Reaction paths and equilibrium end-points in solid-solution aqueous-solution systems

Equations are presented describing equilibrium in binary solid-solution aqueous-solution (SSAS) systems after a dissolution, precipitation, or recrystallization process, as a function of the composition and relative proportion of the initial phases. Equilibrium phase diagrams incorporating the concept of stoichiometric saturation are used to interpret possible reaction paths and to demonstrate relations between stoichiometric saturation, primary saturation, and thermodynamic equilibrium states.

The concept of stoichiometric saturation is found useful in interpreting and putting limits on dissolution pathways, but there currently is no basis for possible application of this concept to the prediction and/ or understanding of precipitation processes.

Previously published dissolution experiments for (Ba, Sr)SO₄ and (Sr, Ca)C̈O_3orth. solids are interpreted using equilibrium phase diagrams. These studies show that stoichiometric saturation can control, or at least influence, initial congruent dissolution pathways. The results for (Sr, Ca)CO_3orth. solids reveal that stoichiometric saturation can also control the initial stages of incongruent dissolution, despite the intrinsic instability of some of the initial solids. In contrast, recrystallisation experiments in the highly soluble KCl-KBr-H₂O system demonstrate equilibrium. The excess free energy of mixing calculated for K(Cl, Br) solids is closely modeled by the relation G^E = χ_KBrχ_KClRT[a₀ + a₁(2χ_KBr−1)], where a₀ is 1.40 ± 0.02, a₁, is −0.08 ± 0.03 at 25°C, and χ_KBr and χ_KCl are the mole fractions of KBr and KCl in the solids. The phase diagram constructed using this fit reveals an alyotropic maximum located at χ_KBr = 0.676 and at a total solubility product, ΣΠ = [K⁺]([Cl⁻] + [Br⁻]) = 15.35.