Heterogeneous nucleation of calcium oxalate trihydrate in artificial urine by constant composition

Calcium oxalate is a major component of most human kidney stones. To improve our understanding of kidney stone formation, crystallization of calcium oxalate in a urine-like solution was investigated using constant composition technique. Artificial urine with a calcium oxalate relative supersaturation of 10 was incubated at 37°C in the reaction vessel of a constant composition system with or without seed crystals of calcium oxalate monohydrate (COM). Reaction was allowed to proceed until its rate reached an equilibrium. Crystals were isolated and identified. A mixture of COM and calcium oxalate trihydrate (COT) crystals was produced in the absence of COM seeds. Addition of COM seeds resulted in their growth, formation of more COM crystals and unexpectedly, the nucleation of COT crystals. Seeded systems are generally considered to produce crystal growth only. Our results, however, indicate the occurrence of both nucleation and growth under these circumstances. It is proposed that low supersaturation in concert with high ionic strength as present in the artificial urine and exists in mammalian urine may lead to the coexistence of stable COM and unstable COT.