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 demineral ization of dentinal root surfaces for periodontal regeneration and in the etching and conditioning of enamel or dentin for bonding restorati ve 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 hydr oxyapatite (1 g) was studied at 22 degrees C after a given reaction pe riod (from 3 hr to 11 days) by immediate spectrophotometric monitoring of the concentrations of the filtrates (214 nm). The concentrations o f calcium, phosphate, and hydrogen ions were also determined in the sa me solutions. The interaction: (i) is a time-independent ionic-exchang e 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 repre senting the reactive process may be quantitatively or qualitatively ex plained on the basis of the supersaturation of the solutions with resp ect to calcium citrate and its slow precipitation. The physico-chemica l analysis of the needle-shaped birefringent crystals of the precipita te from the supersaturated solutions confirms the precipitate to be Ca -3(citrate)(2) . 4H(2)O.