In vitro study on the neutralization potential of particulate hydroxyapatite in erosive acids

Hydroxyapatite (HAP; Ca₅(PO₄)₃(OH)) is used in oral care formulations as a remineralizing agent and acts as a reservoir for calcium and phosphate ions. Consequently, HAP can serve as a neutralizing agent during acidic attacks caused by erosive beverages or cariogenic processes. This in vitro study aims to investigate the neutralization potential of HAP particles, used in oral care products, with a focus on changes in pH value, Ca²⁺ release, and structural integrity.

5.0% (w/v) particulate HAP was added to hydrochloric acid, phosphoric acid, citric acid, and lactic acid (37 °C, 0.1 M), as well as to Sprite® and Coca-Cola®, all under continuous stirring. pH values were measured after equilibration. Neutralization kinetics of HAP were investigated in Sprite® and Coca-Cola® within the first 30 s after addition of 0.15% (w/v), 0.5% (w/v), and 5.0% (w/v) HAP. Scanning electron microscopy (SEM) and X-ray powder diffraction (XRD) were employed to investigate crystallite morphology and composition. Atomic absorption spectroscopy (AAS) was conducted to analyze Ca²⁺ release.

The investigated HAP shows a significant increase in pH in all systems and neutralizes between 98.0% and 99.9% of the acidic protons in the equilibrium. Up to 2619 mg L^-1 ± 48 mg L^-1 of Ca²⁺ are released in hydrochloric acid, while the porous particle structure remains unaffected. Kinetic investigation in beverages shows that already 0.15% (w/v) HAP neutralizes most of the dissociated acid within 20 s (83.9% in Sprite®, 98.7% in Coca-Cola®).

The study shows that HAP used in oral care formulations can efficiently counteract acids by increasing pH value and releasing Ca²⁺ ions, while retaining its functional morphology.