevent hypertension by forming nitro fatty acids that inhibit soluble epoxide hydrolase, which is one explanation for the apparent health effect of the Mediterranean diet. Adding nitrites to meat has been shown to generate known carcinogens; the World Health Organization advises that eating of nitrite processed meat a day would raise the risk of getting bowel cancer by 18% over a lifetime.
The recommended maximum limits by the World Health Organization in drinking water are and for nitrite and nitrate ions, respectively. Ingesting too much nitrite and/or nitrate through well water is suspected to cause methemoglobinemia.
95% of the nitrite ingested in modern diets comes from bacterial conversion of nitrates naturally found in vegetables. However, potentially cancer-causing nitroso compounds are not made in the pH-neutral colon. They are mostly made in the acidic stomach.