Formation constants beta(2) of calcium and magnesium fluorides at 25 degrees C

Until now, scientific work on the reactions of the complexation in water of fluoride ions with calcium or magnesium ions considered only soluble order 1 complexes (CaF+ and MgF+). The precipitation of the compounds CaF2 and M gF2 did not take into account the preliminary formation in solution of thes e order 2 complexes (CaF20 and MgF20). We therefore studied their formation using a direct potentiometric method in the F- ion-selective electrode at 25 degreesC. This experiment, together with a computerized thermodynamic st udy, allowed us to determine the values of formation constants at ionic str ength equal to zero: log beta (CaF20) = 5.7 and log beta (CaF20) = 3.2, as well as limiting solubilities: S-CaF2 = 10(-4.8) and S-MgF20 = 10(-5) mol l (-1). Then, some examples were studied showing the necessity to take into a ccount the formation constant of CaF2: the supersaturation of solutions, th e validity of results from the fluoride selective electrode, the difference between released fluoride in distilled water and in artificial saliva in r estorative dentistry, the fluoride concentration in hard waters and its hea lth consequences. (C) 2000 Elsevier Science B.V. All rights reserved.