Geoscience Reference
In-Depth Information
energy-producing processes, but it also occurs naturally when sediments with a high
detrital carbon content are eroded:
C
+
O
2
⇔
CO
2
(7.7)
sediment
air
atmosphere
5. Atmospheric gases are scarcely soluble in water, except for CO
2
. The solubility of gases
is described by Henry's law, which establishes a simple proportionality between the
partial pressure
P
i
of the gas
i
above the solution and its concentration in the solution.
For CO
2
, we write:
[H
2
CO
3
]
=
k
CO
2
P
CO
2
(7.8)
where
k
CO
2
is the coefficient of solubility. The concentration of the H
2
CO
3
species in
natural waters is very low and the species actually present is dissolved CO
2
,butforall
practical purposes, the present notation is sufficient.
6. The solubility of solids precipitating from solutions is expressed using another coeffi-
cient of the mass action law, the product of solubility
K
s
. For carbonate precipitation
Ca
2
+
+
CO
2
3
⇔
CaCO
3
, the saturation condition is written:
Ca
2
+
CO
2
3
=
K
s
(7.9)
7. The condition of electrical neutrality: this condition is normally written by calculating
the charge balance of fully dissociated species
Alk
, which is known as alkalinity (not to
be confused with basicity, which characterizes a solution with pH
>
7):
2
Ca
2
+
2
Mg
2
+
=
Na
+
+
K
+
+
Alk
+
+···
2
SO
2
4
−
Cl
−
−
NO
3
−
−···
(7.10)
But since the neutrality of the solution must be maintained, we can also write:
2
CO
2
3
≈
OH
−
−
H
+
+
HCO
3
+
Alk
(7.11)
in which the minor borates and phosphates have been omitted. Alkalinity being a con-
centration of electrical charges, it is expressed in equivalent per kilogram (eq kg
−
1
).
This is a measure of the neutralizing power of a solution: by adding HCl to a highly
alkaline solution, the solution bubbles intensely as the carbonate ions are driven off and
replaced by Cl
−
ions. Notice that while the pH of a solution can vary with temperature
or CO
2
pressure, alkalinity does not change and for this reason it is said to be conserva-
tive. Alkalinity is conservative like [Na
+
]or[Cl
−
] but unlike [HCO
3
]or[OH
−
]. The
sum [OH
−
]-[H
+
], known as caustic alkalinity, is normally negligible compared with
carbonate alkalinity [HCO
3
]
2[CO
2
3
].
8. All the thermodynamic constants depend on temperature and, to a lesser extent, on
pressure. Those analogous to an equilibrium coefficient may be expressed by a similar
+
