Geology Reference
In-Depth Information
How can we tell from these figures that the main ionic species have
been accounted for?
There are 19.68 mmol L
1
of monovalent positive charge and 19.40
mmol L
1
of monovalent negative charge.
What is the ionic strength of this solution?
I
¼
1
=
2
½ð
5
:
05
2
2
Þþð
3
:
55
2
2
Þþð
0
:
68
1
2
Þþð
1
:
80
1
2
Þ
þð
3
:
70
1
2
Þþð
1
:
80
1
2
Þþð
6
:
25
2
2
Þþð
1
:
40
1
2
Þ
¼
1
=
2
68
:
78
¼
34
:
39 in terms of concentrations inmmolL
1
Ionic strength is expressed in terms of mol L
1
I
¼
0.0344.
What is the activity coecient of each ion?
By substituting I
¼
0.0344 into the Davies equation,
g monovalent ions
¼
0.843,
g divalent ions
¼
0.504.
Short-range (
o
0.5 nm) interactions between ions in solution result in
the formation of ion pairs or complexes with other ions or with organic
molecules. An ion pair is a transient entity formed by coulombic
attraction between ions of opposite charge, with each ion retaining its
own water of hydration. In a complex, the ions form covalent or
coordinate bonds to form an entity with its own hydration shell.
An example of an ion pair could be between calcium and sulfate ions
in solution:
Ca
21
þ
SO
2
CaSO
4
"
(5.33)
4
An example of a complex could be between copper ions and an organic
molecule:
COOH
COO
OM
+ Cu
2+
(5.34)
OM
Cu
COOH
COO
