Calcium phosphate phases

Calcium phosphate precipitation may also be involved in the fixation of phosphate fertilizer in soils. Studies of the uptake of phosphate on calcium carbonate surfaces at low phosphate concentrations typical of those in soils, reveal that the threshold concentration for the precipitation of the calcium phosphate phases from solution is considerably increased in the pH range 8.5 -9.0 (3). It was concluded that the presence of carbonate ion from the calcite inhibits the nucleation of calcium phosphate phases under these conditions. A recent study of the seeded crystal growth of calcite from metastable supersaturated solutions of calcium carbonate, has shown that the presence of orthophosphate ion at a concentration as low as 10-6 mol L" and a pH of 8.5 has a remarkable inhibiting influence on the rate of crystallization (4). A seeded growth study of the influence of carbonate on hydroxyapatite crystallization has also shown an appreciable inhibiting influence of carbonate ion.(5). 

A disadvantage of the conventional precipitation method in which the supersaturation was allowed to decrease during the reactions, was that different calcium phosphate phases could form and subsequently dissolve during the course of the reactions. In the present work, the constant composition method was used to investigate the influence of sodium chloride, potassium chloride, and potassium nitrate, as background electrolyte upon the rate of crystallization of HAP in solutions supersaturated only with respect to this phase. These experiments were made in solutions containing totaj... 

Fluorapatite is a highly insoluble calcium phosphate phase. The solubility product of stoichiometric fluorapatite at 37°C is 3.19 0.14x10 " mol 1 (for Cas(P04)3F as reported by Moreno et al. [53]) and appears significantly lower than that of HA in the same conditions (7.36 0.93 x 10 ° mol for Ca5(P04)30H). Asuggested explanation for this very low solubility product is that cohesive forces are stronger in fluorapatite than in other apatites due to smaller unit-cell dimensions. The complete solid solution Ca-,o(P04)6(OH)2-xFx can be obtained. Initial solubility determinations have shown a solubility minimum for x close to 1 [54], related to the formation of hydrogen bonding between F and OH ions. These results were subsequently... 

According to Bloomfield and Mead (1974), The ultimate goal of all workers on casein is to reconstitute micelles with native properties from the separated constituents of skim-milk. This assertion reflects the large number of studies in the literature on the precipitation and association properties of the caseins.There are, however, legitimate scientific goals in this kind of work other than the creation of artificial casein micelles, such as the elucidation of the mechanisms by which phosphoproteins profoundly influence the nucleation and growth of calcium phosphate phases. 

Figure 1. Calcium concentrations and pH values of solutions saturated with respect to various calcium phosphate phases in the ternary system. Ca(OH)2-HjP0>-H20 at 25°C (2).

Figure 2. Growth of HAP seed crystals. Plots of total calcium as a function of time. Approximate saturation levels with respect to the various calcium phosphate phases also are shown.

As the pH is increased (see figure 1), other calcium phosphate phases may be stabilized as precursors to the formation of apatite and the growth of HAP seed crystals in stable supersaturated solutions is much more complex (, 1, 49). ... 

Measurements of the specific surface area, SSA, of the products grown at various times indicate that the initial formation of a microcrystalline or amorphous precursor leads to a rapid increase in SSA. The development of these phases is also observed by scanning electron microscopy, and dissolution kinetic studies of the grown material have indicated the formation of OCP as a precursor phase ( , 7). The overall precipitation reaction appears to involve, therefore, not only the formation of different calcium phosphate phases, but also the concomitant dissolution of the thermodynamically unstable OCP formed rapidly in the initial stages of the reaction. In the presence of magnesium ion the overall rate of crystallization is reduced and lower Ca P ratios are observed for the first formed phases (51). 

The results of these studies confirm the suggestions made earlier that the nature of the calcium phosphate phase which forms in the precipitation process is dependent not only upon the supersaturation, ionic strength, and pH, but also upon the rate at which reactions proceed. 

At pH values below about 6.0 the influence of HEDP and fluoride, an important environmental constituent is particularly interesting. It has been shown that the nature of the precipitating calcium phosphate phase can be controlled not only by the concentration of inoculating HAP seed ( )but also by the presence of additives. Thus fluoride ion accelerates the crystallization of calcium phosphate probably through the formation of fluorapatite (Ca5(P04)3 F, FAP) (, 60). Under... 

Sediment phosphorus extraction analyses show that hydrous iron oxides (extracted by (NH4)2C204) play a major role in the transport of sediment phosphorus. In northern areas of the Genesee River watershed calcium carbonate formation also appears to be Involved in phosphorus fixation. Ion activity product calculations for water column samples from the Genesee River consistently exhibit subsaturation with respect to the stable calcium phosphate phase, hydroxyapatite. Calcium carbonate, which can serve as a substrate for phosphate mineralization, shows an ion activity product below the solubility product in the Genesee River except during the summer low-rainfall season. 

