The structure consists of tetrahedral and octahedral centers linked by sharing corners. The structure differs from that of the corresponding phosphorus 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.
As coordination
unit cell
cell packing
Synthesis
Historical
Macquer found a crystallizable salt which he called ‘sel neutre arsenical’. This salt was the residue obtained after distillingnitric 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’, although this term was actually used for arsenic trichloride. The products that Paracelsus and Libavius found were all impure alkali arsenates. 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. 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: 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:
Safety
Like all arsenic compounds, the pentoxide is highly toxic. Its reduced derivative arsenite, which is an As 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, and is subject to strict reporting requirements by facilities which produce, store, or use it in significant quantities.
