Introduction to Soil Chemistry
Introduction to Soil Chemistry
Introduction to Soil Chemistry
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CHAPTER<br />
5<br />
ELECTRICAL MEASUREMENTS<br />
In making measurements using electricity, all its parameters—voltage, amperage,<br />
resistance, capacitance, frequency, and dielectric characteristics—can be<br />
used singly or in combination <strong>to</strong> obtain information about various conditions<br />
in the medium through which electrons are moving. All soils contain many<br />
ions of many different sizes and complexities: simpler hydrated K + , more<br />
complex NO 3 - , organic ions, and charged solids. All contribute <strong>to</strong> the electrical<br />
characteristics of a soil and its solution.<br />
One easily demonstrated electrical characteristic of moist soil is seen in the<br />
production of electricity when two different metals, namely, copper and zinc,<br />
are inserted in<strong>to</strong> it (assuming that the reaction is spontaneous). This is not<br />
unexpected because any salt containing solution, adsorbed in media such as<br />
paper or cloth, and placed between these same two electrodes, will cause a<br />
spontaneous reaction, which will produce electricity. The source of this flow of<br />
electrons is a chemical, oxidation–reduction reaction, represented as the<br />
following two half-reactions, for copper and zinc:<br />
0 2<br />
Oxidation: Zn Æ Zn + 2e<br />
+ -<br />
+ -<br />
Reduction: Cu 2e<br />
Æ Cu<br />
2 0<br />
(5.1)<br />
Production of electricity in this way was the original method used <strong>to</strong> produce<br />
electricity (i.e., an electric current) for scientific experiments. Thus, if copper<br />
and zinc are inserted in<strong>to</strong> wet soil, electricity will be produced by the same<br />
process, although a mixture of salts will be involved. Figure 5.1 shows copper<br />
and zinc strips inserted in<strong>to</strong> a moist soil and connected <strong>to</strong> a voltmeter, which<br />
displays the resulting voltage, demonstrating the existence of salts in the soil.<br />
This simple setup is not, however, used <strong>to</strong> measure soil characteristics such as<br />
salt content or pH.<br />
This electricity generation characteristic, along with the salts, their movement<br />
through soil, and the diffuse double layer must be kept in mind when<br />
making any soil measurement using electricity or electrodes.<br />
Although all characteristics of electricity have been used <strong>to</strong> investigate soil<br />
and its properties, only a limited number are used routinely.The most common<br />
<strong>Introduction</strong> <strong>to</strong> <strong>Soil</strong> <strong>Chemistry</strong>: Analysis and Instrumentation, By Alfred R. Conklin, Jr.<br />
Copyright © 2005 by John Wiley & Sons, Inc.<br />
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