Geoscience Reference
In-Depth Information
7
The chemistry of natural waters
The external aspects of geochemical cycles, the phenomena that occur at relatively low
temperatures (typically from 0 to
30
◦
C) in the ocean, the atmosphere, and in rivers, are
largely governed by chemical equilibria in solution or at the water-mineral interface. The
cycles themselves imply transfer controlled primarily by water-rock interaction (erosion,
sedimentation, hydrothermal reactions) and by biological activity. A central role is played
by the carbonate system. We will apply these concepts to the geochemistry of erosion and
of the ocean, with a discussion of the impact of these cycles on climates in particular.
+
7.1 Basic concepts
A few important concepts that are part of college chemistry are required.
1. Acidity is the concentration [H
+
](molkg
−
1
) of protons in a solution. The exact form,
H
+
or H
3
O
+
, in which these protons occur is of little significance. A scale of acidity is
defined by the potential pH of the protons in the solution, such that pH =
log [H
+
]. At
25
◦
C, pure water has a pH of 7. A lower pH indicates an acidic solution and a higher
pH a basic solution.
2. Ion behavior is dictated by the dissociation of acids and bases. In an acid-base reaction,
the acid is the proton donor and the base is the acceptor. A strong acid such as HCl or
a strong base such as NaOH become completely dissociated to produce Cl
−
and Na
+
ions, which behave essentially like inert species and are of relevance only in terms of
charge. Weaker acids become partly dissociated by releasing one, two, or possibly more
−
