Page 114 - Geochemical Remote Sensing of The Sub-Surface

Spontaneous potentials and electrochemical cells 91

occurring form of chlorine within the Eh-pH regime of shallow terrestrial waters is CI
(Stumm and Morgan, 1970, p. 320). The confusion results from the over statement of the
concentrations of reactants in this equation which is inherent with standard electrode
potentials. In this case, the concentration of OH is overstated by at least three orders of
magnitude, since the upper limit of pH in terrestrial waters is about 11 (Bass Becking et
al., 1960). The pO2 is overstated by a factor of five, since the maximum pO2 in the
groundwater environment is the same as the mole ratio of oxygen in the atmosphere,
which is about 0.2.
Notwithstanding this, provided the user is aware of these problems, Table 3-1 can be
useful in the estimation of the approximate voltages of natural reactions. In addition, it
can be used to show the range of reactions that could occur in nature and that might be
responsible for spontaneous potential currents in the Earth. In the terrestrial
environment, materials such as C12 or Fe(s), which are capable of oxidising or reducing
water to 02 or H2 respectively, rarely occur in large quantities. Therefore, reducing
agents that occur in half-reactions above the H+-H2 reaction in Table 3-I and oxidising
agents that occur below the O2-H20 reactions are not likely to participate in natural
voltaic cells.
In the preceding and following discussions, the effect of kinetics and rate-limiting
factors on redox reactions are largely ignored. Electrode potentials have been treated as
the sole factor that will determine whether one reaction is favoured over another. In
reality, there are many processes that affect rates of reaction, such as diffusion of species
across phase boundaries, high resistance to current flow between electrodes and solution,
high activation energies and slow intermediate reactions. Such processes may result in
one reaction being favoured over another that has a much higher reaction potential, or in
negligible reaction rates for reactions that might otherwise occur according to their
electrode potentials. In the Earth, it is more justifiable to make extrapolations from
thermodynamic data than would be the case in a sterile laboratory setting because of the
ubiquitous occurrence of biological catalysis. In nature, thermodynamic reactions that
can occur typically do occur because organisms have developed systems for
"piggybacking" on these reactions to obtain metabolic energy from them. Examples
abound of biologically-mediated reactions in nature that have reaction rates that are
orders of magnitude faster than they would be in a sterile laboratory setting. As a result,
in a very general sense, kinetic inhibitors to thermodynamic reactions tend to be
minimised in natural environments relative to the laboratory.

SPONTANEOUS POTENTIAL IN EARTH MATERIALS

The existence of spontaneous potentials in the Earth has been known for at least 150
years and their measurement has been used systematically in the search for buried ore
deposits since the 1920s (Parasnis, 1979). Spontaneous or "self' potentials (SP) are
natural voltage differences between two points in the Earth which result in electrical