Scanning electron microscopic study of the effect of citrate and pyrophosphate on calcium oxalate crystal morphology

The effects of citrate and pyrophosphate on the morphology of calcium oxalate monohydrate crystals formed in a supersaturated calcium oxalate solution were examined by scanning electron microscopy. Within the normal range of urinary concentrations, citrate and pyrophosphate affected the morphology of the crystals. The length/width ratio of the crystals decreased at higher concentrations of citrate and pyrophosphate. Citrate seems to have a dual mechanism of inhibition of crystal growth (binding to the crystal surfaces) and of complexation (with calcium in the solution). Pyrophosphate is believed to bind preferentially to calcium in the solid phase rather than in the solution phase, as it affected crystal morphology at very low concentrations and was detected on the crystals by X-ray microanalysis. Pyrophosphate seems to bind to the monohydrate but not to the dihydrate.