Solubility of the three calcium oxalate hydrates in sodium chloride solutions and urine-like liquors

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 degrees C in aqueous NaCl (0.02-0.20 mol.dm(-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 GOT, 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.