Solubility Criteria for the Existence of Hydroxyapatite

Abstract

The solubilities of synthetic basic calcium phosphate precipitates in carbon dioxide free aqueous systems were studied over a wide range of conditions. T h e pH of the suspensions and the calcium and phosphate concentrations in the solutions were determined both after precipitation atid after dissolution of the solid phases. Solubility criteria applied to these measurements indicated that hydroxyapatite has a definite solubility product.

Solubility of Ca5{P04)3OH in the System Ca{OH)2-H3P04-H20 at 5, 15, 25, and 37 °C

“…Clark [1] has reported a value of similar magnitude, 20 X 10-59 , for a sample of hydroxyapatite prepared by precipitation at 90°C.…”

“…(All quantities were calculated on the molar basis.) The ionic a ctivity product is defin ed for all Ca5(P04hO H as (1) where the first fac tor (activity of ionic calcium) a nd the second (activity of orthophosphate ion) are defin ed as follows :…”

“….077 (1) .282 (5) .820 (13) .823(14) 3.46(6) 3.43 (6) a T is sum of ion pair c oncentrations. :; The standard errors li sted in the ta ble were computed for eac h system with the aid of the covariance matri x (eq {A -l a}) of the adjus ted observations and the nl)propri atc d erivatives of e<lu<otions {8-9} whic h d efin e T and the ion pair concentrntions.…”

Solubility of hydroxylapatite and some phosphate rocks

“…Furthermore, in reviewing literature, it was found that there was much controversy about the solubility of hydroxylapatite. One researcher felt that hydroxylapatite possessed a solubility product constant (Clark, 1955). Others felt that it did not possess a solubility product constant (Levinskas and Neuman, 1955), and still others felt that the solubility of hydroxylapatite was controlled by a complex that formed on the surface of the solid (Rootare et al, 1962;LaMer, 1962).…”

“…Levinskas and Neuman found that the molar ratio of calcium to phosphorus in solutions in equilibrium with hydroxylapatite was not independent of the solid to solution ratio and that the solubility of hydroxylapatite was greater in 0.165 N sodium chloride than in 0.165 N potassium chloride. Clark (1955) obtained a reproducible solubility product constant for hydroxylapatite by reacting dilute solutions of calcium hydroxide and orthophosphoric acid under an N^ atmosphere at 90° C. for 120 hours and equilibrating the resulting systems under at 25° C. for 96 hours. The solubility product calculated from analysis of the supernatant solution was not significantly different from the solubility product calculated from analyses on a solution obtained when some of the solid was equi librated at 25° C. for 24 hours with pure water under an atmosphere.…”