Solubilities of metal carbonates

Solubilities in aqueous media of sparingly-soluble metal carbonates play an important role in chemical processes whether they occur in a laboratory, on an industrial scale, or in the geologic environment. Solubility phenomena (i.e. dissolution and precipitation reactions) of metal carbonates control procedures for preparing, separating and purifying chemicals. Moreover, the interactions of the hydrological cycle with the cycle of carbonate rocks, the naturally occurring dissolution of carbonate minerals in water as well as their precipitation on the ocean floor and in the sediments of rivers and lakes can be described by the principles of solubility, although gigantic quantities of material may be involved. The experimental methods for the determination of carbonate solubilities are reviewed and the results are discussed within the frame-work of equilibrium thermodynamics. It will be shown that Gibbs functions of pure stoichiometric carbonate phases can be determined by accurately measuring solubilities in aqueous electrolyte systems. In cases where solid-solid phase transformations and recrystallisations are kinetically inhibited, the methods developed were successfully applied to metastable equilibria. Occasionally, solubility data were even employed to estimate activity coefficients of components forming a series of carbonate solid solutions. Whenever possible the activity coefficients of the reacting species were controlled by use of a constant ionic medium. Solubility constants obtained at various fixed ionic strengths were fitted to the Pitzer equations and extrapolated to infinite dilution. Provided the standard potentials of the respective metal ion/metal electrode are known over a range of ionic strengths the values of solubility constants at infinite dilution can be calculated independently. In any case the optimised thermodynamic constants were incorporated in a comprehensive computer model which permits a wide variety of solubility calculations. These are illustrated by a case study of a multicomponent system with industrial relevance

Graphical representation of solid-solute phase equilibria in aqueous solution

Solid-solute phase diagrams are useful tools for the depiction of stable and metastable equilibria between binary mixed crystals with common ions and aqueous solutions. Generalized Gibbs-Duhem equations provide the common basis and suggest natural co-ordinates for the graphical representation of thermodynamic variables. Diagrams applicable even to highly soluble electrolytes can be constructed by plotting the osmotic coefficient of the solvent times the total molality πΣm vs. the mole fractions x of the dissolved and solid components, respectively. According to Schmalzried and Pelton's topological classification the resulting phase diagrams are of Type II. They can be used for practical purposes, e.g., to predict the separability of the components by fractional crystallization

Solid-solute phase equilibria in aqueous solution. IV. Calculation of Phase Diagrams

The thermodynamic principles of conventional (T-x, P-T) phase diagrams and solubility (log ΣK-x) diagrams depicting solid-solute phase equilibria in aqueous solution are derived from a unifying point of view. It is shown that thermodynamic quantities necessary for the construction of conventional phase diagrams can be obtained from solubility measurements. The unary system calcite-aragonite and the binary system aragonite-strontianite, where solubility data are available over the whole compositional range, have been selected as examples. In the latter case, the constraint of constant composition of the solid phase leading to a metastable equilibrium with the respective solute species is an essential point in the thermodynamic derivation and was observed experimentally as well

Solid-solute phase equilibria in aqueous solution: VII. A re-interpretation of magnesian calcite stabilities

It is proposed to model magnesian calcites thermodynamically as dilute solid solutions of MgCO3 in CaCO3 according to the Unified Interaction Parameter Formalism (Bale and Pelton, Met. Trans. A21A, 1997–2002, 1990). In this case both stoichiometric saturation and precipitation data of synthetic magnesian calcites taken from literature can consistently be explained

Solid-solute phase equilibria in aqueous solution. III. A new application of an old chemical potentiometer

Incongruent dissolution of mixed crystals leading to a stable thermodynamic equilibrium as well as congruent dissolution leading to a metastable equilibrium termed ‘stoichiometric saturation’ are governed by the same principles of phase theory, when in the latter case the constraint of constant composition of the solid phase is taken into consideration. A simple galvanic cell was used as chemical potentiometer and it was found that reliable thermodynamic data can be obtained on one-component phase transitions such as aragonite → calcite as well as on ‘stoichiometrically saturated’ homogeneous solid mixtures of Co-Mn carbonates. With this method data for modelling the solubility behavior of solid solutions become accessible