Calcium X phosphate product

Block GA, Hulbert-Shearon TE, Levin NW, Port FK. Association of serum phosphorus and calcium x phosphate product with mortality risk in chronic hemodialysis patients a national study. Am J Kidney Dis 1998 31 607-17. 

Chertow GM, et al. Long-term effects of sevelamer hydrochloride on the calcium x phosphate product and lipid profile of haemodialysis patients. Nephrol Dial Transplant 2000 15 559. 

The combination of a reduced plasma calcium concentration (due to vitamin D deficiency) and a reduced plasma phosphate concentration (due to secondary hyperparathyroidism) produces a calcium X phosphate product below the threshold for calcification of new bone. 

The first is that calcium, and probably phosphate ions as well, are concentrated by hard tissue cells, and subsequently secreted in matrix vesicles. In this way the (calcium x phosphate) ion product may be boosted above the solubility level of the more soluble, but spontaneously nucleable, calcium phosphates, such as amorphous calcium phosphate. Under appropriate conditions, this substance may undergo solid phase transformation to biological apatite, which cannot itself be formed by homogeneous nucleation. This type of mineralization may occur in extrafibrillar situations at an early stage of the process, e.g. calcospherites may be formed where maximum ordering of the inorganic phase has not yet been reached. 

PRACTICE EXAMPLE B A handbook lists K p = 1 X 10 for calcium hydrogen phosphate, a substance used in dentifrices and as an animal feed supplement. Write (a) the equation for the solubility equilibrium and (b) the solubility product constant expression for this slightly soluble solute. 

Calcium Phosphates. The alkaline-earth phosphates are generally much less soluble than those of the alkaH metals. Calcium phosphates include the most abundant natural form of phosphoms, ie, apatites, Ca2Q(P0 3X2, where X = OH, F, Cl, etc. Apatite ores are the predominant basic raw material for the production of phosphoms and its derivatives. Calcium phosphates are the main component of bones and teeth. After sodium phosphates, the calcium salts are the next largest volume technical- and food-grade phosphates. Many commercial appHcations of the calcium phosphates depend on thek low solubiHties. 

Hydroxyapatite, Ca2Q(PO (OH)2, may be regarded as the parent member of a whole series of stmcturaHy related calcium phosphates that can be represented by the formula M2q(ZO X2, where M is a metal or H O" Z is P, As, Si, Ga, S, or Cr and X is OH, F, Cl, Br, 1/2 CO, etc. The apatite compounds all exhibit the same type of hexagonal crystal stmcture. Included are a series of naturally occurring minerals, synthetic salts, and precipitated hydroxyapatites. Highly substituted apatites such as FrancoHte, Ca2Q(PO (C02) (F,0H)2, are the principal component of phosphate rock used for the production of both wet-process and furnace-process phosphoric acid. 

In this case, we start with a known material for which we have already used x-ray analysis to determine the nature of the fired product. We start with the reactions given in 7.3.8. for the reactions of calcium phosphate, since this also illustrates how assays are calculated. The steps include ... 

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... 

Milk serum is supersaturated with calcium phosphate, the excess being present in the colloidal phase, as described above. The balance between the colloidal and soluble phases may be upset by various factors, including changes in temperature, dilution or concentration, addition of acid, alkali or salts. The solubility product for secondary calcium phosphate, [Ca2+][HPOr] is about 1.5 x 1(T5 or pKs = 4.85. 

Figure 1. Crystals of calcium phosphate (arrows) in an unstained smear of a cheese sauce product. A small sample of the product was placed on a slide and observed by Kohler Illumination. If quantification is necessary, sample preparation is easily standardized to allow for particulate enumeration, (x 80)...

The results of the characterization of the product synthesized with Fourier transformation infra-red spectrophotometer (FTIR), X-ray diffractometer (XRD), scanning electron mirror microscope (SEM) and the transmission electron mirror microscope (TEM) illustrate that the product synthesized by the process of double decomposition-precipitation with calcium nitrate and di-ammonium phosphate as the reactants in the SCISR consists of well dispersed particles of about 15 nm in diameter and 50-70 nm long, having a very regular shape and appearance and is confirmed to be hydroxylapatite. 

The fact that milk and milk products are rich in calcium is known by most people. Casein is the main protein of milk, accounting for about 80% of milk protein. Casein has a molecular weight of about 23,000 and, thus, is a relatively small protein however, the casein in milk occurs as a large complex of casein molecules having an overall molecular weight of about 100 x 10. Casein is phosphorylated on residues of serine. These phosphate groups bind calcium ions. The calcium ions help maintain the stability of the large complex. 

By use of releasing agents Considering the reaction M-X-i-R = R- Xh-M, it becomes evident that an excess of the releasing agent (R) will lead to an enhanced concentration of the required gaseous metal atoms (M) which will be of special significance if the product R-X is a stable compormd. Hence in the determination of calcium in presence of phosphate the addition of excess of strontium chloride to the test solution will lead to the formation of strontium phosphate and the calcium can then be determined in an acetylene-air flame without any interference due to phosphate. Also addition of EDTA to a calcium solution before analysis may increase the sensitivity of the subsequent flame spectrophotometric determination which may be due to the formation of an EDTA complex of calcium which is readily dissociated in the flame. 

---