Biomedical Engineering Reference
In-Depth Information
adsorbed protons seems reasonable and, perhaps, this effect will be
better elucidated in future.
7.3
Summary on the Dissolution Models
There is an old tale on six blind men, who wanted to determine what
an elephant looked like. An elephant was delivered and the men were
allowed to feel different parts of its body. The blind man who felt a leg
said the elephant was like a pillar; the one who felt the tail said the
elephant was like a rope; the one who felt the trunk said the elephant
was like a tree branch; the one who felt an ear said the elephant was
like a hand fan; the one who felt the belly said the elephant was like a
wall; and the one who felt the tusk said the elephant was like a solid
pipe. Thus, six blind men appeared to be in complete disagreement.
Then, a wise man explained to them: “All of you are right. The reason
every one of you is telling it differently is because each one of you
touched the different part of the elephant. So, actually the elephant
has all the features you mentioned.” The story is widely used to
indicate that the reality may be viewed differently depending upon
one's perspective, suggesting that what seems the absolute truth
may be relative due to a deceptive nature of half-truths.
As the complete physical picture that explains widely observed
variations in dissolution behavior is still lacking, a similar approach
must be adapted to the aforementioned dissolution models of calcium
apatites. Undoubtedly, each from the aforementioned models appears
to have limitations and drawbacks of its own and none of them is
able to describe the apatite dissolution in general. Moreover, most of
the models were elaborated for apatite dissolution in either slightly
acidic or nearly neutral (4 < pH < 8) aqueous solutions, relatively
small values of solution undersaturation and temperatures of 25
and 37°C only. Nothing is known about the validity of these models
for apatite dissolution in strong inorganic acids (HCl, HNO
, H
SO
,
3
2
4
H
) for solution pH < 2 and temperatures above 70°C. Moreover,
in such “tough” experimental conditions, the numerical value of
solution undersaturation by HA and FA becomes uncertain due to
impossibility to determine their solubility [1-4]. The latter cases
serious problems for application of current versions of calcium-
rich layer formation and polynuclear models to apatite dissolution
in strong acids, because both models operate with the solution
supersaturation/undersaturation phenomena. The abovementioned
PO
3
4
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