Critical evaluation of solubility data: enthalpy of formation of siderite

Abstract

A general method for the critical evaluation of solubility data of sparingly-soluble metal carbonates has been proposed, which allows the optimization of the thermodynamic properties of the solid phase. This method was applied to siderite, FeCO₃(s), leading to a new optimized value for the standard enthalpy of formation, Δ_fH(FeCO₃) = (−752.0 ± 1.2) kJ mol⁻¹. The generally applicable Gibbs energy minimization program ChemSage has been used for the simultaneous analysis of all relevant literature data of the solubility of siderite valid at various temperatures. The activity coefficients of the ionic species as well as the osmotic coefficient of the solvent were calculated by means of the Davies approximation. Whereas the values for Δ_fH(FeCO₃) given in the published thermodynamic databases deviate from each other by more than 10 kJ mol⁻¹, it was shown that the results of solubility measurements agree remarkably well with each other and with the most reliable values obtained by other methods. The large variety of literature data for Δ_fH(FeCO₃) can be attributed to a discrepancy in the auxiliary data for the iron(II) ion. The reliability of a recent selection of the thermodynamic properties of Fe²⁺ was confirmed by the present evaluation of solubility data.