Iron Arsenites

Both ferrous and ferric compounds are known, but the existence of ferrous orthoarsenite has not been established. Ferrous metarsenite, Fe(AsO₂)₂, has been obtained by the action of potassium tetrarsenite, K₂As₄O₇.2H₂O, upon a dilute solution of ferrous sulphate. It is greenish in colour, becoming brown on exposure to air. It decomposes when heated. When a hot, saturated solution of arsenious acid containing ferrous iodide is allowed to cool, crystals of the compound FeI₂.4As₂O₃.12H₂O are obtained, which slowly oxidise in air, and in vacuo over concentrated sulphuric acid yield up all their combined water. The iodide character of the salt is suppressed, from which it may be inferred that the iodine has combined with the arsenious oxide to yield a complex negative radical.

Ferric Orthoarsenite, FeNaO₃, was described by Reichard 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:

a:b:c = 0.9405:1:0.6234 β = 105°10.5'

The pentahydrate, FeNaO₃.5H₂O, found with the preceding salt, crystallises in the rhombic system.

Ferric arsenite is reduced to a slight extent when heated in an inert atmosphere; if heated in hydrogen under pressure it decomposes, forming iron arsenide, iron oxide and arsenious oxide.

On adding freshly precipitated ferric arsenite to potassium hydroxide solution until no more dissolves, and subsequently evaporating, the soluble potassium salt, 6K₂O.5Fe₂O₃.9As₂O₃.24H₂O, is obtained as a reddish-brown amorphous substance, which dissolves in water, yielding an alkaline solution.

Ferric orthoarsenite cannot be prepared directly from ferric hydroxide and arsenious oxide. The brown product obtained by shaking freshly precipitated ferric hydroxide with an aqueous solution of arsenious oxide has been described as a basic ferric arsenite of composition 4Fe₂O₃.As₂O₃.5H₂O. 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.

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

Ferric Pyroarsenite, Fe₄(As₂O₅)₃.7H₂O, has been described as a yellow substance obtained by adding ferric hydroxide to a hot saturated solution of arsenious oxide in concentrated sodium hydroxide. On cooling, excess of arsenious oxide separates, and after keeping for 24 hours a yellow precipitate of the above composition is formed. The existence of the pyroarsenite has not been confirmed, however.