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

Abstract

Ground enamel samples with surface areas of 18 and 28 m2/g, lost 8 and 12% more CO2 respectively, than calcium, phosphorus, or weight, on treatment with potassium and sodium acetate (pH 4.0-5.5) and sodium phosphate (pH 4.5-7) buffers. The excess loss of CO2 for a given amount of enamel dissolved, tended to be independent of pH, molarity, or solid/solution ratio, but dependent (1) on the buffer system used, with phosphate more effective than acetate, and sodium more effective than potassium, and (2) on the surface area of the sample. Under no experimental conditions could all the CO2 be removed preferentially without completely dissolving the sample. Under identical conditions, 100-200 mesh enamel failed to lose CO2 preferen-tially. It was suggested that the mechanism of CO2 removal may be heteroionic exchange with hydronium or phosphate ions. Assuming that CO2 is in a statistically even distribution in the crystals, it was proposed that the extent of preferential removal of CO2 from any apatite would be proportional to the ratio of CO2 in the surface to the CO2 in the total volume of the crystal, and should be a function of surface area.