Precipitation of calcium carbonates and phosphates. I. Spontaneous precipitation of calcium carbonates and phosphates under physiological conditions

Abstract

The slow spontaneous precipitation of calcium carbonates and calcium phosphates was studied under physiological conditions. Calcium carbonate precipitation was prevented by the presence of phosphate ions at concentrations too low for the precipitation of calcium phosphate. The composition of most biological fluids does not allow the deposition of calcium carbonate. The suggestion is made that such deposition will occur under the influence of metabolic processes which locally raise the bicarbonate and lower the phosphate concentration. Aragonite was metastable under the conditions of the experiments. The aragonite present in the early precipitates of calcium carbonate transformed into calcite, when kept in contact with the supernatant solution. The crystallization of apatite is disturbed when bicarbonate is present in the solution. At sufficiently elevated calcium concentrations and bicarbonate/phosphate concentration ratios, an amorphous precipitate of calcium carbonate phosphate formed, which failed to crystallize into apatite when kept in contact with the supernatant solution. The Ca P molar ratio of the apatitic and amorphous precipitates varied between approximately 2.3 and 1.4. This large variation is due to the coprecipitation of carbonate, HPO 4, and Na ions. These and other impurities interfere with the crystallization of the biological apatites and limit the size of the crystallites in bone and dentine. Dental enamel is better crystallized because it contains lesser amounts of these impurities.