Thermogravimetric analysis of kidney stone with monohydrate calcium oxalate

Abstract

Renal calculi with calcium oxalates are represented by the general formula CaC2O4.xH2O, where x is the number of bounded-water molecules, which can vary from 1 to 2. It can be formed on crystalline seed particles of organic or inorganic compounds that work as a nucleating substrate. Therefore, the H2O molecule might be bound or free, depending on if the H2O molecule belongs to the crystal structure or the organic compound among them. Some of the characterization techniques commonly used is not suitable to give the structural information about the H2O molecule. For instance, chemical, infrared spectroscopy and thermogravimetric analysis cannot define by itself whether the H2O molecule is structural or interstitial. Thermogravimetric Analysis (TGA) of kidney stones colleted from 56 donors of Vit´oria City, Esp´ırito Santo State - southeast - Brazil was developed in order to investigate the phase transition diagram of calcium oxalate. X-ray powder diffraction was used as initial technique to classify the initial composition of renal calculi. Rietveld refinement of XRD data of one apparent monophasic (CaC2O4.xH2O or COM) renal calculus revealed the presence of a small amount of hydroxyapatite. The presence of this second phase and the morphology of the stone (ellipsoidal) indicated that this calculus can be classified as non-papillary type and its nucleation process developed in closed kidney cavities. The thermogravimetric Analysis of this (CaC2O4.xH2O or COM) reveals that monohydrate calcium oxalate presents a two phase transitions up to 600 Celcius.