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 mol Á dm À3 ) and in urine-like liquors. Also, for the ®rst time, the solubility of COM was systematically studied as a function of pH in arti®cial 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 arti®cial urine solutions, citrate and magnesium ions promote the solubility of COM, whereas urea and creatinine have no signi®cant effect. Lo Èslichkeit der drei Calciumoxalathydrate in Natriumchloridlo Èsungen und urina Èhnlichen Flu È ssigkeiten 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 Lo Èslichkeit bei 20, 25, 30, 37, und 40 C in NaCl-Lo Èsungen (0.02±0.20 mol Á dm À3 ) sowie in ku Ènstlichen Urinlo Èsungen, die zum Teil auch organische Substanzen enthielten, gemessen. In letzteren wurde die Lo Èslichkeit von COM als Funktion des pH-Werts erstmalig systematisch untersucht. Die Ca 2 -Konzentrationen wurden mit Hilfe einer ionenselektiven Elektrode sowie durch AAS bestimmt. Die thermodynamischen Lo Èslichkeitsprodukte wurden aus Computersimulationen erhalten, in denen die in der wa Èûrigen Lo Èsung vorkommenden Komplexe weitestgehend à Corresponding author beru Ècksichtigt wurden. Beim Vergleich dieser Gro Èûen mit Literaturwerten stellte sich heraus, daû insbesondere die Werte fu Èr COD und COT eine wesentliche Verbesserung gegenu Èber den bisherigen Daten darstellen. Die in dieser Arbeit ermittelten Lo Èslichkeitsprodukte wurden zur Berechnung der Lo Èslichkeit der Calciumoxalathydrate in ku Ènstlichem Urin verwendet. Es zeigte sich, daû Mg 2 und Citrat die Lo Èslichkeit erho Èhen, wogegen Harnstoff und Creatinin keine signi®kante Auswirkung haben.

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The solubility constant of norsethite, BaMg(CO 3 ) 2 , was determined for the ®rst time. Also, a new method was developed which permits the precipitation of pure norsethite from homogeneous solution. The solubility measurements were performed at 25 C in 0.20 mol Á dm À3 sodium perchlorate media of constant ionic strength. It was shown that norsethite dissolves congruently in this solution. The pH variation method was used to obtain a stoichiometric solubility constant which was extrapolated to zero ionic strength by use of the Davies equation.The resulting thermodynamic solubility constant was incorporated in a comprehensive computer model which permits solubility calculations in complex systems. Thus, the conditions for norsethite formation in the environment and in the laboratory can be predicted. Furthermore, the stability of norsethite with respect to witherite (BaCO 3 ) and the various magnesium carbonate phases was calculated.

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