In chemistry an arsenite is a chemical compound containing an arsenic oxoanion where arsenic has oxidation state +3.

The different forms of the anion are the next ones:
  • ortho-arsenite: AsO33-
  • meta-arsenite: AsO2-

Examples of arsenites include sodium arsenite
Sodium arsenite
Sodium arsenite is a compound with formula NaAsO2. It is the sodium salt of arsenous acid. It is a crystalline solid consisting of sodium cations, Na+, and catena-arsenite anions, [AsO2], which are infinite -O-As- chains, similar in structure to selenium dioxide, SeO2.It is a pesticide....

 which contains a polymeric linear anion, [AsO2]n, and silver arsenite, Ag3AsO3, which contains the trigonal, As
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

33− anion.

Arsenite contrasts to the corresponding anions of the lighter members of group 15, phosphite
A phosphite is a salt of phosphorous acid. The phosphite ion is a polyatomic ion with a phosphorus central atom where phosphorus has an oxidation state of +3...

 which has the structure HPO32− and nitrite
The nitrite ion has the chemical formula NO2−. The anion is symmetric with equal N-O bond lengths and a O-N-O bond angle of ca. 120°. On protonation the unstable weak acid nitrous acid is produced. Nitrite can be oxidised or reduced, with product somewhat dependent on the oxidizing/reducing agent...

, NO2 which is bent.
Sodium arsenite is used in the water gas shift reaction
Water gas shift reaction
The water-gas shift reaction is a chemical reaction in which carbon monoxide reacts with water vapor to form carbon dioxide and hydrogen:The water-gas shift reaction is an important industrial reaction. It is often used in conjunction with steam reforming of methane or other hydrocarbons, which is...

 to remove carbon dioxide.
Arsenites are salts of arsenious acid.

Note that in fields that commonly deal with groundwater chemistry, arsenite commonly refers to As2O3
Arsenic trioxide
Arsenic trioxide is the inorganic compound with the formula As2O3. This commercially important oxide of arsenic is the main precursor to other arsenic compounds, including organoarsenic compounds. Approximately 50,000 tonnes are produced annually...

, the acid anhydride of arsenious acid. Its white odorless crystals are toxic and very soluble in water. It occurs in nature as arsenolite
Arsenolite is an arsenic mineral, chemical formula As2O3. It is formed as an oxidation product of arsenic sulfides. Commonly found as small octahedra it is white, but impurities of realgar or orpiment may give it a pink or yellow hue...

 and claudetite
Claudetite is an arsenic mineral chemical formula As2O3.It is named for the french chemist, F. Claudet. Claudetite is formed as an oxidation product of arsenic sulfides and is colorless or white. It can be associated with arsenolite as well as realgar, orpiment and sulfur....

, and is also a byproduct of metal smelting. Its main use is in producing chromated copper arsenate (CCA) to treat timber. It is also used for arsenic pesticides, glass production, pharmaceuticals and non-ferrous alloys.

Bacteria using and generating arsenate

Some species of bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 obtain their energy by oxidizing
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 various fuels while reducing
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

The arsenate ion is AsO43−.An arsenate is any compound that contains this ion. Arsenates are salts or esters of arsenic acid.The arsenic atom in arsenate has a valency of 5 and is also known as pentavalent arsenic or As[V]....

s to form arsenites. The enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

s involved are known as arsenate reductases.

In 2008, bacteria were discovered that employ a version of photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

 with arsenites as electron donor
Electron donor
An electron donor is a chemical entity that donates electrons to another compound. It is a reducing agent that, by virtue of its donating electrons, is itself oxidized in the process....

s, producing arsenates (just like ordinary photosynthesis uses water as electron donor, producing molecular oxygen). The researchers conjectured that historically these photosynthesizing organisms produced the arsenates that allowed the arsenate-reducing bacteria to thrive.

In humans, arsenite inhibits pyruvate dehydrogenase (PDH complex) in the pyruvate acetyl CoA reaction, and binds to the SH group of lipoamide
Lipoamide is a trivial name for 6,8-dithiooctanoic amide. It is 6,8-dithiooctanoic acid's functional form where the carboxyl group is attached to protein by an amide linkage . Sometimes lipoamide is used to refer to protein bound lipoic acid, but this can be misleading as this is technically...

, a participant coenzyme. In this inhibition, arsenite poisoning affects energy production in the body.

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