CaO—As2O5—H2O System at 23 .+‐. 1 °C.

The phase diagram for the ternary system CaO-As2O5-H2O at 23°± 1°C is plotted. The calcium arsenates represented are Ca(H2AsO4)2, CaHAsO4·H2O, Ca5H2(AsO4)4·9H2O, Ca3(AsO4)2·xH2O (x= 3⅔ and 4¼), Ca5(AsO4)3OH (apatite), and Ca4(OH)2(AsO4)2·4H2O. The solubility data indicate the following invariant points: Ca(H2AsO4)2–CaHAsO4·H2O–solution, CaHAsO4·H2O–Ca5H2(AsO4)4·9H2O–solution, Ca5H2(AsO4)4·9H2O–Ca3(AsO4)2·4¼H2O–solution, Ca3(AsO4)2·3⅔H2O–Ca4(OH)2(AsO4)2·4H2O–solution, and Ca4(OH)2(AsO4)2·4H2O–Ca(OH)2–solution. The tricalcium arsenate Ca3(AsO4)2·3⅔H2O is shown to be the only hydrate to dissolve congruently.