Geology Reference
In-Depth Information
Although originally introduced in terms of ion concentration, today the
definition of pH is based on hydrogen ion activity and is
pH
¼
loga
H
where a
H
refers to the relative hydrogen ion activity (i.e., dimensionless,
as is pH). Defined using concentration scales, the pH can be
pH
¼
log c
H
g
H
=
c
or
pH
¼
log m
H
g
H
m
where c
H
and m
H
represent molar and molal concentrations, c1 and m1 are
the respective standard state conditions (1 mol L
1
and 1 mol kg
1
), and
g
H
is the appropriate activity coefficient. Obviously g
H
differs in these two
expressions because c1
a
m1. However, different activity scales may also
be used. In the infinite dilution activity scale, g
H
)
1 as the concentration
of hydrogen ions and all other ions approach 0. For analyses, pH meters
are calibrated using dilute buffers prepared in pure water. Alternatively, in
the constant ionic medium activity scale, g
H
)
1 as the concentration of
hydrogen ions approaches 0, while all other components are maintained
at some constant level. Calibrant buffers are prepared in solutions of
constant ionic composition, and in marine chemistry, this is often a
solution of synthetic seawater. While these two methodologies are equally
justifiable from a thermodynamic point of view, it is important to
appreciate that pH scales so defined are quite different. As a further
consequence, the absolute values for dissociation constants also differ.
The biogeochemical cycle of inorganic carbon in the ocean is ex-
tremely complicated. It involves the transfer of gaseous carbon dioxide
from the atmosphere into solution. Not only is this a reactive gas that
readily undergoes hydration in the ocean, but also it is fixed as organic
material by marine phytoplankton. Inorganic carbon can be regenerated
either by photochemical oxidation in the photic zone or via respiratory
oxidation of organic material at depth. Surface waters are supersatu-
rated with respect to aragonite and calcite, different solid phases of
CaCO
3
, but precipitation is limited to coastal lagoons such as found in
the Bahamas. However, several marine organisms utilise calcium car-
bonate to form shells. Sinking shells can remove inorganic carbon from
surface waters that is then regenerated following dissolution in the
under-saturated waters found at depth. Nonetheless, calcitic oozes of
biogenic origin constitute a major component in marine sediments.
