The Solubility of Magnesium Ammonium Phosphate Hexa-hydrate at 38°c. With Considerations Pertaining to the Urine and the Formation of Urinary Calculi

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Abstract
Since magnesium ammonium phosphate hexahydrate or "triple phosphate" is a fairly frequent constituent of human urinary calculi, the solubility of this compound was studied. Within the pH range of 6.6-7.9, saturated solutions of MgNH4PO46H2O are supersaturated with respect to magnesium and phosphate. Under the conditions of this experiment Mg3(PO4)2 appears to be the form precipitated above pH 6.87-6.88; below this pH the evidence suggests that MgHPO4 is coming out of solution. At pH 7.4 both the onset of precipitation of Mg3(PO4)2 and the final equilibrium are delayed. At pH 7.4 sodium citrate had a pronounced effect in increasing the solubility of MgNH4PO4.6H2O. The higher concentrations of sodium citrate also prevented the precipitation of Mg3(PO4)2 from saturated solutions of the magnesium ammonium salt. These effects can be explained by the existence of a magnesium citrate complex.