Arsenic Pentoxide
Properties
| State | Solid (deliquescent) |
| Color | White |
| Solubility | Very soluble in water (1500 g/L, forms arsenic acid); soluble in alcohol and alkali |
| Melting Point | 315 °C (decomposes to As2O3 + O2) |
About Arsenic Pentoxide
Arsenic pentoxide is the anhydride of arsenic acid (H3AsO4) and the As(V) endpoint of arsenic's main oxidation-state pair. The structural chemistry is more complex than the simple stoichiometric formula suggests: the solid contains both AsO4 tetrahedra and AsO6 octahedra connected through corner-sharing oxygens, an unusual arrangement for a Group-15 oxide where the simpler P2O5-style structure of corner-sharing tetrahedra would be expected. The compound is strongly deliquescent — exposed to ambient humidity it pulls water from the air and reverts to arsenic acid within minutes — which is part of why it's typically prepared in situ rather than stored. The historical use that defined As2O5 was as the arsenate component of chromated copper arsenate (CCA) wood preservative, the dominant treatment for pressure-treated lumber from the 1940s through the 1990s. CCA-treated wood resisted termite damage and fungal decay for 30–50 years, making it the standard for outdoor decking, utility poles, marine pilings, and playground equipment. The EPA voluntarily restricted CCA in residential applications in 2003 after concerns about arsenic leaching into surrounding soil, especially from playground structures where children might contact the wood directly. Modern wood preservatives use copper-quaternary or copper-azole formulations that don't contain arsenic. The compound's other industrial uses — herbicide intermediates, glass-clarifying agent, analytical reagent for iodide determination — are smaller in volume and progressively being phased out in favor of less-toxic alternatives.
Where you'll encounter it
If you've torn out a pre-2003 wooden deck, the lumber was almost certainly CCA-treated, with As2O5 contributing the arsenate ion that did the insecticide and fungicide work. Disposal of that lumber is regulated as hazardous waste in many jurisdictions because the arsenic remains in the wood long after the treatment chemistry has finished, and burning it releases volatile arsenic compounds. In a research or analytical lab, As2O5 is rarely encountered as a working reagent; the standard practice is to use sodium arsenate (Na2HAsO4) or potassium arsenate when arsenate ion is needed, since those salts handle more cleanly and don't reform the strongly hygroscopic acid in air. Where As2O5 still appears in industrial chemistry, it's almost always as an intermediate that's used immediately rather than stored.
Common Uses
- Arsenate component of CCA wood preservative (residential use restricted post-2003)
- Precursor for organoarsenic herbicides and pesticides (largely phased out)
- Decolorizing and clarifying agent in optical-glass production
- Iodometric-titration reagent for analytical chemistry
- Source of arsenate ion for arsenic-doped semiconductor precursors
Safety Information
IARC Group 1 human carcinogen — chronic exposure produces lung, skin, and bladder cancers at exposure levels well above acute-toxicity thresholds. Acutely lethal at oral doses of a few tens of mg/kg; the OSHA PEL is 0.01 mg/m³ as arsenic, low enough that any open handling requires specialized hood configurations. As(V) is more bioavailable than As(III) when ingested because the arsenate ion mimics phosphate and disrupts oxidative phosphorylation by substituting for inorganic phosphate in ATP synthesis. Regulated under EPA RCRA hazardous waste (D004); treat any spill or contamination as a controlled-substance event. GHS H300, H314, H350, H410.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.