Agriculture Reference
In-Depth Information
Table 3.5
Classification of metal ions
A-type metal cations
Transition-metal cations
B-type metal cations
Electron configuration of
inert gas, low
polarizability, 'hard
spheres'
One to nine outer shell
electrons, not
spherically
symmetric
Electron number corresponds
to Ni
0
,
Pd
0
and Pt
0
(10 or 12
outer shell electrons), low
electronegativity, high
polarizability, 'soft spheres'
(
H
+
)
,Li
+
,Na
+
,K
+
,Be
2
+
,
Mg
2
+
,Ca
2
+
,Sr
2
+
,Al
3
+
,
Sc
3
+
,La
3
+
,Si
4
+
,Ti
4
+
,
Zr
4
+
,Th
4
+
V
2
+
,Cr
2
+
,Mn
2
+
,
Fe
2
+
,Co
2
+
,Ni
2
+
,
Cu
2
+
,Ti
3
+
,V
3
+
,
Cr
3
+
,Mn
3
+
,Fe
3
+
,
Co
3
+
Cu
+
,Ag
+
,Au
+
,Tl
+
,Ga
+
,
Zn
2
+
,Cd
2
+
,Hg
2
+
,Pb
2
+
,
Sn
2
+
,Tl
3
+
,Au
3
+
,In
3
+
,Bi
3
+
Ligands
Ligands
F
>
O
>
N
=
Cl
>
Br
>
S
>
I
>
Br
>
Cl
=
N
>
O
>
F
I
>
S
OH
−
>
RO
−
>
RCOO
−
CO
3
2
−
NO
3
−
PO
4
3
−
SO
4
2
−
ClO
4
−
Source
: Stumm and Morgan (1996). Reproduced by permission of Wiley, New York.
involved. However general computer programs are available to perform such cal-
culations using successive approximation (Melchior and Bassett, 1990; Mangold
and Tsang, 1991; Sposito, 1994). WHAM (Tipping, 1994, 2002) gives particular
attention to reactions involving humic substances.
An important component of equilibrium calculations is the conversion between
ion activities, which equilibrium constants refer to, and ion concentrations, which
mass balance and electrical neutrality equations refer to. The conversion is made
with
activity coefficients
defined by the relation:
a
i
=
γ
i
C
i
(
3
.
3
)
Various empirical relations are available for calculating individual ion activity
coefficients [discussed by Stumm and Morgan (1996) for natural waters and
Sposito (1984a, b), for soil solutions]. In the calculations in this topic I used
the Davies equation:
=−
AZ
2
√
I
0
.
3
I
log
γ
1
+
√
I
−
(
3
.
4
)
2
C
i
Z
i
2
)
,
Z
is ionic charge and
A
=
1
.
82
×
10
6
(εT )
−
1
.
5
,where
ε
is the dielectric constant (
A
≈
0
.
5forwaterat25
◦
C).
This relation is valid for
I<
0
.
5m.
1
where
I
is ionic strength
(
=
