Calcium Pyroarsenate, Ca₂As₂O₇

Calcium Pyroarsenate, Ca₂As₂O₇, in orthorhombic prisms, may be obtained by fusing calcium oxide with potassium arsenate.10 When treated with cold water these break up into minute crystals of the hexahydrate Ca₂As₂O₇.6H₂O. By leaving in contact with water the salt is gradually transformed into dicalcium orthoarsenate, CaHAsO₄.2H₂O.

Several double orthoarsenates with the alkalies are known: CaKAsO₄, CaNaAsO₄, and Ca₃Na₆(AsO₄)₄, Ca(NH₄)AsO₄. 7H₂O, Ca(NH₄)AsO₄.6H₂O, and Ca₃(NH₄)H₂(AsO₄)₃.3H₂O. Lefevre also isolated a calcium sodium pyroarsenate, Ca₄Na₄(As₂O₇)₃.