Arsenic acid
Arsenic acid, also called arsoric acid, is the chemical compound with the formula H3AsO4. It is the arsenic analogue of phosphoric acid and is usually found in solution; it can form stable crystalline hydrates, such as the hemihydrate (H3AsO4·0.5H2O) and the dihydrate (H3AsO4·2H2O).
Arsenic atom in arsenic acid is tetrahedrally coordinated to four oxygens. It is a triprotic acid, with three ionization steps described by pKa values of about 2.19, 6.94, and 11.5. The final ionization produces the arsenate ion (AsO4^3−). Unlike phosphoric acid, arsenic acid acts as an oxidizer in reactions.
Preparation and forms: Arsenic acid is made by dissolving arsenic trioxide (As2O3) in concentrated nitric acid, with dinitrogen trioxide produced as a by-product. The hemihydrate and dihydrate crystals can form depending on crystallization conditions. It can also form slowly when arsenic pentoxide is dissolved in water, or when meta- or pyroarsenic acid is treated with cold water. It can be produced directly from elemental arsenic by moistening it and treating with ozone.
Properties: Molar mass 141.94 g/mol; density about 2.5 g/cm3; melting point 35.5°C; boiling point 120°C (it decomposes on heating). It dissolves in water (about 16.7 g per 100 mL at 25°C) and is soluble in ethanol. It is a colorless, hygroscopic substance.
Hazards and safety: Arsenic acid is extremely toxic and carcinogenic; it is corrosive. The oral LD50 in rabbits is about 6 mg/kg. It should be handled with strict safety measures to avoid ingestion, inhalation, or skin contact.
Uses: It is a precursor to various pesticides and has been used as a wood preservative, a broad-spectrum biocide, a finishing agent for glass and metal, and in making some organo-arsenic compounds. Because of its toxicity, its practical uses are limited.
This page was last edited on 3 February 2026, at 20:22 (CET).