Dissolution and Solubility of the (B a x S r 1 - X)HAsO4· h2O Solid Solution in Aqueous Solution at 25°C and pH 2

Abstract

Six different members of the (Ba x Sr 1 - x)HAsO4·H 2O solid solution were prepared and characterized, and then dissolution of the synthetic solids was studied at 25°C and pH 2 in a series of batch experiments for 4320 h. With the increase in the Ba/(Ba + Sr) mole fraction of the solids, the aqueous pH decreased and the aqueous barium concentration increased. The aqueous strontium and arsenic concentrations had the highest values at Ba / (Ba + Sr) = 0.21. The solubility products for BaHAsO4·H2O and SrHAsO4·H 2O were calculated to be 10 - 5.52 and 10 - 4.62, respectively. The corresponding free energies of formation were determined to be - 1543.99 ± 0.18 kJ/mol and - 1537.94 ± 0.02 kJ/mol. The solid solution had a minimum solubility product of 10 - 5.61 at Ba / (Ba + Sr) = 0.82. The Guggenheim coefficients were determined to be a o = 1.55 and a 1 = - 4.35. The Lippmann diagram was a typical Lippmann diagram for a nonideal solid solution with a negative enthalpy of mixing. The system shows an "alyotropic" minimum at the aqueous Ba / (Ba + Sr) activity fraction of 0.87 where the solutus and solidus curves meet. At the end of the dissolution experiment, the dissolution followed the saturation curve for the pure endmember BaHAsO 4 ·H2O and approached the intersection with the minimum stoichiometric saturation curve on the Lippmann diagram. © 2014 Xuehong Zhang et al.