Solubility and stability of lead arsenates at 25 degrees C.

Lead arsenates were the most extensively used arsenical insecticides in the past. Their residues, especially the water-soluble fractions in environment may pose a potential human health risk. The solubility products (K(sp)) and free energies of formation (Delta G(f) degrees) of PbHAsO(4).2H(2)O(c), Pb(5)(AsO(4))(3)OH.H(2)O(c), Pb(3)(AsO(4))(2)(c) and Pb(8)As(2)O(13)(8PbO.As(2)O(5))(c) at 25 degrees C were determined by precipitation and dissolution experiments. The composition of precipitates depended on the Pb/As molar ratios and pH values. The solid precipitates were lamellar PbHAsO(4).2H(2)O(c) at pH = 3-5 and Pb/As = 1.0, columnar Pb(5)(AsO(4))(3)OH.H(2)O(c) at pH = 12-13 and Pb/As = 1.5, Pb(5)(AsO(4))(3)OH.H(2)O(c) with little amounts of Pb(8)As(2)O(13)(c) at pH = 12-13, Pb/As = 1.67 and 2.0. The mixture of PbHAsO(4).2H(2)O(c) and Pb(3)(AsO(4))(2)(c) formed at pH = 6.51 and Pb/As = 1.5. Based on the analytical results, the solubility products (K(sp)) of PbHAsO(4).2H(2)O(c), Pb(5)(AsO(4))(3)OH.H(2)O(c), Pb(3)(AsO(4))(2)(c) and Pb(8)As(2)O(13)(c) were calculated by the computer program PHREEQC as 10(-10.70)(10(-7.81)-10(-14.32)), 10(-81.75)(10(-74.37)-10(-84.83)), 10(-33.83)(10(-32.05)-10(-34.49)) and 10(-174.24)(10(-156.64)-10(-178.35)), respectively. Correspondingly, the free energies of formation (Delta G(f) degrees) of these lead arsenates were calculated to be -1274.36 kJ/mol, -2928.33 kJ/mol, -1563.12 kJ/mol and -3909.69 kJ/mol.