Geoscience Reference
In-Depth Information
H 2 O. Skinner (1974) [226] found that the field of stability of HAp alone with water in
the triangle CaO
H 2 O was confined to a narrow band joining the representative
point of stoichiometric HAp to the water representative point of the phase diagram.
There are several other phase equilibria studies on the apatite system and the reader
can refer to the works [204,227] . Andrade et al. (1997) [228] carried out a detailed ther-
modynamic analysis of the system Ca
P 2 O 5
P
H 2 O based on the development of the
Eh
log 10 a Ca ) diagrams at different temperatures
and different activities of phosphorous and calcium in aqueous solution. The HAp pre-
cipitated at ambient
pH and Pa C
pH (where Pa Ca 52
temperature (30 C) was hydrothermally treated at 150 C.
H 2 O system at 25 C
and 300 C, for 1.67 mol activity of the Ca and 1 mol activity of pressure in the
aqueous solution. Figures 5.61 and 5.62 present
Figures 5.59
5.63 show the Eh
pH diagrams of the Ca
P
the Pa Ca
pH diagrams (where
Pa Ca 52
1.67 aP and show that the pH value
for the point of minimum solubility of the HAp clearly decreases with an increase in
the temperature of the system. At each temperature level, HAp predominates in the
higher pH range, while Ca 3 (PO 4 ) 2 ,Ca 2 P 2 O 7 ,andCaH 6 P 2 O 9 have their predominate
log 10 a Ca ), for the same system with a Ca 5
Figure 5.59 Eh
pH diagram of
the Ca P H 2 O system at 25 C
for 1.67 mol activity of Ca and
a Ca 5 1.67a P [228] .
2.0
1.5
CaO 2
1.0
0.5
Ca 2+
Ca 2 P 2 O 7
Ca 5 (PO 4 ) 3 OH
0.0
-0.5
-1.0
-1.5
CaH 2
Ca (OH) 3 (S)
-2.0 202468 0 2 4 6
pH
Figure 5.60 Eh pH diagram of
the Ca P H 2 O system at 300 C
for 1.67 mol activity of Ca and
a Ca 5 1.67a P [228] .
2.0
1.5
1.0
CaO 2
C a 2+
0.5
0.0
Ca 5 (PO 4 ) 3 OH
-0.5
-1.0
-1.5
-2.0 202468 0 2 4 6
pH
CaH 2
Search WWH ::




Custom Search