Isoionic Isotope Exchange with Hydroxylapatite and the Dilution Effect

Isoionic isotope exchange data of ⁴⁵Ca²⁺ and ³²PO₄ ^3- in a saturated solution with hydroxylapatite at 25 °C, collected by Avnimelech, have been reinterpreted on the basis that the processes consisted of three pools: hydration shell, surface layer and "recrystallization of crystallites." These processes are regarded as strictly separable. This theoretically allows: (i) a quantitative evaluation of the constants involved in the processes; (ii) an estimation of the exchange capacities of hydration and surface layers (fast pools) which may be pH-dependent; and (iii) a determination of the isoionic exchange rate constants. The constants are essentially the same for Ca²⁺ and PO₄ ^3- at a given pH if the cross-sectional areas of the ions are taken as 23 Ų and 33 Ų respectively; they, perhaps, increase linearly with the hydrogen ion concentration of the solutions. The effects of three consecutive abrupt dilutions of the radioactive ions without changing the concentrations (or pH) of the nonradioactive components of the solution are predictable by a mathematical model based on the complete reversibility of the two fast pools and the kinetics of the irreversible third pool.