A calcium hydroxyapatite precipitated from an aqueous solution: An international multimethod analysis

Abstract

In this paper a pure calcium hydroxyapatite prepared from aqueous solutions at low temperature, was analysed by a large number of techniques in six Institutes. The techniques employed (frequently in more than one laboratory) were X-ray diffraction, IR analysis, BET measurement, chemical analysis, differential thermal analysis, magic angle spinning NMR, TEM, size distribution measurements, crystal growth and crystal dissolution measurements. Several data were obtained not only at room temperature but in the range up to 900 °C. The results show that the apatite is very pure. The impurities CO 3 and NO 3 are both below 0.1%. The important ion HPO 2- 4 is present in amounts of ≈ 2 mol% of total phosphate and covers most likely the outer surface of the crystals. Five different techniques were employed to determine the HPO 2- 4 content. H 2O is present in adsorbed form in about 2.0 wt%. The Ca/P ratio is between 1.63 and 1.66. The OH - content is about 10% by weight lower than the stoichiometric value. Crystal sizes have mean values of 33 nm width and 133 nm length. BET surface is 37 m 2 g -1. X-ray powder diffraction yields a and c lattice parameters of 9.428 and 6.882 Å at RT and after a brief heating to 900 °C in N 2. If the apatite is heated at 850 °C for 18 h in air, partial decomposition takes place to form α and β Ca 3(PO 4) 2. 1H MAS-NMR shows that the H 2O is present in a very mobile form and is presumably adsorbed in several monolayers. The absolute quantitation of the structural hydroxyl content by 1H MAS-NMR reveals a slight deficiency, as do IR results. Kinetic data suggest that the dissolution mechanism is controlled by polynuclear surface nucleation catalysed by hydrogen ions, the interfacial surface free energy σ being 50 mJ m -2 at pH = 7. Growth kinetics analysed for a polynuclear mechanism lead to the value σ = 87 mJ m -2 at pH = 7.