Studies on the Inorganic Carbon Dioxide Component of Human Enamel. II. The Effect of Acid on Enamel CO₂

¹ Eastman Dental Dispensary, Rochester, New York

When ground enamel samples with surface areas of 18 and 28 m²/gm (by ethane absorption measurement) were treated with acetate and phosphate buffers of pH 4-7 over a range of molarity, and solid-to-solution ratios, the residue was found to have up to 40-44 per cent less CO₂ than the original enamel. Under similar experimental conditions, enamel of larger particle size (100-200 mesh) failed to lose CO₂ in detectable amounts. The buffers were sodium phosphate, sodium acetate, and potassium acetate, in the order of their effectiveness in increasing the removal of CO₂ from the residue (with less dissolution but a greater net rise in pH). This was interpreted to mean that the exchange of H₃O⁺ for CO₂ in the enamel was increased by sodium and by phosphate ions. However, in the same buffer, increasing the dissolution increased the CO₂ losses from the residue. The factors which increased the dissolution were original pH, pH rise with increasing solid-to-solution ratios, and molarity. A constant preferential removal of CO₂ was found that was statistically independent of the amount of dissolution or the factors influencing it but was a function of the available surface. The 18 m²/gm gave a constant of 8 per cent; the 28 m²/gm, a constant of 12 per cent; while the 100-200 mesh gave a constant of < 1. On the basis of these results, it was proposed that the CO₂ in enamel is statistically in an even distribution in the unit cells of the apatite lattice and that the phenomenon of a constant preferential removal in apatites is a reflection of the removal of replaceable or labile CO₂ by H₃0⁺, phosphorus, or sodium from surface unit cells and is a direct function of the ratio of surface area to total volume.