The areas per 043ion and Ca2+ ion on the surface of calcium hydroxyapatite [Caio(OH)2(P04)6] at the point of zero charge were found to be 33.1 ± 2.7 and 23 ± 2.1, respectively; these values are in reasonable agreement with those expected from crystal parameters. The presence of P043-(or its hydrolysis products) Ca2+ Na+, K+, CHCh, or barbiturate ion in suspension of hydroxyapatite did not alter the area per P043or Ca2+.These areas did however change with pH. At pH values sufficiently less than 7.5 the area per P043-ion decreased.
The kinetics of exchange of 32P and 45Ca in aqueous solution with synthetic, stoichiometric hydroxyapatitie was followed up to 200 hours. Seventy percent of the total exchange occurred in 1 hour, and isotopic equilibrium was reached at approximately 40 hours. Using McKay (13) plots, the exchange reaction was shown to be heterogeneous. Diffusion of isotope through solution “compartments” was considered to be rapid in relation to exchange occurring at the solid surface. The nature of the exchange curve was explained by the differing energies of activation of exchange for surface atoms located at different positions on the crystal landscape. The use of well‐crystallized, stoichiometric hydroxyapatite resulted in the establishment of a well‐defined isotopic equilibrium (11) with little evidence of loss of isotope through intracrystalline diffusion or recrystallization.
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