Boron Arsenate, BAsO₄

Boron Arsenate, BAsO₄, has been prepared by evaporating to dryness an aqueous solution of boric and arsenic acids and igniting the residue. The product is a mixed anhydride. The crystal structure resembles that of boron phosphate, being of the silica type, the arsenic and boron atoms being surrounded tetrahedrally by oxygen, each atom of which belongs to two tetrahedra. The space group is S₄²; each cell, space-centred, has two molecules and has a = 4.459 ± 0.006 and c = 6.796 ± 0.006 A., c/a = 1.524; arsenic is in position a and boron in position c.

The arsenate is reduced when heated with aluminium or calcium silicide. Two hydrates have been obtained, BAsO₄.3H₂O and BAsO₄.6H₂O, and by extracting either of these with liquid ammonia, the compound BAsO₄.3H₂O.NH₃ has been isolated. This appears to be an acid salt of a complex arsenoboric acid of structure H₂[BNaO₃(OH)₄], corresponding with the trihydrate, and the hexahydrate appears to be a trihydrate of this acid. Potentiometric titration indicates that in aqueous solution a mixture of boric and arsenic acids is obtained.