The solubility and stability of calcium arsenates at 25 • C was determined by both precipitation and dissolution experiments. Ca 3 (AsO 4 ) 2 ·3H 2 O(c), Ca 3 (AsO 4 ) 2 ·2 1 / 4 H 2 O(c), Ca 5 (AsO 4 ) 3 (OH)(c) and Ca 4 (OH) 2 (AsO 4 ) 2 ·4H 2 O(c) were identified in our experiment over a wide range of pH and for Ca/As molar ratios between 1.25 and 4.0. The solids precipitated at pH = 3 ∼ 7 and Ca/As = 1.5 were phase-pure and well-crystallized Ca 3 (AsO 4 ) 2 ·xH 2 O(c) and had relatively larger grain size than those formed at pH > 7. Based on the analytical results and using the computer program PHREEQC, the solubility products for Ca 3 (AsO 4 ) 2 ·3H 2 O(c), Ca 3 (AsO 4 ) 2 ·2 1 / 4 H 2 O(c), Ca 5 (AsO 4 ) 3 (OH)(c) and Ca 4 (OH) 2 (AsO 4 ) 2 ·4H 2 O(c) were calculated as K sp of 10 −), respectively. Correspondingly, the free energies of forming ( G o f ) of these calcium arsenates were calculated to be −3787.87 kJ/mol, −3611.50 kJ/mol, −5096.47 kJ/mol and −4928.86 kJ/mol.

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