AsO33

AsO3^3-, commonly written as AsO3^3-, is the arsenite anion. It represents arsenic in the +3 oxidation state within a three‑oxygen, tetrahedral framework that bears a lone pair on the central atom. It is the conjugate base of arsenous acid, H3AsO3, and in aqueous solution exists as several protonation states (H3AsO3, H2AsO3−, HAsO3^2−, and AsO3^3−) depending on pH.

In terms of structure, arsenite is typically described as a three‑coordinate, trigonal pyramidal species around arsenic, with resonance among the As–O bonds leading to bond orders around 1.5 on average. The exact geometry can be influenced by ligands and crystal environment in solid salts.

AsO3^3− forms salts with various cations, including alkali metals such as sodium and potassium. It is a key inorganic arsenic species in reducing environments, where arsenite often predominates over arsenate (AsO4^3−). In groundwater and other natural waters, arsenite is more mobile under certain conditions and can contribute to environmental toxicity.

Chemically, arsenite is a reducing agent that can be oxidized to arsenate by oxidants such as oxygen. It readily interacts with biological thiol groups, inhibiting enzymes and disrupting cellular processes. Arsenite compounds are generally more toxic to humans and wildlife than arsenate compounds, and exposure is associated with a range of health effects, including cancer risk with long-term exposure.

Historically, arsenite has been used in pesticides and wood preservatives, but concerns about toxicity and environmental persistence have led to regulatory restrictions. Current remediation strategies for arsenite-contaminated water often involve oxidizing As(III) to As(V) to promote adsorption and removal.