Introduction to Soil Chemistry

voltammetry 105
directly proportional to the partial pressure of the gas to which the electrode
is exposed.
Electrodes are available for most common gases, including oxygen, carbon
dioxide, and ammonia. For oxygen and carbon dioxide their natural concentration
in air can be used in standardization while other gases require standard
gas concentrations. Because of the importance of oxygen in biological
processes, a number of different types of oxygen sensing electrodes have been
developed. Some may be compatible with pH meters; others will not. It is thus
important to make sure that the correct electrode is obtained for the instrument
to be used.
There are three concerns in using gas sensing electrodes in soil: (1) some
electrodes have membranes that have a limited shelf life and must be changed
regularly; (2) the membrane is relatively delicate, and so electrodes must be
placed in soil carefully and cannot be subject to movement; and (3) many
membranes must be kept moist to function properly and thus cannot be used
in dry soils or situations where the soil may dry out during measurement [7].
5.4. VOLTAMMETRY
Voltammetry is the oxidation or reduction of a species at an electrode. In this
case the electrode is the source or the sink for the electrons being exchanged
during the reaction. By measuring current (amperage) and potential (voltage)
in a system, either stirred or unstirred, one can obtain information about not
only the species present but also the amount present. In a typical experiment,
only the oxidized form of a component might be present and the negative
potential might gradually increase until a spike is observed. This is the potential
at which the oxidized species is being reduced. Because each oxidized
species has a different potential where it is reduced, this spike can be used to
identify the species. Numerous analytical techniques are based on this electrochemical
phenomenon. Some common examples are stripping voltammetry,
cyclic voltammetry, and polarography.
The standard potentials of practically all oxidation and reduction reactions,
especially those common in the environment and soil, are known or can easily
be determined. Because of the specificity and relative ease of conducting
voltammetric measurements, they might seem well suited to soil analysis.
There is only one major flaw in the determination of soil constituents by
voltammetric analysis, and that is that in any soil or soil extract there is a vast
array of different oxidation–reduction reactions possible, and separating them
is difficult. Also, it is not possible to begin an investigation with the assumption
or knowledge that all the species of interest will be either oxidized or
reduced.
In a well aerated soil it is expected that all species will be in their highest
oxidation states; however, this does not happen for reasons elucidated in previous
chapters. In a well-aerated soil, both ferrous and ferric iron can exist