Geology Reference
In-Depth Information
pK
1
o
pH
o
pK
2
: log
f
H
2
CO
3
g¼
log
f
H
þ
gþ
log C
log K
1
d log
f
H
2
CO
3
g=
dpH
¼
1
pH4pK
2
: log
f
H
2
CO
3
g¼
2log
f
H
þ
gþ
log C
log K
1
K
2
d log
f
H
2
CO
3
g=
dpH
¼
2
The slopes for the other lines are obtained in the same way. Approx-
imating the charge balance expression (3.62) by {H
1
}
{HCO
3
} gives
the equilibrium pH for a closed aqueous system containing carbonic acid.
B
{H
+
} ~ {HC O
-
}
4
H
2
CO
3
*
6
H
2
CO
3
8
10
12
0
2
4
6
8
10
12
14
pH
Alkalinity. An important definition arising from the considerations of
carbonate equilibria is that of alkalinity, which is derived with reference
to the charge balance expression for carbonic acid.
{H
1
}
¼
{HCO
3
}
þ
2{CO
2
3
}
þ
{OH
} (3.62)
Alkalinity is described as the acid neutralizing capacity of an aqueous
system or equivalently as the amount of base possessed by the system.
Alkalinity
¼
{HCO
3
}
þ
2{CO
2
3
}
þ
{OH
}
{H
1
}
(3.63)
Species other than carbonate can contribute to alkalinity and an
alternative definition is
Alkalinity
¼f
HCO
3
gþ
2
f
CO
2
3
gþf
NH
3
gþf
HS
gþ
2
f
S
2
g
þf
H
3
SiO
4
gþ
2
f
H
2
SiO
2
4
gþf
Org
gþf
HPO
2
g
4
þ
2
f
PO
3
4
gþf
OH
gf
H
þ
gf
H
3
PO
4
g
ð
3
:
64
Þ
