Effect of citrate and pyrophosphate on the stability of calcium oxalate dihydrate

The effect of citrate and pyrophosphate on the stability of calcium oxalate dihydrate (COD) was studied in an aqueous solution over 7 days. COD was precipitated from 1.0 x 10(-2) mol/l calcium oxalate solution with the addition of citrate and/or pyrophosphate and separated by centrifugation immediately, 48 h and 7 days after the precipitation. The percentage of COD in the precipitate, determined by X-ray diffraction, increased with a rise in citrate or pyrophosphate concentration and reached > or =90% at a citrate concentration of 1.0 x 10(-3) mol/l or a pyrophosphate concentration of 2.0 x 10(-4) mol/l. The resulting COD was completely transformed into calcium oxalate monohydrate within 48 h when the precipitate was composed of <90% COD. Nearly pure COD produced with pyrophosphate was stable over 7 days while that with citrate underwent partial transformation within 48 h. An additive effect of citrate and pyrophosphate was found on the stability of COD. It was concluded that a pyrophosphate concentration above a critical point was sufficient to prevent solution-mediated transformation of COD, and this critical point might be lowered to the physiological range with the presence of citrate.