Interaction of Citric Acid with Hydroxyapatite: Surface Exchange of Ions and Precipitation of Calcium Citrate

The use of citric acid is efficacious and distinctive in the demineralization of dentinal root surfaces for periodontal regeneration and in the etching and conditioning of enamel or dentin for bonding restorative resins. To decipher the role of citric acid in these applications, it is important that one have a basic understanding of its interaction with synthetic hydroxyapatite. The uptake or removal of citrate ions from aqueous solutions of citric acid (4 to 100 mmol/L, 10 mL) by hydroxyapatite (1 g) was studied at 22°C after a given reaction period (from 3 hr to 11 days) by immediate spectrophotometric monitoring of the concentrations of the filtrates (214 nm). The concentrations of calcium, phosphate, and hydrogen ions were also determined in the same solutions. The interaction: (i) is a time-independent ionic-exchange process with the substrate when the initial acid concentrations are dilute (4 to 12.5 mmol/L), and (ii) is a reactive process that is time-dependent for higher acid concentrations. The exchange process shows an adsorption of about one citrate ion per (100) face of the unit cell of hydroxyapatite for a maximally exchanged surface. The curves representing the reactive process may be quantitatively or qualitatively explained on the basis of the supersaturation of the solutions with respect to calcium citrate and its slow precipitation. The physico-chemical analysis of the needle-shaped birefringent crystals of the precipitate from the supersaturated solutions confirms the precipitate to be Ca3(citrate) 2·4H2O.