Precipitation and Solubility of Calcium Hydrogenurate Hexahydrate

Solid phases formed in the quaternary system: uric acid—calcium hydroxide —hydrochloric acid—water aged for 2 months at 310 K were studied to determine conditions for calcium hydrogenurate hexahydrate, Ca(C(5)H(3)N(4)O)(2) · 6H(2)O precipitation. The precipitates were identified by chemical and thermogravimetric analyses, x-ray powder diffraction, infrared spectroscopy, light microscopy, and scanning electron microscopy. In the precipitation diagram the concentration region in which calcium hydrogenurate hexahydrate precipitated as a single solid phase was established. The solubility of calcium hydrogenurate hexahydrate was investigated in the pH range from 6.2 to 10.1 at different temperatures. The total soluble and ionic concentration of calcium (atomic absorption spectroscopy and Ca-selective electrode), total urate concentration (spectrophotometry), and pH were determined in equilibrated solutions. The data are presented in the form of tables and chemical potential diagrams. By using these data the thermodynamic solubility products of calcium hydrogenurate hexahydrate, K(s) = a(Ca(2+)) · a(2)(C(5)H(3)N(4)O(3)(−)), were determined: [Formula: see text] The formation of calcium hydrogenurate hexahydrate crystals in urinary tract of patients with pathologically high concentrations of calcium and urates (hypercalciuria and hyperuricosiuria) is possible.