Beryllium Arsenates

Beryllium Arsenates are less stable than those of magnesium and readily form basic salts. Beryllium orthoarsenate, Be₃(AsO₄)₂.15H₂O, is obtained by drying in air the precipitate formed on mixing aqueous solutions of sodium monohydrogen arsenate and beryllium sulphate. The precipitate itself is probably beryllium monohydrogen orthoarsenate, BeHAsO₄, but this is readily hydrolysed and unless immediately removed from the solution forms a basic salt. It has been obtained in the form of a dihydrate, BeHAsO₄.2H₂O, as a viscous solid by dissolving beryllium hydroxide in arsenic acid and precipitating with alcohol, and in the anhydrous form by heating arsenic pentoxide and beryllium hydroxide together in a sealed tube at 220° C. Beryllium dihydrogen orthoarsenate, Be(H₂AsO₄)₂, is obtained as colourless hygroscopic tablets by the action of a hot saturated solution of arsenic acid on beryllium hydroxide and evaporation in vacuo.

By stirring beryllium oxide or carbonate with aqueous arsenic acid under carefully regulated conditions of concentration, acidity and temperature, Ephraim and Rosetti have been able to isolate two arsenates of definite composition in a crystalline state, namely, 4.5BeO.As₂O₅.9H₂O and 4BeO.As₂O₅.10H₂O; whether these are to be considered basic salts or mixed acid salts has not been determined.

Several double arsenates with alkali metals and ammonium have been described, but these are extremely unstable, undergoing ready hydrolysis to form basic salts.