Selenite (SeO3²⁻): the selenite ion and its chemistry

Structure and properties

Selenite is a trigonal-pyramidal oxyanion related to sulfite (SO3²⁻) and tellurite (TeO3²⁻). It behaves as a moderately weak oxidizing agent and participates in redox reactions typical of selenium(IV) compounds. Many metal selenites are crystalline salts; sodium selenite is a frequently encountered example. Preparations and synthetic routes are discussed below and introductory material is available at selenous acid reference.

Formation and common salts

Selenite salts are commonly produced by combining metal oxides or hydroxides with selenium dioxide (SeO2) or by neutralizing selenous acid with bases. Heating a metal oxide with selenium dioxide is a classical route to metal selenites. Examples of salts include sodium selenite and various transition‑metal selenites; a general industrial or laboratory example is described at selenium dioxide chemistry and specific salt information at sodium selenite.

Uses, occurrence and environmental behaviour

Selenite occurs in natural waters and soils as part of the environmental cycling of selenium. In biology and nutrition selenium is an essential trace element; selenite and other inorganic selenium species can be converted in organisms to organic selenium compounds. In analytical chemistry, selenite is relevant for detection and quantification of selenium. Where present, its redox behaviour can influence mobility and toxicity in the environment. For practical handling and reactivity notes consult chemical safety and reactivity.

Distinctions and safety

• Related ions: Selenate (SeO4²⁻) contains selenium in +6 and is a stronger oxidant than selenite; selenide (Se²⁻) is selenium in −2 and behaves quite differently chemically.
• Mineral name: The word "selenite" is also used for a transparent form of the mineral gypsum (a calcium sulfate). Context is important to avoid confusion between the mineral and the SeO3²⁻ ion.
• Safety: Like other selenium(IV) compounds, selenites are biologically active and can be toxic at elevated concentrations; they require appropriate laboratory handling and environmental controls.

This article summarizes common properties and roles of the selenite ion, its preparation, and its place among other selenium species in chemistry and the environment.