Dissolution and Solubility of the Erythrite/Annabergite Solid Solution [(CoxNi1-x)3(AsO4)2·8H2O] at 25 ºC

Abstract

Nine different members of the erythrite/annabergite solid solution [(Co_xNi_1-x)₃(AsO₄)₂·8H₂O] were prepared and characterized by various techniques and then dissolution of the synthetic solids was studied at 25 ºC and pH 2 in a series of batch experiments for 2880 h. The result indicated that the erythrite/annabergite solid solution had a minimum aqueous arsenate concentration or a minimum solubility at the mole fraction of erythrite in the solid solution of x = 0.70. The Guggenheim parameters a_o and a₁ were determined to be -1.79 and 1.23 from the alyotropic composition and alyotropic solubility at x = 0.70. A Lippmann diagram for the ideal case when a₀ = 0.0 and for the non-ideal case when a₀ = -1.79 and a₁ = 1.23 was constructed using the thermodynamic solubility products for erythrite of 10^-33.68 and annabergite of 10^-32.34. The experimental data were plotted on Lippmann phase diagrams to show the evolution of the aqueous composition during the dissolution of the erythrite/annabergite solid solution.