Extensive seeded calclte growth experiments in the presence of phosphate ion indicate that the phosphate ion adsorbs onto the crystal surface as a monolayer. At a concentration of 10" M, phosphate ion can strongly inhibit calcite formation however, short term experiments show that this monolayer adsorption removes insignificant amounts of phosphorus from solution. In experiments lasting several days a further decrease in solution phosphate concentration occurs, presumably caused by nucleation of a surface calcium phosphate phase on the calcite seed. 

Figure 7.20. Solubility of the metal phosphates. The solubilities of AIPO4 and FeP04 have been calculated on the basis of the equilibria assuming that FeP04S(s) or AlP04(s) can be converted into Fe(OH)3(s) [or a-FeOOH(s) or A1(OH)3(s).] The solubility of the calcium phosphate phases has been calculated under the assumption that [Ca 1 = 10" M and that F is regulated by the solubility of CaF2(s).

Santos AR, Wind RA, Bronnimann CE (1994) H CRAMPS and H- P HetCor experiments on bone, bone mineral, and model calcium phosphate phases. J Magn Reson B 105 183-187... 

In contrast to calculus, dental enamel contains over 96% w/w inorganic mineral [50]. The main constituent is a single calcium phosphate phase, HAP, the structure of which contains minor impurities such as magnesium, sodium, carbonate and chloride [50]. Dental caries is a disease of bacterial origin. Certain plaque bacteria can ferment sugars and other carbohydrates from the diet to produce lactic acid and other short chain organic acids [51], If the concentration of acid depresses the pH adjacent to the tooth surface below about pH 5.5, then the enamel dissolves. 

From a mechanistic viewpoint it is reasonable to anticipate an inverse clinical relationship between calculus and caries. Calculus formation is essentially a mineralisation process. The development of a caries lesion is the result of the net demineralisation of tooth enamel by plaque acid. These processes both involve crystalline calcium phosphate phases in contact with liquid, saliva and/or plaque fluid, containing their constituent ions. The oral environment also contains other salivary constituents and bacteria, which either inhibit or promote crystal growth or dissolution. 

It has been demonstrated that the release of citric acid from PHEMA hydrogels hinders the formation of calcium phosphates, especially hydroxyapatites. Because of this inhibitory effect, the calcium phosphate phases formed during in vitro calcification were mainly present as non-apatite phases, possibly MCPM and DCPD. The porous morphology of the outer surface of the spherical calcium phosphate deposits could be due to the dissolution of precipitates in the presence of citric acid. The results obtained after subcutaneous implantation ofPHEMA and PHEMA containing citric acid in rats confirmed the resistance of PHEMA-citric acid to calcification. The calcium phosphate deposits which formed in vivo consisted mainly of Ca2+ and OH deficient hydroxyapatites. However, it is not yet known whether or not the differences between the calcium phosphate phases found in vivo and in vitro arise from the presence of proteins/peptides in the in vivo calcifying medium. 

The knowledge of the presence of calcium phosphates in bone (De Fourcroy et al., 1788 Parr, 1809 von Bibra, 1844), teeth (Davy, 1814), blood and milk (De Fourcroy, 1804), urine (De Fourcroy et al., 1788) as well as urinary and renal calculi (Colon, 1770 Pemberton, 1814) was solidly established by the early nineteenth century. Additional historic evidence for this has been painstakingly recorded by the prolific chronicler of calcium phosphates, Dorozhkin (2012), quoting no less than 279 references on the history of calcium phosphate research. Among these treasures there appears faint indication that several calcium phosphate phases, important for biomineralisation, were already known, suspected or suggested early on such as amorphous calcium phosphate, ACP (Brande and Taylor, 1863) and octacalcium phosphate, OCP as well as dicalcium phosphate dihydrate, DCPD (brushite) (Warington, 1866). 

From a thermodynamical point of view, only two calcium phosphate phases are stable at ambient temperature and pressure in SBF, monetite (DCPA, dicalcium phosphate anhydrate, CaHP04) (Nancollas, 1977 Jinawath et al., 2001)... 

Hence, these calcium phosphate phases are members of the same family and their expression in aqueous precipitates is strictly governed by extraneous factors such as supersaturation, temperature and pH level. It is well known that OCP is a transitional state during the formation of HAp in vitro (Brown and Chow, 1986) and that its nucleation rate is substantially higher than that of HAp (Lu and Leng, 2005 Ito et al., 2014). In vivo proliferation and differentiation studies on murine bone marrow stromal ST-2 and primary calvarial osteoblastic cells support the conclusion that OCP promotes appositional bone formation more significantly than hydrolysed Ca-deficient hydroxyapatite, at least in its early stages up to 12 weeks (Suzuki et al., 2006). 

The reactions (5.3a) and (5.3b) lead to local ionic supersaturation that causes precipitation of calcium phosphate phases with low solubility product and high thermodynamic stability (Drevet and Benhayoune, 2012). The type of calcium phosphate precipitated is a function of the pH of the solution adjacent to the cathode. At a pH value below about 6.5 brushite (calcium diphosphate dihydrate) is stable according to... 

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