Electrochemistry of human enamel: Selectivity to potassium in solutions containing calcium or phosphate ions

Further studies have been made at 25 ± 1 °C of the steady-state electromotive forces (emfs) developed across the enamel caps of whole teeth and synthetic hydroxyapatite membranes in concentration cells using KH₂PO₄, K₂HPO₄ and K₃PO₄ solutions at their natural pH values, and of the variation in the emf of enamel and Permutit C20 cation-exchange membranes in 0.100/0.010 M KCl concentration cells when small concentrations of calcium and phosphate are added separately (as CaCl₂ and K₂HPO₄) to both KCl solutions. Over the concentration range examined, the cationic selectivity of both enamel and HA cells with the potassium phosphate gradients was always greater than that with KCl gradients, the selectivity with K₂HPO₄ and K₃PO₄ gradients being virtually ideal. The results thus appear to substantiate the inferences drawn in previous work that (a) the mineral fraction is responsible for the observed effects and (b) that enamel is a “non-leaky” membrane. In KCl concentration cells with small calcium additions, a marked decrease in the membrane potential was observed which, by comparison with the results for the synthetic cation-exchange membrane C20, is attributed to the preferential selectivity of the enamel for Ca²⁺ ions under these conditions. With small concentrations of added phosphate, a decrease in potential was observed which could be ascribed to changes in the K⁺ ion gradient in the cells.