Geology Reference
In-Depth Information
Worked example 5.8 - activity of ions pairs in soil solution
In a soil solution the activity of Ca
21
¼
10
3
mol L
1
and SO
4
2
ions
¼
5
10
4
mol L
1
, what is the activity of the ion pair CaSO
4
0
?
CaSO
4
Ca
21
þ
SO
2
K
¼
5.25
10
3
"
(5.35)
4
½
Ca
2
þ
½
SO
2
4
¼
5
:
25
10
3
½
CaSO
4
½
CaSO
4
¼
ð
10
3
Þð
5
10
4
Þ
¼
9
:
5
10
5
molL
1
5
:
25
10
3
Alternatively, this calculation can be done using logarithms.
CaSO
4
"
Ca
21
þ
SO
2
log K
¼
2.28
(5.36)
4
log
½
CaSO
4
¼
log
½
Ca
2
þ
þ
log
½
SO
2
4
ð
2
:
28
Þ
¼ð
3
:
00
Þþð
3
:
30
Þð
2
:
28
Þ¼
4
:
02
½
CaSO
4
¼
9
:
5
10
5
molL
1
5.5.2
Ion Exchange (Physisorption)
Ion exchange is the process where ions held at a charged surface by
coulombic bonding are exchangeable with other ions of the same charge
in solution in contact with the surface. The reaction of major importance
in soils is cation exchange - i.e. positively charged ions (cations) held at
negatively charged surfaces. Anion exchange can occur, but is of minor
importance.
The solid phase surfaces on which cation exchange occurs are:
Silicate clays
permanent negative charge on clay faces
pH-dependent charge on clay edges
Humified organic matter
pH-dependent charge
(always overall negative)
Hydrous oxides
pH-dependent charge
(overall positive or negative)
