Dissolution and Solubility of the (BaxSr1-x)HAsO4·H2O Solid Solution in Aqueous Solution at 25°C and pH 2

Abstract

Six different members of the (BaxSr1-x)HAsO4·H2O 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 atBa/(Ba+Sr)=0.21. The solubility products for BaHAsO4·H2O and SrHAsO4·H2O were calculated to be10-5.52and10-4.62, 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 of10-5.61atBa/(Ba+Sr)=0.82. The Guggenheim coefficients were determined to beao=1.55anda1=-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 aqueousBa/(Ba+Sr)activity fraction of 0.87 where thesolutusandsoliduscurves meet. At the end of the dissolution experiment, the dissolution followed the saturation curve for the pure endmember BaHAsO4·H2O and approached the intersection with the minimum stoichiometric saturation curve on the Lippmann diagram.