Selenous

Selenous is a chemical term referring to selenium in the +4 oxidation state. It is most often encountered in connection with selenous acid (H2SeO3) and the selenite ion (SeO3^2−), which together define the principal selenous species. In contrast, selenium in the +6 state is described as selenic.

Chemically, selenous compounds tend to be less oxidizing than their selenic counterparts and often act as reducing agents in redox reactions. In aqueous solution, selenous acid behaves as a weak polyprotic acid, and salts derived from the selenite ion form with alkali and other cations to give a range of soluble and insoluble selenium(IV) salts. Selenites can participate in various transformations, including reactions with oxidants, reducing agents, or organic substrates to introduce selenium-containing fragments.

Occurrence and production are tied to selenium’s natural abundance and cycling. Selenium occurs as a trace element in the Earth’s crust and biological systems. Selenous acid can be prepared from selenium-containing oxides or elemental selenium treated with water and oxidants under controlled conditions; neutralization of selenous acid yields selenite salts such as sodium selenite (Na2SeO3) and potassium selenite (K2SeO3).

Health and safety aspects are important, as selenous compounds are toxic in higher concentrations and can be corrosive. They require careful handling and appropriate safety procedures in laboratory and industrial settings. In biology, selenium in small amounts is an essential nutrient, but its +4 form is among the forms that can be hazardous if exposures are not managed.