Introduction to Soil Chemistry

104 electrical measurements
In addition to the simple Eh–pH graph shown in Figure 5.3, threedimensional
Eh–pH graphs can be produced. Known quantities of a pollutant
can be added to a number of different soil suspensions and its degradation at
different combinations of Eh and pH measured. In this way the optimum conditions
for the decomposition of the pollutant in question can be determined.
An excellent example of this is the decomposition of pentachlorophenol and
hexahydro-1,3,5-trinitro-1,3,5-triazine in soil and water under various Eh–pH
conditions as illustrated in the papers by Petrie et al. and Singh et al. [9,10].
In soil analyses knowledge of Eh–pH data can be used in three ways. It will
provide information as to the form or species of pollutant present (see also
Chapter 10). It can also be used to determine which extraction procedure is
best suited for extraction of a component from a soil sample. Potential changes
in species, movement in the environment, and conditions suitable for bioremediation
or natural attenuation can also be derived from this type of
measurement.
In this discussion so far all the systems are well defined, at equilibrium and
at a constant 25°C. None of these conditions occur in soil in the environment.
Soil is not a pure system and all the components affecting redox reactions are
seldom known, defined, or understood, and a host of different redox couples,
many unknown, are likely to be present. Unless it is possible to take into
account all couples present, it is not possible to describe the exact redox conditions
in a soil without measuring it.
Even though very small soil samples may be well defined, large samples and
field size areas are not. Soil is never at equilibrium even though it may appear
to be so over short periods of time. Reactions occur, and microorganisms continue
to function in soil samples after they are taken. In tropical conditions
soil temperatures will vary significantly even where there is little change in air
temperature. Heating by sunlight and cooling by radiation will always occur.
Variable shading by trees and other vegetation will add to soil temperature
variability, and both rain and its subsequent evaporation will have a cooling
effect on soil.
In spite of the limitations, Eh–pH data will provide information about the
condition of a soil in terms of it being in an oxidizing or reducing condition.
Thus it will indicate the prominent redox conditions of species present. It will
also indicate what changes in Eh or pH may be desirable to effect the desired
extraction or analysis for the compound or species of greatest concern [9–11].
5.3.4. Gas Electrode
The gas electrode is similar to the ISE electrodes and usually works on the
same basic principles. Here the electrode looks much like a standard ISE electrode
and is, except that it has a gas-permeable, water-impermeable membrane
in the tip. Gas present in the environment passes through the membrane and
reacts with reagents in the interior, producing a chemical change that is
directly related to the development of a potential. This potential is thus