Solid‐Solute Phase Equilibria in Aqueous Solution. I. Solubility Constant and Free Enthalpy of Formation of Huntite

Abstract

AbstractSolid binary carbonate phases often dissolve congruently in aqueous media, even when they are not stable under the prevailing conditions. From the thermodynamic point of view these dissolution reactions lead to a metastable equilibrium with fixed composition, when neither diffusion in the solid state nor recrystallization from the solution occurs. The solubility of natural huntite. CaMg3(CO3)4, was studied potentiometrically at 50°C and I = 3.0 mol kg−1 (Na)ClO4. Since it dissolved congruently the solubility constant log *Kpso = log{[Ca2+]0.25 [Mg2+]0.75 p[H+]−2} = 10.36 ± 0.03 was used to calculate its standard free enthalpy ΔfG0298 (Ca0.25Mg0.75CO3) = −1045.0 ± 2.4 kJ mol−1; thus huntite is considerably less stable than the corresponding mechanic mixtures of calcite or dolomite and magnesite respectively. Stable and metastable equilibria in carbonate systems can be graphically presented by phase diagrams with In *Kpso and mole fractions as variables, where *Kpso is the total solubility constant defined by *Kpso = ([Ca2+] + [Mg2+]) p [H+]−2.