Geology Reference
In-Depth Information
freshwaters, which are frequently of low ionic strength, ion charge is a
key parameter. Greater coulombic interactions occur between oppo-
sitely charged ions with high charge.
Inner sphere complexation involves interactions between metal ions
and other species in solution which possess lone pairs of electrons. Inner
sphere complexation involves the transfer of at least one lone pair of
electrons. Those species which possess electron lone pairs are termed
ligands and reactions may involve inorganic or organic ligands.
3.2.3.2 Hydrolysis. In aqueous systems, hydrolysis reactions are an
important example of inner sphere complexation for many metal ions.
The interaction between hydrated metal ions and water can be written as
a series of equilibria for which the equilibrium constants are denoted
*K
n
.
y
Al
3
þ
Al(OH)
2
þ
(aq)
þ
H
(aq)
*K
1
¼
10
4.95
(aq)
þ
H
2
O
(1)
"
(3.28)
Al(OH)
2
þ
Al(OH)
2(aq)
þ
H
(aq)
*K
2
¼
10
5.6
(aq)
þ
H
2
O
(1)
"
(3.29)
Al(OH)
2(aq)
þ
H
2
O
(1)
Al(OH)
3(aq)
þ
H
(aq)
*K
3
¼
10
6.7
"
(3.30)
Al(OH)
4(aq)
þ
H
(aq)
*K
4
¼
10
5.6
Al(OH)
3(aq)
þ
H
2
O
(1)
"
(3.31)
Hydrolysis and, more generally, complex formation equilibria may be
described by cumulative stability constants, b
n
. Using the Equations
(3.28)-(3.31)
Al
3
þ
Al(OH)
2
þ
(aq)
þ
H
(aq)
*b
1
¼
*K
1
¼
10
4.95
(aq)
þ
H
2
O
(1)
"
(3.32)
y
The hydrolysis of hydrated metal ions can also be written as the interaction between the hydrated
metal ion and the hydroxyl ligand. For example
Al
31
(aq)
þ
OH
(aq)
" Al(OH)
21
(aq)
K
1
where K
1
and *K
1
are related as follows
Al
31
(aq)
þ
OH
(aq)
"
Al(OH)
21
(aq)
K
1
H
1
(aq)
þ
OH
(aq)
K
w
H
2
O
(l)
"
so
Al(OH)
21
(aq)
þ
H
1
(aq)
*K
¼
K
1
K
w
The values for *K
n
are those obtained by Wesolowski and Palmer;
32
all values are given in unitless
form but fundamentally are defined in terms of the products of the activities of ions on the molar or
molal scales.
Al
31
(aq)
þ
H
2
O
(l)
"
