Dicalcium phosphate dihydrate hydroxyapatites

Dicalcium Phosphate Dihydrate (DPD). Dicalcium phosphate cHhydrate is completely nonreactive at room temperature. At 65—71°C and in the presence of water, it dehydrates and decomposes into hydroxyapatite and acidic monocalcium phosphate, or a free phosphoric acid (18). It is used to some extent in cake mixes in combination with faster acting acid. Its primary function is to provide acidity late in the baking cycle and thus produce a neutral and palatable product. DPD has an NV of 33. It provides sufficient acidity only in products requiring long baking times. 

Despite the importance of the precipitation of calcium phosphates, there is still considerable uncertainty as to the nature of the phases formed in the early stages of the precipitation reactions under differing conditions of supersaturation, pH, and temperature. Although thermodynamic considerations yield the driving force for the precipitation, the course of the reaction is frequently mediated by kinetic factors. Whether dicalcium phosphate dihydrate (CaHPO HoO, DCPD), octacalcium phosphate (Ca HfPO, 2.5 H20, OCP), hydroxyapatite (Cag (PO fOH), HAP), amorphous calcium phosphate (ACP), or a defect apatite form from aqueous solution depends both upon the driving force for the precipitation and upon the initiating surface phase. Thermodynamically, the relative supersaturation, o, is given by... 

The deposition of sparingly soluble calcium phosphates on PHEMA hydrogels, both in vitro and in vivo, can be modified by the incorporation of citric acid into the PHEMA hydrogels. In the presence of citrate anions the formation of hydroxyapatites was prevented. The calcium phosphate deposits which formed in vitro on PHEMA were mainly monocalcium phosphate monohydrate and dicalcium phosphate dihydrate. The types of deposits formed in vivo were quite different from those formed in vitro. The in vivo deposits formed on PHEMA were mostly hydroxyapatites deficient in calcium and hydroxyl ions. Citrate anions were also observed to prevent significantly the deposition in vivo of protein onto PHEMA hydrogels. 

Monocalcium phosphate monohydrate Dicalcium phosphate Dicalcium phosphate dihydrate Octocalcium phosphate Precipitated hydroxyapatite ... 

Calcium phosphate cements are combinations of tetracalcium phosphate and a phosphate more acid than hydroxyapatite, preferably anhydrous dicalcium phosphate or dicalcium phosphate dihydrate. Upon contact with water these two phosphates dissolve until the singular point for both compoimds is reached, which corresponds to a pH value of 7.4. At this point the liquid phase is oversaturated with respect to hydroxyapatite, which starts to precipitate, causing setting and hardening ... 

Dicalcium phosphate dihydrate [CaHP04.2H20] Dicalcium phosphate anhydrate [CaHPOJ Octacalcium phosphate [CagH2 (P04)6.5H20] Tricalcium phosphate [Ca3 (P04)2] Hydroxyapatite [Ca o (P04)6 (OH)2] Tetracalcium phosphate [Ca4 (P04)20]... 

In neutral or alkaline solutions, the order of stability (insolubility) is hydroxyapatite > P-tricalcium phosphate > octacalcium phosphate > dicalcium phosphate (monetite) > dicalcium phosphate dihydrate (brushite) > monocalcium phosphate. Below pH 4.8, however, monetite and brushite are the most stable and insoluble phases, although these acid salts dissolve incongruently in water (see below). 

This compound is structurally related to hydroxyapatite and is produced as an intermediate during the hydrolysis of dicalcium phosphate dihydrate. It is also involved in the formation of bones and teeth (Chapter 11.1). 

Octacalcium phosphate can be isolated by controlled hydrolysis of dicalcium phosphate dihydrate in 0.5 M sodium acetate at 40°C (5.57), or from calcium acetate and disodium phosphate at pH 5-6. The hydrolysis of octacalcium phosphate is spontaneous and is accelerated by high temperature, high pH and by F. If an excess of Ca " is present, the sole hydrolysis product is hydroxyapatite. 

Ca-P has a Ca P ratio of 1.67 (hydroxyapatite). With a decrease of the Ca to P ratio, other Ca-defident phases occur like tricalcium phosphate (TCP, Ca P ratio 1.50), octacalcium phosphate (OCP, Ca P ratio 1.33), dicalcium phosphate anhydrous (DCPA or Monetite, Ca P ratio 1.00), and dicalcium phosphate dihydrate (DCPD or Brushite, Ca P ratio 1.00). Most of these caldum deficient compounds are used as raw material for sintering procedures for ceramics, or as ingredients for example, Ca-P cements. TCP and HA will be discussed more in detail, since these materials are most used for biomedical applications. 

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