Biomedical Engineering Reference
In-Depth Information
congruently or incongruently might also depend on both their initial
chemical composition and the solution properties.
To conclude, one should notice that all the aforementioned cases
of incongruent dissolution of both FA and HA describe the formation
of a surface layer of a CDHA and/or acidic calcium orthophosphates,
which is calcium-depleted when compared to the bulk apatite. Thus,
there is a disagreement between the incongruent dissolution and
calcium-rich layer formation model. Perhaps, this might be due to
the differences in experimental conditions.
7.2.5
Chemical Model
A chemical dissolution model was proposed as well [91-93]. This
model was developed from a self-evident supposition that it would
be highly unlikely if apatite were dissolved by detachment of “single
molecules” equal to the unit cells and consisting of 18 ions. Moreover,
in the crystal lattice all ions are shared with neighboring unit cells
and often cannot be attributed to the given “single molecule”.
Therefore, based on the experimental results obtained on the one
hand and an analysis of the data found in references on the other
hand, a sequence of four successive chemical reactions:
+
+
Ca
(PO
)
(F, OH) + H
O + H
= Ca
(PO
)
(H
O)
+ HF, H
O (7.1)
5
4
3
2
5
4
3
2
2
+
2+
2Ca
(PO
)
(H
O)
= 3Ca
(PO
)
+ Ca
+ 2H
O
(7.2)
5
4
3
2
3
4
2
2
+
2+
Ca
(PO
)
+ 2H
= Ca
+ 2CaHPO
(7.3)
3
4
2
4
+
2+
4
−
CaHPO
(7.4)
was proposed to describe the chemical processes of apatite
dissolution [91-93]. According to this model, Eqs. (7.1)-(7.4) should
be used instead of the well-known net-reactions (7.5) and (7.6):
+ H
= Ca
+ H
PO
4
2
+
2+
4
−
Ca
(PO
)
(F, OH) + 7H
= 5Ca
+ 3H
PO
+ HF, H
O
(7.5)
5
4
3
2
2
→
2+
4
3−
−
−
(7.6)
Chemical model (7.1)-(7.4) was initially created for description
of apatite dissolution in strong acidic media [92] and appeared to be
a further development of some ideas proposed earlier. For example,
already in 1960s reactions (7.5) and (7.6) had been suggested “not
to give the mechanism but merely express net-reactions only” [94,
p. 333]. Later, Pearce, [95] and Chow [31] discussed which of net-
reactions (7.5) or (7.6) described the dissolution process better.
Ca
(PO
)
(F, OH)
5Ca
+ 3PO
+ F
, OH
5
4
3
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