Arsenic pentoxide is the

inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemis ...

with the formula As₂O₅. This glassy, white,

deliquescent Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance' ...

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 Arsenic trioxide, sold under the brand name Trisenox among others, is an inorganic compound and medication. As an industrial chemical, whose major uses include in the manufacture of wood preservatives, pesticides, and glass. As a medication, ...

(As₂O₃). All inorganic arsenic compounds are highly toxic and thus find only limited commercial applications.

Structure

The structure consists of tetrahedral and octahedral centers linked by sharing corners. The structure differs from that of the corresponding

phosphorus(V) oxide Phosphorus pentoxide is a chemical compound with molecular formula P4 O10 (with its common name derived from its empirical formula, P2O5). This white crystalline solid is the anhydride of phosphoric acid. It is a powerful desiccant and dehydrat ...

; 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 Antimony pentoxide (molecular formula: Sb2O5) is a chemical compound of antimony and oxygen. It contains antimony in the +5 oxidation state. Structure Antimony pentoxide has the same structure as the ''B'' form of niobium pentoxide and can be deri ...

, as antimony substitutes for arsenic only in its octahedral sites.

Synthesis

Historical

Paracelsus Paracelsus (; ; 1493 – 24 September 1541), born Theophrastus von Hohenheim (full name Philippus Aureolus Theophrastus Bombastus von Hohenheim), was a Swiss physician, alchemist, lay theologian, and philosopher of the German Renaissance. He ...

Macquer found a crystallizable salt which he called ‘sel neutre arsenical’. This salt was the residue obtained after distilling

nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...

from a mixture of

potassium nitrate Potassium nitrate is a chemical compound with the chemical formula . This alkali metal nitrate salt is also known as Indian saltpetre (large deposits of which were historically mined in India). It is an ionic salt of potassium ions K+ and ...

and

arsenic trioxide Arsenic trioxide, sold under the brand name Trisenox among others, is an inorganic compound and medication. As an industrial chemical, whose major uses include in the manufacture of wood preservatives, pesticides, and glass. As a medication, it ...

. Previously Paracelsus heated a mixture of arsenic trioxide and potassium nitrate. He applied the term ‘arsenicum fixum’ to the product. A.

Libavius Andreas Libavius or Andrew Libavius was born in Halle, Germany c. 1550 and died in July 1616. Libavius was a renaissance man who spent time as a professor at the University of Jena teaching history and poetry. After which he became a physician a ...

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.

Scheele Scheele is a surname of Germanic origin. Notable people with the surname include: *Carl Wilhelm Scheele (1742–1786), German-Swedish pharmaceutical chemist * George Heinrich Adolf Scheele (1808–1864), German botanist * Karin Scheele (b. 1968) ...

prepared a number of arsenates by the action of arsenic acid on the alkalies. One of the arsenates that he prepared, was arsenic pentoxide. The water in the alkalies evaporated at 180˚C, and the arsenic pentoxide was stable below 400˚C .

Modern methods

Arsenic pentoxide can be crystallized by heating As₂O₃ under oxygen. This reaction is reversible: :As₂O₅ As₂O₃ + O₂ Strong

oxidizing agents An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ). In other words, an o ...

such as

ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the l ...

hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3 ...

, and

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 Orpiment is a deep-colored, orange-yellow arsenic sulfide mineral with formula . It is found in volcanic fumaroles, low-temperature hydrothermal veins, and hot springs and is formed both by sublimation and as a byproduct of the decay of anothe ...

, a typical arsenic sulfide ore: :2 As₂S₃ + 11 O₂ → 2 As₂O₅ + 6 SO₂

Safety

Like all inorganic 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 Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. When present as a deprotonated catalytic residue, some ...

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 The Emergency Planning and Community Right-to-Know Act of 1986 is a United States federal law passed by the 99th United States Congress located at Title 42, Chapter 116 of the U.S. Code, concerned with emergency response preparedness. On October ...

(42 U.S.C. 11002), and is subject to strict reporting requirements by facilities which produce, store, or use it in significant quantities.

References

External links

NIOSH Pocket Guide to Chemical HazardsNTP Report on Carcinogens – Inorganic Arsenic CompoundsESIS: European chemical Substances Information System
* {{oxygen compounds Arsenic(V) compounds Oxides IARC Group 1 carcinogens