Biomedical Engineering Reference
In-Depth Information
Additional analyzes of ACPs prepared from aqueous solutions at
pH ~7.4 at wide variations (from 5 : 1 to 1 : 5) in starting Ca/PO
4
molar ratios showed that the compositional Ca/P ratio decreased
only slightly from ~1.5 due to the presence of small amounts (<15%)
of HPO
4 2−
ions [137, 206-208]. This is an indirect confirmation
of the existence of an amorphous TCP. The presence of HPO
4 2−
in
ACP is easy to detect by infrared spectroscopy most notably by
an absorption peak at about 890 cm
, due to a P-O(H) stretching
mode of protonated orthophosphate species (Fig. 2.7) [133, 140].
Furthermore, in infrared spectra of HPO
−1
4 2−
-contained ACPs there is
a large band of a weak intensity near 2300 cm
−1
, which corresponds
4 2−
to H-O(P) stretching in HPO
ions [140]. More to the point, the
amount of HPO
in ACP strongly depends on the solution pH: the
lower the Ca/P ratio for any given ACP precipitate, the greater is its
HPO
4 2−
4 2−
content (Fig. 2.8).
Figure 2.7
Infrared spectra after baseline subtraction for ACPs prepared
at: A - pH 10.0, B - pH 7.0, C - pH 6.5, and D - pH 6.0. Reprinted
from Ref. [133] with permission. The complete (from 400 cm −1
to 4000 cm −1 ) infrared spectrum of ACP is available in Ref.
[31].
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