Arsenic pentoxide

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Arsenic pentoxide (As2O5) is an inorganic compound. It is a white, glassy powder that readily absorbs moisture from the air (deliquescent). It is less common and less stable than arsenic trioxide (As2O3). Like all inorganic arsenic compounds, it is highly toxic, and its use is limited.

Structure
Arsenic pentoxide is made up of two kinds of arsenic–oxygen units: four-coordinate arsenic in tetrahedra (AsO4) and six-coordinate arsenic in octahedra (AsO6). These units share corners to form a connected solid structure. The material can form solid solutions with phosphorus pentoxide (P2O5) and with antimony pentoxide (Sb2O5) up to certain substitution levels.

Synthesis
Historically, scientists like Scheele and Macquer developed ways to obtain arsenic pentoxide through reactions of arsenic compounds with acids and bases. In modern methods, As2O5 can be produced by oxidizing arsenic trioxide (As2O3) with oxygen or with strong oxidizers such as ozone, hydrogen peroxide, or nitric acid. Arsenic pentoxide can also form when arsenic-containing minerals are roasted (for example, orpiment, As2S3). The conversion is reversible in some conditions: As2O5 can relate to As2O3 and O2.

Key properties
- Formula: As2O5
- Molar mass: 229.84 g/mol
- Appearance: white, hygroscopic powder
- Density: about 4.32 g/cm3
- Melting point: about 315°C (decomposes upon heating)
- Solubility: highly soluble in water (around 60 g per 100 mL at 0–20°C; lower solubility at higher temperatures); soluble in alcohol
- Other: fairly reactive and unstable in some environments

Hazards and safety
Arsenic pentoxide is extremely toxic. Inhalation or ingestion can be harmful or fatal, and exposure is a major health concern. In the United States it is classified as an extremely hazardous substance, with strict reporting and safety requirements for facilities that handle it.

Related compounds
Arsenic has related oxides and acids, such as arsenic trioxide (As2O3) and arsenic acid (H3AsO4). Arsenic pentoxide can form part of various arsenic-containing oxide families, but its use is limited due to toxicity.