Uncertainty exists about the interaction of monofluorophosphate (MFP) ion with apatitic substrates due to the complex interfacial chemistry. Not only is it necessary to consider a series of different ions in the aqueous phase; the solid may additionally show non-homogeneity. The situation is better described as a dynamic as opposed to a static equilibrium. The kinetics of the hydrolysis of MFP have been the subject of a series of studies. Our own experiments utilizing techniques analysing for both fluoride and MFP often showed significant differences in calculated and observed fluoride concentrations. Calcium ion at very low pH increases hydrolysis. This points to the inadequacy of etching procedures in obtaining enamel samples in connection with enamel MFP interactions. Postulated mechanisms of action in MFP apatite interaction include exchange with ortho-phosphate, a hydrolysis step leading to formation of orthophosphate and fluoride, and permanent replacement of HPO42- without a hydrolysis step. Our work does not support the idea of hydrolysis. We believe that observed increases in orthophosphate arise from other than hydrolytic cleavage of MFP ions and can be attributed to the direct interchange of MFP groups for orthophosphate groups on the surface of the apatitic substrate. Our recent observations suggest that MFP may adopt a cyclic trimolecular structure in acidic solutions similar to that of trimetaphosphate. The hydrolysis of this structure is suggested in explanation of the difficulties in resolving the rates of release of F- and orthophosphate from MFP.

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© 1983 S. Karger AG, Basel
1983
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