Selective Removal of Mineral and Organic Components of Bovine Enamel by Phosphoric Acid

Purpose: To follow the chemical composition of bovine enamel during phosphoric acid-induced demineralization. Materials and Methods: Enamel samples were ground into a fine powder, selecting the 150- and 200-µm fractions in order to obtain a more homogeneous study material. They were immersed in diluted phosphoric acid (0.1%) for increasing durations ranging from 1 to 1440 min. The chemical composition of the solution and enamel powder was determined after each sequential treatment by means of atomic absorption (AA) and Fourier transform infrared (FTIR) spectroscopy. Results: AA data revealed that the amount of calcium mobilized to the solution by the acid treatment was higher at shorter exposure times. However, FTIR data showed that the degree of mineralization of the enamel remained constant during the treatment, indicating that the mineral and organic components were lost at the same rate. Interestingly, poorly crystalline phosphate and carbonate-rich mineral components were preferentially removed and were presumably the main source of calcium released by the acid exposure. FTIR results also demonstrated that organic components rich in hydrophobic groups were preferentially removed during acid treatment. Conclusion: Etching with phosphoric acid produces a nonhomogeneous demineralization of bovine enamel, with the selective removal of poorly crystalline mineral and hydrophobic organic components. Keywords: calcium, demineralization, FTIR, hydroxyapatite, phosphoric acid