ELECTRICAL MEASUREMENTS - Introduction to Soil Chemistry - page 100

Standardization is an essential component of soil pH measurement, and
care must be taken to ensure good buffers. If the buffers show any indication
of contamination, such as material floating in them, soil, or microbial growth,
they must be discarded and new buffers prepared. All pH buffers will support
microbial growth, which will interfere with electrode function and change the
buffers pH.
Measurements made using pH meters and electrodes are temperature-
sensitive; that is, the pH reading obtained depends on the temperature of the
solution. Some pH meters have a temperature probe such that a temperature
correction is automatically made during the measurement. However, if this is
not the case, the pH meter must be set to the proper temperature if accurate
measurements are to be obtained.
Fouling of the reference electrode or the reference side of a combination
electrode is a common problem in soil pH measurements. Fouling can be
caused by salts, organic matter, and clay. Each electrode manufacturer will
provide specific cleaning procedures that will help keep electrodes function-
ing properly; however, in many cases no amount of cleaning will be effective
and the electrodes will need to be replaced.
Depending on the extraction method to be used or developed, determina-
tion of pH using a standard method should be used. In different countries
and geographic areas different standard methods will be in common use; for
example, in Ohio (USA), a 1 : 1 soil : water suspension is used while in
Zimbabwe a 1 : 1 soil : 0.01 M CaCl 2 is commonly used. The pHs determined by
the standard method, used in a particular area, are used in many other pro-
cedures and methods and are used to make recommendations and predictions
about the environment. Changing the method will mean that the validity of
all these relationships will need to be reestablished or new relationships deter-
mined. For these reasons it is generally not advantageous to spend time and
money developing a new methodology for determining soil pH unless there is
a highly significant economic benefit in doing so [4-6].

5.3.2. Ion-Selective Electrodes

Ion measurements, using ion-selective electrodes (ISEs), 2 are very similar to
pH measurements and typically are carried out using a pH meter capable of
accepting ion-selective electrodes. Each ion requires a specific electrode, some
of which will be combination electrodes similar to combination pH electrodes,
while others will be only the sensing electrode, called a half-cell . For these half-
cell electrodes, special reference electrodes, which have a high flow of refer-
ence electrode solution, are used. There are two important differences
between pH and ion-selective measurements: (1) the latter may require that
the solution be separated from the soil before measurement is made and (2)

2 Ion-selective electrodes were originally called ion-specific electrodes , and this term may still be
encountered.