Introduction to Soil Chemistry

potentiometry (electrodes in soil measurements) 99
Figure 5.6. A pH electrode in a soil suspension. The electrode is connected to a computer that
shows the readout.
on both electrodes and pH meters; thus, when purchasing pH and reference
electrodes, one should ensure that they have the correct connectors for the
meter being used. Even robust electrodes must be treated with care and kept
wet with the appropriate solution as directed by the manufacturer. Regardless
of the make, pH meters are usually robust and should need little maintenance,
although care should be taken to keep them dry.
Although maintenance of pH meters is low, standardization is essential and
must be done on a regular bases. The starting point in standardization is to
adjust the meter and electrodes using a standard buffer solution of pH 7.00
and adjusting the meter to this pH. The second step is to set a second point,
which is in the range of pH levels expected to occur in the measurements.Thus,
if the pH values are all expected to be acidic, the second point will commonly
be set using a buffer of 4.02; if they are expected to be basic, then a pH 10.00
buffer will be used. If both acidic and basic pH values are likely, the meter can
be standardized using all three buffers.
Standard buffers can be purchased as already prepared solutions or as
powders dissolved in distilled or deionized water. In this latter case the powder
is typically dissolved in 100mL of distilled or deionized water for use. Buffers
may be color-coded, such as green for pH 7.00, orange for pH 4.02, and blue
for pH 10.00.
In practice the electrode is first rinsed with distilled water and placed in the
pH 7.00 buffer and the meter adjusted to read 7. In some cases the meter will
automatically make the adjustment needed. The electrode(s) is (are) rinsed
and placed in the second buffer and the meter adjusted. This two-point standardization
is usually enough for most soil pH measurements; however, if it is
not, the third buffer can be used in the same way.