THE SOLUBILITY OF (BA,SR)SO4 PRECIPITATES - THERMODYNAMIC-EQUILIBRIUMAND REACTION-PATH ANALYSIS

The solubility of (Ba,Sr)SO4 precipitates, varying in SrSO4 mole fract ion from 0.05-0.90, was investigated at room temperature with an equil ibration period extending to almost three years. The data show that on or before 315 days of equilibration the precipitates reach a reversib le equilibrium with the aqueous solution. The reversibility of this eq uilibrium was verified both by the attainment of steady-state concentr ations with time and by heating the samples to perturb the equilibrium and then observing the slow return to the initial equilibrium state. The dissolution of the (Ba,Sr)SO4 precipitates does not, in general, f ollow limiting reaction paths as defined by the Lippmann solutus or st oichiometric dissolution curves. In addition, activity coefficient cal culations for the BaSO4 and SrSO4 components of the solid phase, using either total bulk analysis or near-surface analysis of the component mole fractions, do not satisfy the Gibbs-Duhem equation, demonstrating that a single solid-solution phase does not control both the aqueous Ba and Sr concentrations. Instead, our long-term equilibration data ca n be explained by the unavoidable formation of small amounts of barite and substitution of Sr into a solid-solution phase with the BaSO4 com ponent of the solid-solution phase never reaching thermodynamic equili brium with the aqueous phase.