Overview
Arsenic acid is an inorganic oxyacid with the molecular formula H3AsO4. It contains arsenic in the +5 oxidation state and behaves as a triprotic acid, able to release up to three protons in solution. The substance and its salts are commonly referred to as arsenates. For a basic definition see chemical compound.
Structure and properties
Structurally it resembles phosphoric acid: a central arsenic atom tetrahedrally coordinated to four oxygen atoms, three of which bear hydrogen as hydroxyl groups. In water it undergoes stepwise deprotonation to give hydrogen arsenate and arsenate anions. It may act as a mild oxidizing agent under some conditions and is soluble in water. Elements present include hydrogen and arsenic bound through oxygen; related anions are commonly called arsenate.
Preparation and reactions
Arsenic acid can be prepared by oxidizing lower-valent arsenic compounds (for example arsenous acid or arsenic trioxide) with suitable oxidizing agents. It participates in typical acid–base equilibria and forms a range of salts (arsenates) and esters. It may be interconverted with other arsenic oxyacids by redox chemistry; consult a general reference on arsenic chemistry for experimental details (more).
Uses, hazards and regulation
Historically arsenic compounds have seen use in pesticides, wood treatment, glass manufacturing and as intermediates in chemical synthesis. Because inorganic arsenic species are toxic and associated with chronic health risks, many uses are now restricted and subject to environmental regulation. Handling requires appropriate precautions to limit inhalation, ingestion and environmental release.
Notable distinctions
Arsenic acid is distinct from arsenous acid (H3AsO3), which contains arsenic in the +3 oxidation state and has different reactivity and toxicity. Its salts, the arsenates, are chemically similar in role to phosphates in some reactions but differ in biological and environmental behaviour. For practical or safety guidance, consult regulatory and material safety resources before working with arsenic compounds.