Introduction to Soil Chemistry

potentiometry (electrodes in soil measurements) 97
trode potentials for the oxidation of oxygen and the reduction of hydrogen at
common soil pH levels.
Although the reduction of hydrogen and the oxidation of oxygen determine
the lower and upper limits of electrochemical redox measurements in soil, it
is still possible to have both hydrogen and oxygen produced by biological
processes. Photosynthetic production of oxygen by algae and phototrophic
bacteria occurs in all soils. Likewise, the biological production of hydrogen gas
occurs under anaerobic conditions. Thus it is possible to find oxygen and/or
hydrogen gas being produced in soil.
In soil analysis, pH, selective ion, oxidation–reduction (redox), electrical
conductivity cells, and oxygen electrodes are commonly used. For each of these
measurements a different specific electrode along with a separate or integral
reference electrode will be needed. In some cases, with extended use or long
exposure to soil or soil–water suspensions, electrodes may become polarized.
When this happens, erroneous results will be obtained and depolarization will
need to be carried out using the electrode manufacturers’ directions [3].
5.3.1. pH
In soil one can conceive of the presence of three “types” of protons (see
Chapter 6, p. 116). However, those in solution, associated with water molecules
forming hydronium ions, are the only measurable protons. Other protons will
be associated with cation exchange sites, are exchangeable, and contribute to
soil buffering. They cannot be measured directly but can be exchanged with
cations from salts or buffer solutions, and once in solution they can be measured.
The third “type” of proton is bonded to either inorganic or organic soil
components and normally will be regarded as being covalently bonded.
However, they may be part of a functional group from which they may be
easily removed and thus become part of the protons in solution. Both organic
acid and phenolic groups are examples of compounds, which have protons,
that fall into this category.
The absolute pH of soil cannot be known absolutely; however, standardized
methods for measuring soil pH have been developed.
A combination pH electrode, as illustrated in Figure 5.4 and shown in
Figure 5.5 (A), is most commonly used in determining soil pH. However, two
separate electrodes, one pH sensing (i.e., the H + glass bulb in Figure 5.5) and
the other a reference electrode, may be used and may be best in cases where
fouling of the reference electrode is a particular problem. Care must always
be taken to avoid scratching or breaking the pH sensing bulb when making a
pH measurement, because while some pH electrodes have robust pH sensing
electrodes, others are quite delicate. Figure 5.6 shows a pH electrode in a soil
suspension, which is connected through a card, which is a pH meter, to a laptop
computer that provides data output.
Electrodes may be attached to a pH meter, which can be analog, digital, or,
as mentioned above, to a computer. Several different connectors are used