Civil Engineering Reference
In-Depth Information
Four of the calcium sites are coordinated with eight oxygens each, whereas the remaining
six are seven-coordinated. Six of these seven bonds are with oxygen, whereas the seventh
is with an hydroxyl ion. The theoretical Ca/P ratio of this stoichiometric compound is
1.67, but because of a possible calcium deficiency, the stability region of this phase may
extend down to Ca/P=1.50. Such a deficiency may be associated with the formation of up
to one vacancy per unit cell on one of the seven-coordinated calcium sites, and a charge
balance in such an instance is maintained by the removal of one of the hydroxyl oxygens.
The remaining proton becomes bound to an oxygen that is also coordinated with a
phosphate. Thus the composition of hydroxyapatite having a Ca/P ratio of 1.50 may be
expressed by the formula Ca
9
.(PO
4
)
5
.(HPO
4
).OH. The structure of hydroxyapatite is
closely related to that of fluorapatite, in which the OH
−
ion is replaced by F [Ca
5
.(PO
4
)
3
.F].
Table 12.1
Phases in the system CaO-P
2
O
5
-H
2
O.
Phase
Abbreviation
Chemical formula
C/P
Ca(H
2
PO
4
)
2
Monocalcium phosphate
MCP
0.5
CaHPO
4
Dicalcium phosphate
DCP
1.0
CaHPO
4
. 2H
2
O
Dicalcium phosphate dihydrate
DCPD
1.0
Ca
8
H
2
(PO
4
)
6
.5 H
2
O
Octacalcium phosphate
OCP
1.33
Ca
3
(PO
4
)
2
Tricalcium phosphate
TCP
1.5
Ca
5
(PO
4
)
3
OH
Hydroxyapatite
MAP
1.67
Ca
4
(PO
4
)
2
O
Tetracalcium phosphate
TeCP
2.0
In water suspensions all calcium phosphates have the potential to convert to
hydroxyapatite. Such a reaction, however, would be associated with the liberation of free
phosphoric acid, and a decline of pH with all calcium phosphates that are more acid than
hydroxyapatite. It would stop as soon as the singular point with hydroxyapatite was
obtained, where an equilibrium would be established:
(12.2)
On the other hand, in suspensions of tetracalcium phosphate—the only calcium
phosphate that is more basic than hydroxyapatite—the conversion would be associated
with the liberation of free lime, and with an increase of pH:
(12.3)
Thus a complete conversion to hydroxyapatite, without any changes in the pH value of
