256 -T- A. GRAY AND M. D. FRANCIS 



why fluorapatite should have a lower initial rate of solubility in acid 

 than hydroxyapatite when the reaction of hydrogen ion with the 

 phosphate group must be responsible for the breakdown of the 

 apatite lattice. Measurements of the solution phase in equilibrium 

 with fluorapatite mineral in acidic buffers gave the solubility product 

 for calcium fluoride ( Table IV ) . The measured solubility product of 



TABLE IV. Equilibrium Solubility Constants of Calcium Fluoride 

 AND Fluorapatite Determined in Lactate Buffer 



"The value of the Ksp determined acros.s uU systems (10) was 2.8 (±1.2) X IQ-i". 

 '' A naturally occurring mineral form of fluorapatite [Caio(P04)6F2]. 



calcium fluoride in lactate buffer is given for comparison because 

 it was found that the value was lower, corresponding to literature 

 values, when determined in water or unbuffered acidic solutions. 



Although pH of the solubilizing solution and composition of the 

 enamel may have only an indirect effect in determining how fast 

 enamel dissolves (or an incipient carious lesion forms), they will 

 have a major role in determining the phase at any equilibrium and, 

 therefore, the rate at which the equilibrium is reached. Furthermore, 

 it is necessary to determine the fate of the phosphate phase laid 

 down during a low pH condition when the external pH conditions 

 return to values once more establishing apatite as the stable phase. 

 Unfortunately, solubility product measurements cannot answer this 

 question, but new physical techniques, such as back-scattering elec- 

 tron diffraction and infrared absorption spectroscopy by attenuated 

 total reflection, give promise of confirming the presence of these thin 

 coatings which have been discovered by chemical means. 



Among the problems which remain to be solved in the description 



