A study of the phase transformation of calcium oxalate trihydrate-monohydrate

The precipitation of calcium oxalates may involve transformation from a metastable hydrate such as calcium oxalate trihydrate, COT, to the thermodynamically stable monohydrate, COM. A highly reproducible seeded crystal growth method has been used to study this transformation reaction over a range of temperatures and in ionic media such as the chlorides of sodium, potassium, and magnesium. After the initial dissolution of COT, the calcium concentration remains constant for prolonged periods while appreciable nucleation and growth of COM takes place concomitantly with COT dissolution. The transformation is markedly dependent on temperature, composition, and concentration of ionic media. In urines, the hydrate conversion is appreciably slower and the rate depends upon the method of pretreatment. In these cases, evidence is advanced for the formation of other phases such as the calcium phosphates.