Arsenous acid (arsenious acid) — properties, occurrence, and significance

Chemical structure and properties

The central atom is arsenic (As) bound to three hydroxyl groups (–OH) with a lone electron pair on the arsenic, giving a trigonal pyramidal geometry. Because arsenic is in oxidation state +3, arsenous acid is chemically classed with arsenites rather than arsenates. It is a weak acid that partially dissociates in solution to form arsenite anions and related species; speciation depends on pH and concentration.

Origins, history, and occurrence

Arsenous acid arises when arsenic trioxide dissolves in water: this equilibrium is central to understanding how trivalent arsenic moves in natural and industrial settings. Historically, trivalent arsenic compounds have been encountered in pigments, agricultural chemicals, and in certain medicinal preparations, though their toxicity limited safe use. Natural sources include mineral weathering and geothermal fluids, while human activities can mobilize arsenite into groundwater and soils.

Uses, risks, and environmental importance

Arsenous acid and the arsenite species it produces are more toxic and often more mobile in water than pentavalent arsenic forms (arsenates). This makes them a principal concern for drinking-water safety and environmental remediation. Typical approaches to reduce arsenous acid in water involve oxidation to less mobile arsenate followed by removal, or adsorption and filtration methods targeted at trivalent species. Occupational and laboratory handling require strict controls because of acute and chronic toxicity.

Notable distinctions and facts

• Arsenous acid (As(OH)3) vs. arsenic acid (H3AsO4): the former contains As(III), the latter As(V).
• The term "arsenite" describes the anionic derivatives formed when As(OH)3 loses protons; these anions play a central role in toxicity and transport (arsenite chemistry).
• Analytical speciation—distinguishing As(III) from As(V)—is essential in water testing and risk assessment.

For more detailed chemical and safety information consult specialized references and regulatory guidance (compound data, oxidation state details, hydroxyl group chemistry, and analytical sources at elemental arsenic and environmental arsenic).