Geology Reference
In-Depth Information
Note that the negligible contribution due to dissolved CO
2
is ignored.
The necessary expressions are derived from the definitions of the K
0
1
and K
0
2
given previously.
K
1
HCO
3
H
fg
H
2
CO
3
½
ð
4
:
13
Þ
¼
K
2
¼
K
0
1
K
0
2
CO
2
H
fg
HCO
3
H
f
2
H
2
CO
3
½
3
Substituting into the expression for the summation of all carbonic
species gives
P
CO
2
¼
[H
2
CO
3
]
þ
[HCO
3
]
þ
[CO
3
2
]
¼
[H
2
CO
3
]
þ
K
0
1
{H
1
}
1
[H
2
CO
3
]
þ
K
0
1
K
0
2
{H
1
}
2
[H
2
CO
3
]
¼
[H
2
CO
3
](1
þ
K
0
1
{H
1
}
1
þ
K
0
1
K
0
2
{H
1
}
2
)
Thereafter, the fractional contribution of each species to the total can
be calculated using
½
H
2
CO
3
P
½
CO2
¼
ð
1
þ
K
1
f
H
þ
g
1
þ
K
1
K
2
f
H
þ
g
2
Þ
1
1
:
½
HCO
3
P
CO
2
¼
K
1
H
fg
2
:
½
1
1
þ
K
1
H
fg
1
þ
K
1
K
2
H
f
þ
2
½
CO
2
3
P
½
CO2
¼ð
K
1
K
2
f
H
þ
g
2
Þ
ð
1
þ
K
1
f
H
þ
g
1
þ
K
1
K
2
f
H
þ
g
2
Þ
1
Substituting the values for K
0
1
and K
0
2
and using the previously
calculated pH of 8.21, the fractional contribution of each species is
0.006, 0.907 and 0.087 for H
2
CO
3
, HCO
3
, and CO
3
2
, respectively.
3
:
Consider now the effect of altering the pCO
2
in the water. The
alkalinity should not change in response to variations in CO
2
alone
because the hydration and dissociation reactions give rise to equivalent
amounts of H
1
and anions. CO
2
can be lost by evasion to the atmos-
phere (a process usually confined to equatorial regions) or by photo-
synthesis. This causes the
P
CO
2
to diminish and the pH to rise, an
effect that can be quite dramatic in tidal rock pools in which pH may
then rise to 9. Conversely, an increase in pCO
2
, either by invasion from
the atmosphere or release following respiration, prompts an increase in
