Arsenic pentoxide

Arsenic pentoxide is the inorganic compound with the formula As2O5.[2] This glassy, white, deliquescent solid is relatively unstable, consistent with the rarity of the As(V) oxidation state. More common, and far more important commercially, is arsenic(III) oxide (As2O3). All arsenic compounds are highly toxic and thus find only limited commercial applications.

Diarsenic pentoxide
Names
Other names
Arsenic(V) oxide
Arsenic oxide
Arsenic anhydride
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.013.743
EC Number
  • 215-116-9
RTECS number
  • CG2275000
UNII
UN number 1559
Properties
As2O5
Molar mass 229.8402 g/mol
Appearance white hygroscopic powder
Density 4.32 g/cm3
Melting point 315 °C (599 °F; 588 K) (decomposes)
59.5 g/100 mL (0 °C)
65.8 g/100 mL (20 °C)
8.20 g/100 mL (100 °C)
Solubility soluble in alcohol
Acidity (pKa) 7
Hazards
GHS pictograms
GHS Signal word Danger
H301, H331, H350, H400, H410
P201, P202, P261, P264, P270, P271, P273, P281, P301+310, P304+340, P308+313, P311, P321, P330, P391, P403+233, P405, P501
NFPA 704 (fire diamond)
Flammability code 0: Will not burn. E.g. waterHealth code 4: Very short exposure could cause death or major residual injury. E.g. VX gasReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
0
4
0
Lethal dose or concentration (LD, LC):
8 mg/kg (rat, oral)
NIOSH (US health exposure limits):
PEL (Permissible)
 [1910.1018] TWA 0.010 mg/m3[1]
REL (Recommended)
 Ca C 0.002 mg/m3 [15-minute][1]
IDLH (Immediate danger)
 Ca [5 mg/m3 (as As)][1]
Related compounds
Other cations
 Phosphorus pentoxide
Antimony pentoxide
Related compounds
 Arsenic trioxide
Arsenic acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Structure

The structure consists of tetrahedral {AsO4} and octahedral {AsO6} centers linked by sharing corners.[3] The structure differs from that of the corresponding phosphorus(V) oxide; as a result, although there is still a solid solution with that oxide, it only progresses to the equimolar point, at which point phosphorus has substituted for arsenic in all of its tetrahedral sites. Likewise, arsenic pentoxide can also dissolve up to an equimolar amount of antimony pentoxide, as antimony substitutes for arsenic only in its octahedral sites.[4]

As coordination
unit cell
cell packing

Synthesis

Historical

Paracelsus Macquer found a crystallizable salt which he called ‘sel neutre arsenical’. This salt was the residue obtained after distilling nitric acid from a mixture of potassium nitrate and arsenic trioxide. Previously Paracelsus heated a mixture of arsenic trioxide and potassium nitrate. He applied the term ‘arsenicum fixum’ to the product. A. Libavius called the same product ‘butyrum arsenici’ (butter of arsenic), although this term was actually used for arsenic trichloride. The products that Paracelsus and Libavius found were all impure alkali arsenates.[5] Scheele prepared a number of arsenates by the action of arsenic acid on the alkalies. One of the arsenates that he prepared, was arsenic pentoxide.[6] The water in the alkalies evaporated at 180˚C, and the arsenic pentoxide was stable below 400˚C .[5]

Modern methods

Arsenic pentoxide can be crystallized by heating As2O3 under oxygen. This reaction is reversible:[3]

As2O5 ⇌ As2O3 + O2

Strong oxidizing agents such as ozone, hydrogen peroxide, and nitric acid convert arsenic trioxide to the pentoxide.

Arsenic acid can be generated via routine processing of arsenic compounds including the oxidation of arsenic and arsenic-containing minerals in air. Illustrative is the roasting of orpiment, a typical arsenic sulfide ore:[7]

2 As2S3 + 11 O2 → 2 As2O5 + 6 SO2

Safety
Main article: arsenic toxicity

Like all arsenic compounds, the pentoxide is highly toxic. Its reduced derivative arsenite, which is an As(III) compound, is even more toxic since it has a high affinity for thiol groups of cysteine residues in proteins.

It is classified as an extremely hazardous substance in the United States as defined in Section 302 of the U.S. Emergency Planning and Community Right-to-Know Act (42 U.S.C. 11002), and is subject to strict reporting requirements by facilities which produce, store, or use it in significant quantities.[8]

References
  1. NIOSH Pocket Guide to Chemical Hazards. "#0038". National Institute for Occupational Safety and Health (NIOSH).
  2. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  3. Martin Jansen (1977). "Crystal Structure of As2O5". Angewandte Chemie International Edition in English. 16 (5): 314–315. doi:10.1002/anie.197703142.
  4. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  5. J. W. Mellor. "Comprehensive Treatise on Inorganic & Theoretical Chemistry". Archived from the original on 2012-05-11. Retrieved 2013-03-30.
  6. C.W. Zenger; et al. "Arsenic 149". Archived from the original on 2013-04-09.
  7. Grund, S. C.; Hanusch, K.; Wolf, H. U. "Arsenic and Arsenic Compounds". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a03_113.pub2.
  8. "40 C.F.R.: Appendix A to Part 355—The List of Extremely Hazardous Substances and Their Threshold Planning Quantities" (PDF) (July 1, 2008 ed.). Government Printing Office. Archived from the original (PDF) on February 25, 2012. Retrieved October 29, 2011. Cite journal requires |journal= (help)
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