Arsenic pentoxide

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Arsenic pentoxide is the inorganic compound with the formula As2O5.[1] 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.


The structure consists of tetrahedral {AsO4} and octahedral {AsO6} centres linked by sharing corners.[2] The structure differs from that of the corresponding phosphorus(V) oxide.



Paracelsus Macquer found a crystallizable salt which he called ‘sel neutre arsenical’. This salt was the obtaining residue 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 tricholoride. The products that Paracelsus and Libavius found were all impure alkali arsenates.[3] 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.[4] The water in the alkalies evaporated at 180˚C, and the arsenic pentoxide was stable below 400˚C .[3]

Modern methods

Arsenic pentoxide can be crystallized by heating As2O3 under oxygen. This reaction is reversible:[2]

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:[5] 2 As2S3 + 11 O2 → 2 As2O5 + 6 SO2


{{#invoke:main|main}} 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.


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External links

Template:Arsenic compounds