Heinz Gamsjäger

Low-temperature thermodynamic model for the system Na2CO3−MgCO3−CaCO3−H2O

Abstract A comprehensive low-temperature thermodynamic model for the geochemically important Na2CO3−MgCO3−CaCO3−H2O system is presented. The model is based on calorimetrically determined ΔfH°298 values, S°298 values and C°p(T) functions taken from the literature as well as on μ°298 values of solids derived in this work from solubility measurements obtained in our laboratories or by others. When these thermodynamic quantities were combined with temperature-dependent Pitzer parameters taken from the literature, solubilities calculated for a wide range of conditions agree well with experimental data. The results for several subsystems were summarized by depicting the respective phase diagrams. For the MgO−CO2−H2O subsystem, it was found that the commonly believed stability relations must be revised, i.e., in the temperature range covered, nesquehonite never becomes more stable than hydromagnesite at pCO2 ≤ 1 atm. Although the recommended set of thermodynamic data on sparingly soluble solids was derived from experimental results on mainly NaClO4 systems, it can be incorporated in databanks containing additional Pitzer parameters for modeling more complex fresh- or seawater systems.

Glossary of terms related to solubility (IUPAC Recommendations 2008)

Phenomena related to the solubility of solids, liquids, and gases with one another are of interest to scientists and technologists in an array of disciplines. The diversity of backgrounds of individuals concerned with solubility creates a potential for confusion and miscommunication and heightens the need for an authoritative glossary of terms related to solubility. This glossary defines 166 terms used to describe solubility and related phenomena. The definitions are consistent with one another and with recommendations of the International Union of Pure and Applied Chemistry for terminology and nomenclature.

Thermodynamic Aspects of the Vaterite-Calcite Phase Transition

Although vaterite is the least stable anhydrous calcium carbonate polymorph, it is formed as a metastable phase in some normal and pathological biomineralisation processes. In this work, thermodynamic aspects of the vaterite-calcite phase transition were comprehensively studied. Vaterite samples were prepared by different methods and characterised for the composition, crystal structure, specific surface and grain size. All products were identified to be pure vaterite by careful X-ray diffraction measurements. The enthalpy and Gibbs energy of transition were determined by precise calorimetric and potentiometric measurements. The reliability of the thermodynamic data for the vaterite-calcite phase transition derived from this work was shown by the use of different calorimetric methods to determine the enthalpy of transition and the independent measurements of heat capacity and entropy of vaterite. Our recommended values are ΔtrsG*=−2.9±0.2 kJ mol−1 , Δ trsH *=−3.4±0.2 kJ mol−1 and Δ trsS *=−1.7±0.9 J K−1 mol−1 , where the uncertainties are given as twice the standard deviations.

Solid-solute phase equilibria in aqueous solution. V: The system CdCO3-CaCO3-CO2-H2O

The system CdCO3-CaCO3-CO2-H2O was investigated potentiometrically at 25 °C and 1 atm total pressure. Cd(1−x)CaxCO3 solid solutions were prepared over the entire compositional range. Aqueous solutions were kept at constant ionic strength (omitted from abstract) . Experimental results were depicted in Lippmann (omitted from abstract) diagrams. The solutus was investigated by pH and Cd2+ sensitive electrodes and it turned out that the solid solutions neither dissolved congruently nor attained stable equilibrium with aqueous solutions within one week of observation. Consequently, the excess Gibbs energy of mixing of otavite and calcite was determined by a method of successive approximations. Aqueous solutions were presaturated to the supposed equilibrium composition prior to equilibration with solid solutions, and constant pH values, corresponding to constant log ∑∗Kps0 values, indicated stable equilibrium. A recently proposed technique of sequential Bayesian excess parameter estimation was employed for data evaluation which led to the conclusion that the otavite-calcite solid solution series has ideal mixing properties.

THERMODYNAMICS OF CaCO3 PHASE TRANSITIONS

Thermodynamic quantities of the aragonite → calcite transition, were evaluated using results of calorimetric investigations. (1) Dissolution enthalpies of the CaCO3 polymorphs aragonite and calcite measured near room temperature with different calorimeter, (2) the enthalpy of the spontaneous phase transformation obtained by differential scanning calorimetry, (3) heat capacities and heat capacity differences determined with a heat flux calorimeter as well as previously determined, (4)e.m.f. data on Gibbs-energies of the phase transition were processed simultaneously with an optimization routine developed recently. The optimized data set (25°C) given below corresponds reasonably with CODATA recommendations, however, the precision has markedly improved.

Critical evaluation of solubility data: enthalpy of formation of siderite

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, ΔfH(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 ΔfH(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 ΔfH(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.

The IUPAC-NIST Solubility Data Series: A guide to preparation and use of compilations and evaluations (IUPAC Technical Report)

The IUPAC-NIST Solubility Data Series (SDS) is an ongoing project that provides comprehensive reviews of published data for solubilities of gases, liquids, and solids in liquids or solids. Data are compiled in a uniform format, evaluated, and, where data from independent sources agree sufficiently, recommended values are proposed. This paper is a guide to the SDS and is intended for the benefit of both those who use the SDS as a source of critically evaluated solubility data and who prepare compilations and evaluations for future volumes. A major portion of this paper presents terminology and nomenclature currently recommended by IUPAC and other international bodies and relates these to obsolete forms that appear in the older solubility literature. In addition, this paper presents a detailed guide to the criteria and procedures used in data compilation, evaluation, and presentation and considers special features of solubility in gas + liquid, liquid + liquid, and solid + liquid systems. In the past, much of this information was included in introductory sections of individual volumes of the SDS. However, to eliminate repetitive publication, this information has been collected, updated, and expanded for separate publication here.

Solid-solute phase equilibria in aqueous solution. VI. Solubilities, complex formation, and ion-interaction parameters for the system Na+−Mg2+−ClO 4 − −CO2−H2O at 25°C

AbstractThe stoichiometric solubility constant of eitelite (NaMg0.5CO3+2H+ ⇄ Na++0.5Mg2++CO2(g)+H2O, log*KpsoI=14.67±0.03 was determined at I=3 m (mol kg−1) (NaClO4) and 25°C. The stability of magnesium (hydrogen-)carbonato complexes in this ionic medium was explicitely taken into account. Consequently, trace activity coefficients of free ionic species, calculated from the Pitzer model with ion-interaction parameters from the literature, were sufficient for an evaluation of the thermodynamic solubility constants and Gibbs energies of formation for eitelite (−1039.88±0.60), magnesite (−1033.60±0.40), hydromagnesite (−1174.30±0.50), nesquehonite (−1724.67±0.40), and brucite (−835.90±0.80 kJ-mol−1). The increasing solubilities of nesquehonite and eitelite at higher sodium carbonate molalities were explained by invoking a magnesium dicarbonato complex (Mg2++2CO32− ⇄ Mg(CO3)22−, log βz = 3.90 ± 0.08). A set of ion-interaction parameters was obtained from solubility and dissociation constants for carbonic acid in 1 to 3.5 m NaClO4 media

Solid-Solute Phase Equilibria in Aqueous Solution. XII. Solubility and Thermal Decomposition of Smithsonite

(\theta _{HCO_3^ - ,ClO_4^ - } = 0.081, \theta _{CO_3^{2 - } ,ClO_4^ - } = 0.071, \psi _{{\rm N}a^ + , HCO_3^ - , ClO_4^ - } = - 0.019,\psi _{{\rm N}a^ + , CO_3^{2 - } , ClO_4^ - } = - 0.006,\lambda _{ClO_4^ - ,CO_2 } = - 0.076)