The interactions of biomolecules with the surface of hydroxyapatite (HAP) is of great importance in systems where biological fluids are in contact with hard tissues. The characteristics of the electric double layer (EDL) developed between the surface of HAP and electrolyte solution, play a crucial role in these interactions. In the present study the acid-base properties of HAP in contact with KNO₃ solutions were investigated using potentiometric titrations. The novelty of this work is the analysis for calcium and phosphate of aliquots of samples withdrawn from the system every 0.5 pH unit during the titration procedure. It was thus possible to know precisely the composition of the solution at each point of the titration curve. It was found that, even for rapid titration experiments, a remarkable amount of H⁺ ions (H⁺_dissol.) participates to bulk solution equilibria with species coming from the dissolution of HAP. These ions must be taken into account in the determination of the point of zero charge (pzc) and consequently of the surface charge (σ^o). Finally, it was demonstrated that the intersection point of the titration curve of the suspension (HAP/indifferent electrolyte) with the titration curve of the respective blank solution (indifferent electrolyte), which has been modified to include the H⁺_disol. ions, corresponds to the pzc. Conclusively, a novel and easy methodology for determining the pzc and surface charge of HAP and other sparingly soluble solids is presented.

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1. Study of the the sorption of Cu(II) species on the "TiO₂ / KNO₃" interface.