Characterisation of the protolytic properties of synthetic carbonate free fluorapatite.

The acid/base surface properties of carbonate free fluorapatite (Ca5(PO4)3F) have been characterised using high precision potentiometric titrations and surface complex modelling. Synthetic carbonate free fluorapatite was prepared and characterised by SEM, XRD, FT-IR and FT-Raman. The specific surface area was determined to be 17.7+/-1.2 m2 g(-1) with BET (N2 adsorption). The titrations were performed at 25+/-0.2 degrees C, within the pH range 5.7-10.8, in 0.10 and 0.50 mol dm(-3) NaNO3 ionic media. Experimental data were interpreted using the constant capacitance model and the software FITEQL 4.0. The surface equilibria: [triple bond]S1OH <==> [triple bond]S1O- + H+ lg betaS(-110) (int), [triple bond]S2OH <==> [triple bond]S2O- + H+ lg betaS(-101) (int) well describes the surface characteristics of synthetic fluorapatite. The equilibrium constants obtained were: lg betaS(-110) (int) = -6.33+/-0.05 and lg betaS(-101) (int) = -8.82+/-0.06 at I = 0.10 mol dm(-3). At the ionic strength 0.50 mol dm(-3), the equilibrium constants were slightly shifted to: lg betaS(-110) (int) = -6.43+/-0.05 and lg betaS(-101) (int) = -8.93+/-0.06. The number of active surface sites, N(s), was calculated from titration data and was found to be 2.95 and 2.34 sites nm(-2) for the ionic strengths 0.10 and 0.50 mol dm(-3), respectively. pH(PZC) or the IEP was found to be 5.7 from Z-potential measurements.