3D Solubility Diagram for Calcium Phosphates by Solid Titration

Solid titration has been successfully used to re-examine the solubility of calcium phosphates; the conventional method of adding a large excess of solid is inappropriate due to incongruent dissolution. The results for HAp have been found to differ markedly from the form usually reported. Commonly, the Ca/P ratio of the solution has not been controlled, or has not been allowed for in equilbrium speciation and K_sp calculations. Often, pH has been adjusted using phosphoric acid but this is expected to shift the solubility. The true relationship of the solubility of HAp to other calcium phosphates needs to be established.

Objectives: To ascertain the solubility of calcium phosphates in 100 mM KCl solution at 37 °C as a function of both pH and Ca/P ratio.

Methods: Solid titration, using laser-scattering end-point detection, was used for DCPD, OCP, β-TCP and TTCP. XRD, EDX, SEM and TEM were used to characterize the equilibrium. The HAp solubility surface expressed as log[Ca] against pH and Ca/P ratio was created and compared with results from other work.

Results: HAp was found to be the equilibrium phase under all conditions tested. The HAp was found to be calcium-deficient below pH ~3.9, and close to stoichiometric above that point. DCPD was not detected except as a metastable intermediate with DCPD as titrant. A sharp minimum in the solubility surface was found near Ca/P = 1.67.

Conclusion: The solubility of calcium phosphates is greatly affected by Ca/P ratio and this may underlie the discrepancy between excess-solid and solid-titration results. Solid titration has again been confirmed as a sensitive technique of great reliability and reproducibility. The belief that DCPD is the most stable phase at low pH needs to be re-examined.