Geoscience Reference
In-Depth Information
In closed system, at the end of the reaction, all the available CO
2
has
been consumed. Consequently, the pH is no longer buffered and rises
to nearly 10. This is very important. In the microsites isolated from the
atmosphere (closed soil pores), quartz is liable to be dissolved and the
silica may migrate (see Chap. 3, Fig. 3.8).
+
ions
The
pK
of the dissociation
H
2
CO
3
¤
HCO
3
-
+
H
+
is 6.5 (
K
= 10
-6.5
).
The H
+
-ions are then present in limited quantity and the pH of pure
water in equilibrium with atmospheric CO
2
is 5.65. On the other hand,
laboratory experiments show that introduction of soil organic acids
at concentrations close to those observed in the natural environment
strongly raises the acidity because their
pK
values are below 4. Therefore,
the experimental dissolution of calcite is doubled or tripled in presence of
organic acids (Robert
et al
. 1980). This shows that the protons furnished
by humus and plants participate to a large extent in the solubilization
of calcium carbonate in soils.
A simple way to determine whether the acidity is due to CO
2
or
caused by organic acids is to measure the pH of the soil water before
and after degassing. If it rises by one unit or more, CO
2
was the cause.
If it remains unchanged, organic acids are probably present (Bourrié
1976).
Furthermore, there is the following very important reaction occurring
in soils:
Origin of H
NH
4
+
+ 2
O
2
¤
NO
3
-
+ 2
H
+
+
H
2
O
In other words, nitrification provides H
+
ions that are added to
those produced otherwise and facilitate the decomposition of calcium
carbonate. This is why a high correlation is found between the content
of Ca and that of nitrate in the drainage water (Durand 1979).
The solubility of calcite depends on other cations present in the medium,
particularly Mg and Na. But the phenomena are complex and scientists
are not always in agreement. A given cation may seem to be able to
increase or, on the contrary, to diminish the dissolution of calcite as
function of the pH of the reaction medium. In presence of Mg, the
dissolution pits affecting crystals as seen with the electron microscope
are more numerous (Ruiz-Agudo
et al.
2009).
Role of foreign cations
