Geoscience Reference
In-Depth Information
7.5 Electrolyte chemistry
Most inorganic species in solutions are ionized and develop a local electrostatic poten-
tial, which creates new forms of energy storage. These phenomena add a certain level of
complexity to the theoretical understanding of solution chemistry and force us to use new
parameters known as “activities” instead of concentrations. It also accounts for the remark-
able behavior of particles in natural waters such as gels and flocculation. Let us start by
recalling some elementary concepts of electricity. Imagine that we wire each of two metal
plates to the positive or negative outlets of a battery. This action drives electric charges
of opposite signs at the surface of each plate. We hold the two plates apart by two springs
attached to the wall and measure the forces which tend to bring the two plates together. The
field between the two plates is described by the standard laws of electrostatics (Coulomb
attraction). Let us now introduce a sheet of mica in between the two plates. The effect is to
reduce the force that pulls the plates together. Mica is a dielectric medium: the electrostatic
field created by the plates induces ion pairs in the mica (dipoles) to orient themselves so as
to create a secondary field opposing the primary field of the plates. The ratio of the force
in vacuum to the force with the mica sheet present is known as the dielectric constant
.
Pure water is a strong dielectric. The H
2
O molecules are polar: because of the angle
maintained between the two OH bonds, the hydrogen atoms pull the covalent electron
they each share with the oxygen. The water molecules therefore show a positively charged
apex and orient themselves against the ambient field, e.g. the field created by surfaces. All
of these dipoles reduce the distance at which charges are “felt” in the solution: pure water
therefore screens the mutual attraction or repulsion of neighboring ions. Because of thermal
agitation, the dielectric constant of water decreases when temperature increases. Ions in
solution also interfere with the dielectric properties of the solvent: several H
2
O molecules
attach themselves to each ion to form what is known as a solvation shell. These water
molecules are no longer available to counteract the ambient electrostatic field.
Figure 7.4
shows the example of the hydrated Cr
6
+
ion with its shell of six water molecules. Adding
salt to a solution therefore decreases its dielectric constant
ε
ε
. The mean distance over which
Cr(H
2
O)
6+
Figure 7.4
The hydrated octahedron Cr(H
2
O)
6
6
. The strong field created by the Cr
6
+
cation attracts water
molecules which are no longer available to contribute to the dielectric effect of water.
