NONSTOICHIOMETRIC DISSOLUTION OF HYDROXYA PATITE IN THE PRESENCE OF SIMPLE SALTS

Dissolution behavior of hydroxyapatite (HAP ; 2 g/dl) was studied in t he presence of simple salts (Na2SO4, CH3COONa (=NaAc), or NaCl). Equil ibrium concentrations of phosphate ([Pi]) and calcium ([Ca2+]) ions re leased 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 disso lution of HAP was explained in terms of anion and cation exchanges bet ween constituent ions of HAP and those of added salts under a restrict ion of the solubility product of HAP. The degree of ion exchanges depe nded on the affinity and concentration of added salts. Equilibrium pH of the HAP suspension contaiing Na2SO4 was the highest due to the high est concentration of Pi released from HAP, because Pi consumes H+ in a mother solution through a mechanism of protination. It was concluded that SO42- has a specific affinity for HAP. This fact was confirmed by the facts that zeta potential (zeta) of HAP was the lowest algebraica lly (zeta<0) in the presence of Na.,SO, and that the ion size of SO,'- (i.e, interatomic distance of S-O = 1.49 angstrom) is quite similar t o that of PO43- (i. e., that of P-O = 1.54A) although their valencies are different each other.