Biomedical Engineering Reference
In-Depth Information
Fig. 3.1
(
a
) Solubility isotherms of calcium phosphate minerals in the system Ca(OH)
2
-H
3
PO
4
-
H
2
Oat37
ı
C (reproduced with permission from [
3
]) (copyright 1994, Elsevier). (
b
) Solubility of
HAP by solid titration method compared with conventional excess-solid method. References of
data in conventional method can be seen in original article [
25
]. (
c
) Solid titration isotherms for
calcium phosphates salts (reproduced with permission from ref. [
25
]) (copyright 2009, American
Chemical Society)
been reached regarding changes in the solubility product. One reason for this may be
that solubility measurements are not performed under standardized conditions since
the solubility of the solid phase is affected by the strain introduced into the structure
by ionic substitution and the CO
3
2-
content in the environmental field (solution
phase). The concentrations of calcium and phosphate ions contained in human
body fluid are naturally in a supersaturated state with respect to HAP. Nonetheless,
indiscriminate precipitation of HAP does not occur in the human body. One reason
for this is that high concentrations of CO
2
dissolve into the body fluids and become
a carbonate buffer, thereby raising the solubility of HAP.
The solubility of fluorapatite (Ca
10
(PO
4
)
6
F
2
), in which the OH
of HAP is
completely replaced by F
, is lower than that of HAP. Within the HAP structure,
there is a column of OH
groups parallel to the
c
-axis. However, since the position
of each OH
is slightly staggered relative to the others, there is strain in the
