Arsenic Tetroxide, As₂O₄

Attempts to isolate this oxide have proved unsuccessful, but a number of products have been described which appear to contain arsenic in an intermediate stage of oxidation. Thus, by the action of chlorine on arsenious oxide at a moderate heat Bloxam obtained a transparent glass of composition 2As₂O₃.As₂O₅, and a similar product was formed by heating the pentoxide with an excess of the trioxide. More recently, products approximating in composition to As₂O₄ have been described as resulting when arsenious oxide was heated in oxygen at 400° to 480° C. and under pressures of 127 to 180 atmospheres. If equimolecular proportions of the tri- and pent-oxides are heated together, no sublimation of the former occurs even at 350° C. and combination appears to take place. According to Weidenbach, the product at temperatures between 800° and 1200° C. has the composition 2As₂O₃.As₂O₅, and on cooling yields a clear glass which gradually loses its transparency. The glass is slightly soluble in water, more readily soluble in aqueous alkali or in dilute hydrochloric acid. It may be precipitated from aqueous solution by the addition of alcohol.

Hydrated products have also been obtained. When arsenious oxide (100 g.) is gently warmed with concentrated nitric acid (25 to 30 c.c.), oxides of nitrogen are evolved and, after standing, a mass of microscopic needle-shaped crystals remains, the composition of which is 3As₂O₃. 2As₂O₅.3H₂O. If excess of arsenic acid or of arsenious oxide is present, the products are respectively As₂O₃.As₂O₅.xH₂O or 2As₂O₃.As₂O₅.xH₂O. The addition of water causes separation of arsenious oxide.

Arsenious oxide dissolves in boiling aqueous arsenic acid, but as the solution cools it is reprecipitated and after three weeks only about 1.8 per cent, of that originally dissolved remains in solution.

The solubility is greater in concentrated arsenious acid solution and there is evidence that the tetroxide exists in the form of arsenious arsenate, AsAsO₄.