Geoscience Reference
In-Depth Information
Figure 7.63 Distribution of zinc aqua ions in
alkaline solution
[295]
.
100
Zn (OH)
4
80
60
40
20
Zn (OH)
3
0
0
123
[
C
OH
(mol)]
45
sequestering ability decreases with higher concentrations of salts, or with increasing
pH. Interactions between metal
aqua complexes with the sodium salts of pyropho-
sphoric acid can be written as follows:
NaH
3
P
2
O
7
1
M
ð
OH
Þ
2
!
HNaMP
2
O
7
1
2H
2
O
ð
7
:
23
Þ
ð
pH
2
3
Þ
5
Na
2
H
2
P
2
O
7
1
M
ð
OH
Þ
2
!
HNaMP
2
O
7
1
NaOH
H
2
O
1
ð
7
:
24
Þ
ð
pH
3
9
Þ
5
Na
3
HP
2
O
7
1
M
ð
OH
Þ
2
!
HNaMP
2
O
7
1
2NaOH
ð
7
:
25
Þ
ð
pH
9
Þ
.
where M
Mn, Fe, Co, Ni, Cu, Zn, Pb.
The aqua complexes can form more stable complexes because of the higher des-
olvation energy of metal cations when compared to sodium (3.97 eV). Hence, the
formation of HNaMP
2
O
7
is expected in the system instead of stable sodium salts of
pyrophsophoric acid, namely Na
2
H
2
P
2
O
7
and Na
4
P
2
O
7
.
The hydrothermal growth of HNaMP
2
O
7
(where M
5
Mn, Co, Ni, Zn, Cd, Pb)
crystals clearly shows that there is a gradual change in the crystallinity and mor-
phology from Mn end to the Pb end.
Table 7.25
gives the cations in superionic
pyrophosphates. As evident from
Table 7.25
, higher the entropy values, the lower
will be the degree of crystallinity and the crystal morphology. For example,
5
