E. Königsberger

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.

Complexation of iron (III) and iron (II) by citrate. Implications for iron speciation in blood plasma

Estimates of the concentrations and identity of the predominant complexes of iron with the low-molecular-mass ligands in vivo are important to improve current understanding of the metabolism of this trace element. These estimates require a knowledge of the stability of the iron-citrate complexes. Previous studies on the equilibrium properties of the Fe(III)-citrate and Fe(II)-citrate are in disagreement. Accordingly, in this work, glass electrode potentiometric titrations have been used to re-determine the formation constants of both the Fe(III)- and Fe(II)-citrate systems at 25 degrees C in 1.00 M (Na)Cl and the reliability of these constants has been evaluated by comparing the measured and predicted redox potentials of the ternary Fe(III)-Fe(II)-citrate system. The formation constants obtained in this way were used in computer simulation models of the low-molecular-mass iron fraction in blood plasma. Redox equilibria of iron are thus included in large models of blood plasma for the first time. The results of these calculations show the predominance of Fe(II)-carbonate complexes and a significant amount of aquated Fe(II) in human blood plasma.

A new optimization routine for chemsage

An optimization routine has been incorporated into ChemSage, a software package for Gibbs-energy minimization calculations. Standard Gibbs energies, excess Gibbs functions, and molar volume data can be optimized with regard to several types of experimental information, e.g., phase equilibria, activities, calorimetric, potentiometric, and density data. Parameters for all available solution models in ChemSage can be simultaneously adjusted. The Bayesian estimation technique employed enables the user to provide a priori information for the desired parameters. Examples showing the versatility of the program will be demonstrated.

Solubility of the Three Calcium Oxalate Hydrates in Sodium Chloride Solutionsand Urine-Like Liquors

Summary. Calcium oxalate forms three hydrates (stable monoclinic mono-hydrate (COM), metastable tetragonal dihydrate (COD) and triclinic trihydrate (COT)), which are of medical importance in urinary calculi formation. In this work, the solubility of these calcium oxalate hydrates was determined at 20, 25, 30, 37, and 40°C in aqueous NaCl (0.02–0.20 molċdm−3) and in urine-like liquors. Also, for the first time, the solubility of COM was systematically studied as a function of pH in artificial urine solutions which contain organic compounds. The concentrations of calcium ions were measured continuously using a calcium ion selective electrode and also determined by AAS. The thermodynamic solubility products were obtained from computer simulations in which all possible complexes formed in aqueous solution were taken into account. These values were compared to those available in the literature; it was found that the present results, especially for COD and COT, constitute a major improvement on the previously reported values. The constants determined in this work were used to predict the solubility of calcium oxalate hydrates at different pH in urine-like liquors. It was found that in artificial urine solutions, citrate and magnesium ions promote the solubility of COM, whereas urea and creatinine have no significant effect..Zusammenfassung. Calciumoxalat bildet das stabile monokline Monohydrat (COM) sowie zwei metastabile Phasen, das tetragonale Dihydrat (COD) und das trikline Trihydrat (COT). Da die drei Calciumoxalathydrate bei der Entstehung von Nierensteinen eine wichtige Rolle spielen, wurde ihre Löslichkeit bei 20, 25, 30, 37, und 40°C in NaCl-Lösungen (0.02–0.20 molċdm−3) sowie in künstlichen Urinlösungen, die zum Teil auch organische Substanzen enthielten, gemessen. In letzteren wurde die Löslichkeit von COM als Funktion des pH-Werts erstmalig systematisch untersucht. Die Ca2+-Konzentrationen wurden mit Hilfe einer ionenselektiven Elektrode sowie durch AAS bestimmt. Die thermodynamischen Löslichkeitsprodukte wurden aus Computersimulationen erhalten, in denen die in der wäßrigen Lösung vorkommenden Komplexe weitestgehend berücksichtigt wurden. Beim Vergleich dieser Größen mit Literaturwerten stellte sich heraus, daß insbesondere die Werte für COD und COT eine wesentliche Verbesserung gegenüber den bisherigen Daten darstellen. Die in dieser Arbeit ermittelten Löslichkeitsprodukte wurden zur Berechnung der Löslichkeit der Calciumoxalathydrate in künstlichem Urin verwendet. Es zeigte sich, daß Mg2+ und Citrat die Löslichkeit erhöhen, wogegen Harnstoff und Creatinin keine signifikante Auswirkung haben..

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.

The ionic product of water in highly-concentrated aqueous electrolyte solutions

SummaryThe ionic product of water,

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

K_w = [H^ + ][OH^ - ] = 10^{ - pK_w }

Optimisation of the thermodynamic properties of the Ti–H and Zr–H systems

, has been determined in aqueous NaCl (0.5–5.0M), KCl (3.0M), NaNO3 (3.0 and 5.0M), and KNO3 (2.5M) at 25 °C from high-precision potentiometric titrations carried out in cells with liquid junction using either glass or hydrogen electrodes. Measurements ofKw provide a set of self-consistent data that can be used in the estimation of activity coefficient changes and liquid junction potentials in the study of extremely concentrated electrolyte solutions. Where comparison is possible, results obtained by hydrogen electrode measurements are in excellent agreement (ca ± 0.005 inpKw) with other reliable experimental values and the predictions of thePitzer activity-coefficient model. The glass electrode results are, as expected, routinely lower (by 0.03–0.05pKw units), owing to interference by Na+ ions. This effect virtually disappears in solutions of potassium salts. Comparison of the experimental results with thePitzer predictions shows that knowledge of the ternary interaction parameters is essential to account for specific ionic effects in the concentration dependence ofpKw.ZusammenfassungDas Ionenprodukt des Wassers,