Non-stoichiometric Dissolution of Hydroxyapatite in the Presence of Simple Salts

Abstract

Dissolution behavior of hydroxyapatite (HAP 2 g/dl) was studied in the presence of simple salts (Na2SO4, CH, COONa (=NaAc), or NaCl). Equilibrium concentrations of phosphate ([Pi]) and calcium ([Ca2+]) ions released from HAP increased with concentration of added salts. The order of salts increasing [Pi] was Na₂SO₄>NaAc>NaCl, while that increasing [Ca²⁺] was NaCl>NaAc>Na₂SO₄. The behavior of such an incongruent dissolution of HAP was explained in terms of anion and cation exchanges between constituent ions of HAP and those of added salts under a restriction of the solubility product of HAP. The degree of ion exchanges depended on the affinity and concentration of added salts. Equilibrium pH of the HAP suspension contaiing SO₄^2- was the highest due to the highest concentration of Pi released from HAP, because Pi consumesNa⁺ in a mother solution through a mechanism of protination. It was concluded that SO₄^2- has a specific affinity for HAP. This fact was confirmed by the facts that zeta potential (η) of HAP was the lowest algebraically in the presence of Na, SO₄ and that the ion size of SO₄^2- is quite similar to that of PO₄^3- although their valenc ies are different each other.