Geoscience Reference
In-Depth Information
protons. In this way, carbonic acid H
2
CO
3
dissociates into a bicarbonate ion HCO
3
and
then into a carbonate ion CO
2
3
by losing first one and then two protons. The extent of
dissociation is given by the mass action law:
HCO
3
H
+
[H
2
CO
3
]
=
K
1
(7.1)
CO
2
3
H
+
HCO
3
=
K
2
(7.2)
Both constants of the first (
K
1
) and second (
K
2
) dissociation vary with temperature and,
to a lesser extent, with salinity. The notation p
K
=−
log
K
is commonly used. A special
case of dissociation is that of water by the reaction:
H
+
+
OH
−
H
2
O
⇔
(7.3)
for which the dissociation constant is:
H
+
OH
−
[H
2
O]
=
K
H
2
O
(7.4)
In the case where the solution is dilute, the [H
2
O] concentration of water in the solution
is essentially constant and this relation can be re-written in its more familiar form:
H
+
OH
−
=
10
−
14
K
w
=
(7.5)
3. Ion complexation is the association of ions carrying charges of opposite sign. In the
ocean, for instance, the copper ion Cu
2
+
may be surrounded by different anions OH
−
,
Cl
−
,HCO
3
, which form different species of copper, but also by humic acids from the
soil. In this sense, H
2
CO
3
and HCO
3
may be viewed as carbonate complexes of the
proton. Complexation also obeys the mass action law with its successive constants.
4. Redox reactions relate to electron exchange: a reductant gives up electrons to an oxidant.
Oxidation of Fe
2
+
into Fe
3
+
is a common result of electron acceptance by oxygen
atoms:
Fe
3
+
+
e
−
⇔
Fe
2
+
(7.6)
Here Fe
3
+
is the electron acceptor and Fe
2
+
the donor. Although neither free elec-
trons nor free protons exist in solutions, we can still assign to them a share of the
energy system. The
G
of redox reactions is not really in use and it is customary to
refer to an electron activity, actually to the negative decimal logarithm of this activ-
ity as pe. Alternatively, the electron activity is measured by an electrode potential
E
H
in volts, measured relative to a reference electrode with pe and
E
H
related by the
relationship
E
H
=
0.059 pe at 25
◦
C. Oxidation of organic carbon (coal, petroleum,
bitumen) by atmospheric oxygen is among the most commonly employed of artificial