Introduction to Soil Chemistry
Introduction to Soil Chemistry
Introduction to Soil Chemistry
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100 electrical measurements<br />
Standardization is an essential component of soil pH measurement, and<br />
care must be taken <strong>to</strong> ensure good buffers. If the buffers show any indication<br />
of contamination, such as material floating in them, soil, or microbial growth,<br />
they must be discarded and new buffers prepared. All pH buffers will support<br />
microbial growth, which will interfere with electrode function and change the<br />
buffers pH.<br />
Measurements made using pH meters and electrodes are temperaturesensitive;<br />
that is, the pH reading obtained depends on the temperature of the<br />
solution. Some pH meters have a temperature probe such that a temperature<br />
correction is au<strong>to</strong>matically made during the measurement. However, if this is<br />
not the case, the pH meter must be set <strong>to</strong> the proper temperature if accurate<br />
measurements are <strong>to</strong> be obtained.<br />
Fouling of the reference electrode or the reference side of a combination<br />
electrode is a common problem in soil pH measurements. Fouling can be<br />
caused by salts, organic matter, and clay. Each electrode manufacturer will<br />
provide specific cleaning procedures that will help keep electrodes functioning<br />
properly; however, in many cases no amount of cleaning will be effective<br />
and the electrodes will need <strong>to</strong> be replaced.<br />
Depending on the extraction method <strong>to</strong> be used or developed, determination<br />
of pH using a standard method should be used. In different countries<br />
and geographic areas different standard methods will be in common use; for<br />
example, in Ohio (USA), a 1:1 soil:water suspension is used while in<br />
Zimbabwe a 1 : 1 soil : 0.01 M CaCl 2 is commonly used. The pHs determined by<br />
the standard method, used in a particular area, are used in many other procedures<br />
and methods and are used <strong>to</strong> make recommendations and predictions<br />
about the environment. Changing the method will mean that the validity of<br />
all these relationships will need <strong>to</strong> be reestablished or new relationships determined.<br />
For these reasons it is generally not advantageous <strong>to</strong> spend time and<br />
money developing a new methodology for determining soil pH unless there is<br />
a highly significant economic benefit in doing so [4–6].<br />
5.3.2. Ion-Selective Electrodes<br />
Ion measurements, using ion-selective electrodes (ISEs), 2 are very similar <strong>to</strong><br />
pH measurements and typically are carried out using a pH meter capable of<br />
accepting ion-selective electrodes. Each ion requires a specific electrode, some<br />
of which will be combination electrodes similar <strong>to</strong> combination pH electrodes,<br />
while others will be only the sensing electrode, called a half-cell. For these halfcell<br />
electrodes, special reference electrodes, which have a high flow of reference<br />
electrode solution, are used. There are two important differences<br />
between pH and ion-selective measurements: (1) the latter may require that<br />
the solution be separated from the soil before measurement is made and (2)<br />
2<br />
Ion-selective electrodes were originally called ion-specific electrodes, and this term may still be<br />
encountered.