Geoscience Reference
In-Depth Information
The hydrothermal crystallization of these superionic phosphates has been studied
through another approach, which is quite effective from the point of hydrothermal
solution chemistry.
The crystallization process involves many chemical interactions leading to the
formation of a stable complex. To understand the growth of title compounds, the
authors have described the following possible stages of interactions
[291]
:
1. Acid
base interactions
2. Formation of metal
aqua complexes
3. Interaction between acid
base and metal
aqua complexes.
base interactions. Components such as H
2
O, OH
2
, and PO
3
4
Acid
are consid-
ered as the hard and form pyrophosphate complexes at around 250
C:
2H
3
PO
4
!
H
4
P
2
O
7
1
2H
2
O
ð
7
:
19
Þ
NaOH
1
H
4
P
2
O
7
!
NaH
3
P
2
O
7
1
H
2
O
ð
7
:
20
Þ
As the pH increases, the stability of NaH
3
P
2
O
7
decreases (pH
.
3), and this
leads to the formation of Na
2
H
2
P
2
O
7
:
NaOH
NaH
3
P
2
O
7
!
Na
2
H
2
P
2
O
7
1
H
2
O
ð
7
:
21
Þ
1
The occurrence of
Eq. (7.21)
also increases the pH of the system, but the com-
pound Na
2
H
2
P
2
O
7
is stable in the pH range 3
9. The formation of Na
3
HP
2
O
7
is
favored if pH
9.
.
NaOH
Na
2
H
2
P
2
O
7
!
Na
3
HP
2
O
7
1
H
2
O
ð
7
:
22
Þ
1
aqua complexes in the
hydrothermal system have been studied in detail. The tendency of zinc toward a
tetrahedral coordination is evidently responsible for the existence of [Zn
(H
2
O)
x
(OH)
4
2
x
]
x
2
2
type aqua complexes in aqueous solutions, where the value of
x depends on the pH of the medium
[294]
. In strong alkaline solutions, Zn
Formation of metal
aqua complexes. The zinc metal
2
4
ð
OH
Þ
occurs (and here x
0). By reducing the solution pH to neutral, a dizincate [Zn
2
O
(H
2
O)
x
(OH)
6
2
x
]
x
2
4
may be formed as a result of the association of zincate ion.
The zinc oxide forms aqua complexes when dissolved in alkaline solutions
[295]
. The distribution of zinc aqua ions in alkaline solution is shown in
Figure 7.63
.
Similar aqua complexes can be expected for the other divalent cations, like
Mn
2
1
,Ni
2
1
,Fe
2
1
,Cu
2
1
,Pb
2
1
, and Cd
2
1
, under hydrothermal conditions.
Although the metal cations form octahedral complexes, tetrahedral complexes
are the more stable ones, and a few octahedral complexes retain their constitution
for a long time depending upon their half-life of aqua-ligands
[296]
.
Interactions between acid
base and metal
aqua complexes. Pyrophosphoric
acid forms salts due to the sequestering action of the pyrophosphate radical, the
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