Local concentration controls the hydrate phase of calcium oxalate

Non-additive factors can significantly affect the crystallization pathways and hydrate phases of calcium oxalate. However, little is known about the mechanisms behind these phenomena. Our research investigated various factors affecting calcium oxalate crystallization in the absence of additives. We show that the formation of calcium oxalate hydrate phase in a supersaturated solution is inconsistent with thermodynamic stability. The metastable calcium oxalate trihydrate (COT) forms at lower concentrations (<1.25 mM), while the stable calcium oxalate monohydrate (COM) forms at intermediate concentrations, and the metastable calcium oxalate dihydrate (COD) forms at higher concentrations (>20 mM). The formation of the hydrate phase also depends on the method of mixing calcium and oxalate solutions. By utilizing an automatic titration apparatus, we show that the hydrate phase can be altered by adjusting either the titration rate of the calcium solution or the stirring speed. In addition, the calcium-to-oxalate ratio is important in determining the initial binding between calcium and oxalate ions. All these factors could be explained by the local concentration of calcium and oxalate upon mixing the solutions. The reduction of local concentration promotes the formation of COT. Thus, the hydrate phase is determined by a combination of thermodynamic and kinetic factors. These results significantly enhance our understanding of the crystallization pathway and hydrate phase selection of calcium oxalate.