31P NMR as a spectroscopic monitor of the spontaneous precipitation of calcium phosphates.

High-resolution 31P NMR spectroscopy is shown to be a potentially valuable new method for monitoring the spontaneous precipitation of calcium phosphates from metastable supersaturated solutions. An apparatus capable of pH-statting the sample in a spinning 20 mm NMR sample tube is briefly described. The spontaneous precipitation of dicalcium phosphate dihydrate, CaHPO4 X 2H2O, pH-statted at pH 5, is characterized by a base-uptake curve which follows the decrease in the intensity of the solution 31P resonance. The precipitation of amorphous calcium phosphate at neutral pH, which exhibited an induction period of approximately 10 min, was also studied. No evidence of NMR peaks from transient clusters or the initial colloidal solid phase has been seen.