Ferric orthoarsenite

Ferric Orthoarsenite, FeAs03, was described by Reichard 9 as a rust-yellow powder obtained by adding potassium tetrarsenite to a dilute aqueous solution of ferric chloride. A substance of similar chemical composition has been found in a crystalline deposit formed during the Deacon process of making chlorine. The crystals of the pure salt are monoclinic, their crystallographic elements being 10... 

Ferric orthoarsenite cannot be prepared directly from ferric hydroxide and arsenious oxide.4 The brown product obtained by shaking freshly precipitated ferric hydroxide with an aqueous solution of arsenious oxide has been described 5 as a basic ferric arsenite of composition 4Fe203.As203.5H20. A similar substance is obtained by adding aqueous arsenious oxide or sodium arsenite to ferric acetate solution. If ferric chloride, sulphate or nitrate is used, the ferric salt is not completely precipitated. The product is oxidised in moist air, and decomposes when heated. It is very doubtful whether this is a chemical individual, however, for it has been shown that the removal of arsenious oxide from the solution by the ferric hydroxide is due to adsorption, the amount removed depending upon the conditions and the age of the adsorbent. This subject is discussed more fully on p. 154. 

When a mixture of ferric oxide and arsenious oxide is heated, a mixture of orthoarsenite and orthoarsenate is formed 6 above 225° C., and above 600° C. the latter is the sole product. 

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