Hydroxyapatite stoichiometry

Calcium hydroxyapatites of various stoichiometries catalyze the partial oxidation of methane with oxygen at 600°C to carbon monoxide, carbon dioxide, formaldehyde, hydrogen and water. 

The ASTM F 1185 designation specifies chemical and crystallographic requirements for hydroxyapatite applied to the surfaces of surgical implants. Elemental analyses for calcium and phosphorus will confirm the expected stoichiometry of hydroxyapatite. The calcium and phosphorus contents will be determined by a suitable method such as ion chromatography. A quantitative X-ray diffraction analysis will determine a hydroxyapatite content of at least 95%. The concentration of deleterious trace elements such as arsenic, cadmium, mercury and lead will be assessed for hydroxyapatite derived from natural resources. The analysis of other trace elements may be required, based on the conditions, apparatus or environments specific to the manufacturing techniques and raw materials. Inductively coupled plasma/mass spectroscopy (ICP/MS), atomic absorption (AAS) or the... 

The substitution by other ions reduces the theoretical stoichiometric Ca/P ratio of 1.67 of hydroxyapatite to values for bone-like apatite of less than 1.6 (LeGeros, 1991), even as low as 1.4 (Weng etal., 1994). This non-stoichiometry of biological apatite can be described by the approximate formula (Young and Holcomb, 1982 Hattori and Iwadate, 1990 Liu et al., 2001). 

More complexity is introduced by the fact that deviation from stoichiometry leads to precipitation of calcium-deficient hydroxyapatite (1.33 < Ca/P < 1.67) with the approximate formula Ca HPOJ POJg OH) with 0 < x < 2. If = 2, one obtains OCP, x = 1 leads to TCP and x = 0 to hydroxyapatite. 

It has been suggested that the mineral portions of bone and tooth tissue are calcium-deficient hydroxyapatites (Posner and Perloff, 1957 Neuman and Neuman, 1953). The possible presence of hydrogen bonds in these materials may provide a much needed parameter for an estimation of the stoichiometry of these biological hydroxyapatites under varying conditions (Posner et al, 1960). 

Modification of the magnetite surface with hydroxyapatite was carried out to give high biocompatibility to Fc304 magnetosensitive carriers. It was determined that the hydroxyapatite phase on the surface of the carrier was characterized by the correlation Ca/P = 1.67, which corresponds to the stoichiometry of the reaction of its creation. 

Two aspects of the crystal chemistry of natural and synthetic apatites need to be recognized. First, the HA in bone is nonstoichiometric, has a Ca/P ratio of less than 1.67, and contains carbonate ions, sodium, magnesium, fluorine, and chlorine (Posner, 1985a). Second, most synthetic hydroxyapatites actually contain substitutions for the phosphate and/or hydroxyl groups and vary tom die ideal stoichiometry and Ca/P ratios. Oxyhydroxyapatite [Ca,o(P04) 0], a-tricalcium phosphate (a-TCP), )3-tricalcium phosphate O-TCP) or )8-Whitlockite [Caj(P04)J, tetracalcium phosphate (Ca4P209), and octocalcium phosphate [Cag(HP04)2(P04)4 5H20] have all been detected via x-ray diffraction (XRD), Fourier transform into spectroscopy (FITR), and chemical analyses (Kohn and Ducheyne, 1992 Ducheyne et al., 1986, 1990 Koch et al., 1990). These compounds are not apatites per se since the crystal structure differs from that of actual apatite. 

The literature on hydroxyapatite is very extensive and numerous varieties have been prepared by different methods. Many of these have non-stoichiometric compositions and include calcium-deficient varieties, tricalcium phosphate hydrates and various precipitated products with Ca/P ratios between 1.3 and 2.00 (Ca/P = 1.67 theoretical). While the more impure products may contain quantities of Ca(OH)2, CaHP04 -2H20 or Ca3(P04)2, the non-stoichiometry can in many cases only be accounted for by lattice vacancies (denoted as ), or snbstitntions within the crystal lattice or on its surface. The non-stoichiometry of apatites ranains a subject of much investigation since it is related to physical as well as chanical properties. 

The fact that the reactions and behaviom of hydroxyapatite are to an extent dependent upon stoichiometry and method of preparation, has in the past, led to some of the inconsistencies in the reported data for the supposedly pme compound. 

Because of the lack of stoichiometry, a question that is often raised by those who study the mineral composition of bone is whether the mineral phase is single or multiple. If the crystalline defects of the apatites are considered, it seems quite justified to describe the mineral composition of the bone in terms of a single phase. Thus, the bone salts would be made essentially of an impure hydroxyapatite, in which the impurities found their way either within the lattice or at the surface of the lattice. Such a concept is in keeping with the mineral composition of the body fluids, which are undersaturated in calcium carbonate, calcium citrate, magnesium carbonate, and magnesium bicarbonate. 

Hydroxyapatite Ca5(P04)30H is the main mineral constituent of bones and teeth. The chemical composition of biological apatite is somewhat different from the above given formula. They are usually defined as non-stoichiometric apatites. This non- stoichiometry is caused by the presence of different ions in very small or in... 

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