Introduction to Soil Chemistry

potentiometry (electrodes in soil measurements) 101
the reference electrode and meter must be specially designed for use with this
type of electrode. Both ion-selective and the reference electrodes used with
them are shown in Figure 5.5, letters C and B, respectively. Table 5.1 gives
common ion-selective electrodes useful in soil analysis. This is not an exhaustive,
list and new selective electrodes are being developed on a daily basis.
Ion-selective electrodes are standardized using standard solutions of the ion
dissolved in water or a solution designed to keep all samples at about the same
ionic strength. Standardizing solutions can be purchased or prepared in the
laboratory and typically, as seen in Table 5.1, cover several orders of magnitude
often between 1 and 10 -6 or 10 -7 molar. Measurements are made at the
various concentrations and a standard or calibration curve prepared (see
Chapter 8, Sections 8.8.2, 8.9, and 8.10). Usually the meter can be programed
to read the concentration of the ion directly once a suitable curve is obtained.
Raw data can also be entered into a spreadsheet, which can be programmed
to calculate the amounts of ion present in any units desired.
As with pH measurements, a specific amount of water or ionic strength
adjusting solution is added to soil mixed and allowed to stand. The ionselective
and reference electrodes are then inserted in either the suspension
or solution, filtered from the soil before measuring. When a stable reading is
obtained, it is recorded.
Ion-selective electrodes are subject to interference from ions other than the
one they are designed to measure. The Na + ion-selective electrode is susceptible
to interference from other single positive species (i.e., K + ,NH 4 + ), and the
same situation will hold for ion-selective electrodes designed to measure negatively
charged species (see Table 5.1). Generally the electrode will be less sensitive
to these interfering ions than to the ion it is designed to measure so that
low levels of interfering ions may not make a significant difference in the measurements
being made. In other cases interfering ions can be precipitated or
complexed to remove them from solution before measurement is made.
Interfering ions are problematic when analyzing environmental samples,
particularly soil and soil extracts. Samples of these materials may contain
unknown combinations and concentrations of ions. For these reasons ionselective
electrodes are most useful in two situations. The first is where the soil
composition with regard to ions is well known and routine repetitive analysis
is to be made. The second is where a preliminary screening is to be done and
followed up by detailed laboratory analysis. In this latter case potential interfering
ions will be determined and the validity of the original screening
accessed [7,8].
5.3.3. Redox
Redox is an abbreviation for reduction and oxidation. It is based on the fact
that one component cannot be oxidized without another being reduced.
The most common oxidizing, electron accepting, agent is oxygen. In soil other
electron accepting agents such as ferric iron and nitrate can also serve as