Influence of ageing, pH and various additives on crystal formation in artificial urine

In order to investigate the effect of various factors on urinary crystallization processes, a series of five experiments was carried out using an artificial urine (AU) in a rotary evaporator. The influence of ageing, pH and organic, inorganic and potential inhibitory additives formed the basis of the study. Precipitates were characterized by X-ray powder diffraction, scanning electron microscopy and energy dispersive X-ray analysis. In the ageing experiment, AU aliquots, adjusted to various pH values, were allowed to stand for several days and were not evaporated. Calcium oxalate monohydrate (COM) was formed at low pH, while whitlockite, apatite and struvite occurred at pH greater than 7. In the second experiment, AU aliquots at various pH values, were evaporated. Similar results to those of series 1 were recorded but, in addition, calcium oxalate trihydrate (COT) precipitated in the pH range 3 to 6.5 and brushite at pH greater than 5.5. In series 3, uric acid, creatinine and urea were included in AU aliquots (pH 5.5) which were subjected to evaporation. Uric acid promoted the formation of uric acid dihydrate; however, when present with creatinine, dihydrate formation was inhibited. Urea appeared to inhibit precipitation. In the fourth experiment, MgO, methylene blue and chondroitin sulphate A were independently included in the AU (pH 5.5). Precipitates of calcium oxalate mono-, di- and trihydrates were obtained. In the final experiment fluoride aliquots of variable concentrations were included in the AU (pH 5.5 and 6.5). COT crystals of superior quality to those observed in control solutions were obtained.