Arsenite

The arsenite ion is H2AsO3&minus;, or similar.

An arsenite (compound) is a compound that contains this ion.

The arsenite ion may be formed from arsenic trioxide by adding base. Arsenite has a valency of 3 and is also known as trivalent arsenic.

Its white odorless crystals are toxic and very soluble in water. It occurs in nature as arsenolite and claudetite and is also a by-product of metal smelting. Its main use is in producing copper chrome arsenate to treat timber. It is also used for arsenic pesticides, glass production, pharmaceuticals and non-ferrous alloys.

Addendum
The definition above is incomplete. In fields that commonly deal with groundwater chemistry, arsenite commonly refers to As2O3 and sodium arsenite refers to NaAsO2. Sodium arsenite is used in the water gas shift reaction to remove carbon dioxide.