Chemistry Reference
In-Depth Information
Metal ions with a single positive charge have only one hydrolysis step, while
the divalent or trivalent cations undergo stepwise hydrolysis. For comparison only,
the first step of hydrolysis, characterized by the pK
a1
is considered for each element.
Each step in the hydrolysis reduces the charge as hydroxycations are formed. For
any step of this reaction, a standard hydrolysis constant can be written:
[
M
M
( )
n
−+
+
]
.
(3.66)
pK
=−
pH
log
a
n
10
[
]
Finally, a neutral hydrated metal hydroxide is formed. Usually, such neutral
species will be insoluble and precipitate. Metal hydroxide usually starts to precipi-
tate when the pH of the solution is approximately equal to the pK
a
of the metal
ion (Table 3.11). If the effective electronegativity of the element is high enough, the
hydrolysis process can continue to produce hydroxyanions and even oxoanions. This
is the case of metal ions in their highest oxidation states, which have extremely high
effective electronegativity. For example, Mn(VII) forms the MnO
4
-
oxoanion in
TABLE 3.11
Acid Hydrolysis Constants
(pK
a
's) for Some Aqua Ions at 298 K and
pH Precipitation Range of Hydroxides or Hydrous Oxides
Precipitation
Range
Precipitation
Range
Metal Ion
pK
a
Metal Ion
pK
a
Tl
+
13.2
Sn
2+
3.9
Hg
2
2+
5.0
Pb
2+
7.8
Mg
2+
11.4
>10
Al
3+
5.1
Ca
2+
12.6
Sc
3+
5.1
Ba
2+
13.2
In
3+
4.4
(pK
a2
= 3.9)
Mn
2+
10.6
8-10
Tl
3+
1.1
(pK
a2
= 1.5)
Fe
2+
9.5
6-8
Bi
3+
1.6
Co
2+
8.9
6-8
V
3+
2.8
Ni
2+
10.6
6-8
Ti
3+
2.2
0-2
Cu
2+
6.8
4-6
Cr
3+
3.8
4-6
Zn
2+
8.8
6-8
Co
3+
0.7
Cd
2+
9.0
Fe
3+
2.2
2-4
(pK
a2
= 3.3)
Hg
2+
3.7
(pK
a2
= 2.6)
Source:
Adapted from F. Basolo and E.G. Pearson.
Mechanisms of Inorganic Reactions: A Study
of Metal Complexes in Solution,
2nd edition (New York: John Wiley, 1967); and H.F.
Walton,
Principles and Methods of Chemical Analysis
(New York: Prentice Hall, 1952).
