Geology Reference
In-Depth Information
Finally, the inorganic carbon equilibrium is responsible for buffering
seawater at a pH near 8 on timescales of centuries to millennia.
There are several equilibria to be considered (see also Chapter 3).
Firstly, CO
2
is exchanged across the air-sea interface
CO
2g
Ð
CO
2aq
The equilibrium process obeys Henry's Law, but the dissolved CO
2
reacts rapidly with water to become hydrated as
CO
2aq
þ
H
2
O
Ð
H
2
CO
3
Relative to the exchange process, the hydration reaction forming carbonic
acid occurs quite quickly. This means that the concentration of dissolved
CO
2
is extremely low. The two processes can be considered together as
CO
2g
þ
H
2
O
Ð
H
2
CO
3
The equilibrium constant is then
H
2
CO
f g
p
CO
2
H
2
O
K
CO
2
¼
ð
4
:
5
Þ
f
g
where p
CO
2
is the partial pressure of CO
2
in the marine troposphere.
Carbonic acid undergoes dissociation
H
2
CO
3
þ
H
2
O
Ð
H
3
O
þ
þ
HCO
3
HCO
3
þ
H
2
O
Ð
H
3
O
þ
þ
HCO
2
3
for which the first and second dissociation constants (using {H
1
} rather
than {H
3
O
1
}) are
K
1
¼
H
fg
HCO
3
ð
4
:
6
Þ
f
H
2
CO
3
g
K
2
¼
H
fg
CO
2
3
ð
4
:
7
Þ
HCO
3
The hydrogen ion activity can be established with a pH meter.
However, as discussed above, this measurement must be operationally
defined. On the other hand, the individual ion activities of bicarbonate
and carbonate ions cannot be measured. Instead, ion concentrations are
determined, as outlined below, by titration. Accordingly, the equilib-
rium constants are redefined in terms of concentrations. These are then
known as apparent, rather than true, equilibrium constants and
