Amorphous tricalcium phosphate

Amorphous tricalcium phosphate is obtainable by rapid precipitation at 25°C from saturated solutions of (NH4)2HP04 and Ca(N03)2 kept at pH 10.5 (5.68). This freshly precipitated amorphous tricalcium phosphate, Ca3(P04)2, is believed to be apatitic in nature and its formula can be written as Ca9n(P04)gn2. In the presence of water, some hydrolysis occurs and the material soon becomes crystalline Ca9(P04)5(HP04)(0H), with the channels only half filled by OH. Equation 5.69 has also been given for the preparation of this latter defect apatite [13]. 

By the use of freeze drying techniques, an amorphous phase can, however, be isolated. It is believed that this phase may consist of Ca9(P04)g clusters, 9.5 A diameter, which are randomly packed with up to 20% H2O molecules, to form spherical particles 300-1000 A diameter. 

Heat treatment produces the a and P crystalline varieties. Varieties of amorphous tricalcium phosphate containing other ions such as Mg +, COj and P2O7 have been prepared. The presence of such ions appears to increase the stability against conversion to apatitic crystalline forms. 

The stability of amorphous tricalcium phosphate is reportedly increased by the presence of ATP or certain phosphoproteins such as phosvitin and casein. 

Amorphous tricalcium phosphate may be present immediately prior to the formation of apatite in the bone mineralisation process (Chapter 11.1). On the other hand, it is thought by some that certain phosphoproteins may act as substrates for apatite nucleation directly from supersaturated solutions when bone growth occurs. 

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Heughebaert, J.C. and Montel, G. (1982) Conversion of amorphous tricalcium phosphate into apatitic tricalcium phosphate. Calcif. Tissue Int., 34, S103-S108. 

Phosphoproteins can be extracted from bone and dentine with EDTA. The phosphoproteins in dentine form about 10% of the total protein present, and have a very high serine and aspartine content with about half of the serine residues phosphorylated. Isolated dentine phosphoprotein has been shown to catalyse the formation of apatite from amorphous tricalcium phosphate, and it may act in this way in teeth (Chapter 11.1) [13]. 

Tricalcium phosphate, Ca2(P0 2> is formed under high temperatures and is unstable toward reaction with moisture below 100°C. The high temperature mineral whidockite [64418-26-4] although often described as P-tricalcium phosphate, is not pure. Whidockite contains small amounts of iron and magnesium. Commercial tricalcium phosphate prepared by the reaction of phosphoric acid and a hydrated lime slurry consists of amorphous or poody crystalline basic calcium phosphates close to the hydroxyapatite composition and has a Ca/P ratio of approximately 3 2. Because this mole ratio can vary widely (1.3—2.0), free lime, calcium hydroxide, and dicalcium phosphate may be present in variable proportion. The highly insoluble basic calcium phosphates precipitate as fine particles, mosdy less than a few micrometers in diameter. The surface area of precipitated hydroxyapatite is approximately... 

Tricalcium Phosphate. Commercial tricalcium phosphate (TCP) is actually an amorphous basic calcium phosphate close to hydroxyapatite in composition. Because of its extremely low solubiUty in water, TCP is precipitated almost quantitatively from dilute phosphate solutions with a slurry of hydrated lime. TCP is separated by dmm-, spray-, or flash-drying the TCP slurry, with or without intermediate sedimentation or filtration steps. It is used as an industrial-grade flow conditioner and parting agent. 

Phosphate is sometimes present in MU water sources (say, 1-2 ppm or more) usually as a result of field and factory run-off or from the deliberate addition as a city water threshold agent to prevent corrosion and deposition in the mains. The steady growth in the reuse of secondary water sources such as municipal and industrial waste waters means that phosphate is increasingly likely to be present in MU. If the phosphate remains undetected, it likely will scale and foul FW lines by forming amorphous calcium orthophosphate [tricalcium phosphate Caj(P04)2] sludge before it reaches the boiler section. 

A white, amorphous powder, insoluble in cold water. Tricalcium phosphate is gradually decomposed by boiling water into an insoluble basic salt and an easily soluble acid salt. It is easily soluble in hydrochloric or nitric acid, and without effervescence. 

Calcium phosphate will ordinarily precipitate at concentrations typically exceeding 5 ppm PO4 or less, forming amorphous calcium orthophosphate (tricalcium phosphate) sludge, Ca3(P04>2, in the bulk water and crystalline hydroxyapatite, Caio(OH)2(P04)2, at heat-transfer surfaces. 

Figure 9. Solubility of hydroxylapatite compared to oxyapatite, amorphous calcium phosphate, tetracaleium phosphate (TTCP), a-tricalcium phosphate and P-tricalcium phosphate determined in O.IM potassium acetate at pH 6 (modified from Le Geros et al. 1995).

Tricalcium phosphate (TCP) has a Ca P ratio of 1.5, similar to the amorphous biologic precursors of bone [5]. It can be prepared by sintering Ca deficient apatite (Ca P ratio 1.5). TCP is a polymorph ceramic and exhibits two phases (a- and -whitlockite), known as a- and (S-TCP. Variation in sintering temperature and humidity determine, which phase is being formed a-TCP occurs at dry heat >1300°C and subsequent quenching in water [4]. Solubility and resorbability of both forms is much higher compared to HA. However, a-TCP is unstable in water and reacts to produce HA. a-TCP is used mainly as a compound... 

Loher, S., Reboul, V., Brunner, T.J., Simonet, M., Dora, C., Neuenschwander, P. and Stark, W.J. (2005) Improved degradation and bioactivity of amorphous aerosol derived tricalcium phosphate nanoparticles in poly(lactide-co-glycolide). Nanotechnology, 17(8), 2054-61. 

In the case of calcium carbonate scale, indices are typically calculated at the highest expected temperature and highest expected pH—the conditions under which calcium carbonate is least soluble. In the case of silica, the opposite conditions are used. Amorphous silica has its lowest solubility at the lowest temperature and lowest pH encountered. Indices calculated under these conditions would be acceptable in many cases. Unfortunately, cooling systems are not static. The foulants silica and tricalcium phosphate are used as examples to demonstrate the use of operating range profiles in developing an in-depth evaluation of scale potential and the impact of loss of control. 

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