Perarsenates

Perarsenic acid has not been isolated, but by the action of hydrogen peroxide or of metallic peroxides on the alkali arsenates, products are obtained which contain peroxidic oxygen. Whether these products are to be regarded as true perarsenates or as additive compounds of the arsenates with hydrogen peroxide has not been satisfactorily elucidated.

The alkali "perarsenates" may be obtained by evaporating under reduced pressure an aqueous solution of the arsenate containing hydrogen peroxide. The product with normal sodium orthoarsenate has been formulated 3Na₃AsO₄.5H₂O₂.16H₂O. By the action of sodium peroxide on a cooled solution of sodium monohydrogen orthoarsenate in aqueous alcohol, a precipitate is obtained which, after drying in vacuo in the presence of phosphorus pentoxide, yields a product which was described by Alvarez as the perarsenate, NaAsO₄. Coons, however, found that the precipitate varied in composition and, after excess of water was removed over concentrated sulphuric acid under reduced pressure, approximated to 2Na₃AsO₄.7Na₂O₂.2NaOH.75H₂O. He also found that when the monohydrogen salt was treated with 5 per cent, hydrogen peroxide no compound containing active oxygen was formed, but with 30 per cent, hydrogen peroxide a residue whose composition corresponded with the formula 25Na₂HAsO₄.32H₂O₂.16H₂O was produced. Both the above products were probably mixtures.

When an alkaline earth peroxide is dissolved in arsenic acid and the solution is evaporated under reduced pressure at a low temperature, a perarsenate is said to be produced, and if an alkali sulphate is added to the solution before evaporation, a solution of the alkali perarsenate is obtained. The latter may be obtained with scarcely any loss of active oxygen by careful evaporation to dryness under reduced pressure. Using the above method of desiccation, however, Coons could not obtain any compound containing active oxygen either by this method or by the electrolytic method.

When an alkali perarsenate is added to aqueous solutions of metallic salts, precipitates containing active oxygen are obtained; thus salts of the alkaline earths, zinc, cadmium, silver, mercurous mercury, lead and bismuth yield white precipitates, mercuric salts give red precipitates, copper blue, manganese pink, nickel greenish-white and ferrous salts bluish-green. With auric chloride oxygen is liberated, and with ferric chloride ferric hydroxide is precipitated.

Magnesium ammonium orthoarsenate can combine additively with hydrogen peroxide.