Geology Reference
In-Depth Information
hydrous oxides. Control of solution concentration by dissolution of an
oxide does not apply to calcium, as the solid phases are too soluble.
When calcium carbonate is present, which is insoluble, it can control the
Ca
21
concentration in soil solution.
Worked example 5.10 - control of ion concentrations in soil solution by
solubility of a solid phase
(a) Amorphous iron oxide is commonly found in soil and its dissolu-
tion to release Fe
31
ions can be represented by the equation
Fe(OH)
3(amorph)
þ
3H
1
Fe
31
þ
3H
2
O K
¼
10
3.54
"
(5.46)
½
Fe
3
þ
=½
H
þ
3
¼
10
3
:
54
log Fe
3
þ
¼
3
:
54
3pH
At pH6
;
log Fe
3
þ
¼
14
:
46
;
½
Fe
3
þ
¼
3
:
5
10
15
mol L
1
At pH3
;
log Fe
3
þ
¼
5
:
46
;
½
Fe
3
þ
¼
3
:
5
10
6
mol L
1
At pH1
;
log Fe
3
þ
¼
0
:
54
;
½
Fe
3
þ
¼
3
:
5mol L
1
agreeing well with the observed behaviour of Fe
31
, which is soluble
only under very acid conditions. This can be confirmed by using K
sp
¼
10
36
for Fe(OH)
3
in Worked example 5.5.
(b) Calcium carbonate dissolves to release Ca
21
ions:
CaCO
3
þ
2H
1
Ca
21
þ
CO
2
þ
H
2
O K
¼
10
9.74
"
(5.47)
Log Ca
21
þ
2pH
¼
9.74
log CO
2
For a soil atmosphere with pCO
2
¼
10
2.5
(see Worked example 5.5)
Log Ca
2
þ
¼
12
:
24
2pH
At pH7
;
log Ca
2
þ
¼
1
:
76
;
½
Ca
2
þ
¼
0
:
02mol L
1
At pH5
;
log Ca
2
þ
¼
2
:
24
;
½
Ca
2
þ
¼
200mol L
1
