Neutralization Potential of Hydroxyapatite Particles in Acids and Acidic Beverages

Objectives: Hydroxyapatite is used in oral care products as a remineralizing agent and acts as a reservoir for calcium and phosphate ions. Consequently, it can act as a neutralizing agent in acidic attacks caused by erosive beverages or cariogenic processes. The aim of this in vitro study was to investigate the neutralization potential of hydroxyapatite particles in different acids and beverages.
Methods: Hydroxyapatite particles were immersed in phosphoric acid, citric acid and lactic acid (38 °C, 0.1 M, 5.0 wt%), as well as lemonade and cola (38 °C, 5.0 wt%). To assess the general neutralization potential of the chosen systems, the pH-values of the acids were measured before addition of hydroxyapatite and after equilibration (24 h). The neutralization kinetics of hydroxyapatite particles in lemonade and cola were investigated by continuous measurement of the pH-values within the first two minutes of neutralization. Scanning electron microscopy (SEM) was performed to investigate the morphology of particles before and after the treatment.
Results: pH-measurements showed a significant pH-value increase from 1.6 to 3.0 pH-values in the acidic systems. This equates to a reduction of > 97.6% of protons in solution. During neutralization, stoichiometric amounts of calcium were released into the solution. Kinetic measurements showed a significant reduction of free acid in lemonade and cola already after 20 seconds (Δc_H+/c_H+,0 = - 99.4% [cola]; - 93.2% [lemonade]). SEM analysis revealed the structural integrity of hydroxyapatite particles after acidic treatment.
Conclusions: The results show that hydroxyapatite particles rapidly neutralize most of the free protons in acidic systems. The large surface area resulting from porosity of the particles allows a significant increase of the pH-value within 20 seconds, which makes synthetic hydroxyapatite an efficient neutralizing agent in mouthwashes and toothpastes.