Introduction to Soil Chemistry

112 titrimetric measurement
pH
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Base-acid titration curve
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0 5 10 15 20
Acid added (mL)
Figure 6.1. Curve obtained titrating a standard base with a standard acid.
taking the first or second derivative of the data. Both the first and second
derivatives give an inflection point where the greatest slope of the titration
curve occurs, thus showing the endpoint.
Although this explanation of titration has been simple, the same basic idea
is applied to all forms of titation. In each case there is a slight change in pH,
millivoltage, or ISE reading with added titrant followed by a sharp change
through the endpoint followed again by a slight change.
Soil and soil suspensions are colored and difficult to see through; thus it is
difficult to directly titrate them.There are typically only two cases where direct
titrations of soil are carried out. Titration of soil has been used to determine
the amount of amendment needed to bring the soil to a desired pH.The second
direct titration is in the determination of soil organic matter where organic
matter is oxidized with chromate and the unreacted chromate is titrated (actually
called a backtitration) to determine, by subtraction, the amount of dichromate
reduced and thus the amount of organic matter present.
In other titrations the component to be titrated is separated from soil and
subsequently titrated. The simplest of these is the determination of soil
ammonia. However, all forms of nitrogen in soil are important, and so methods
of converting these to ammonia, distilling it, and determining its concentration
by titration constitute an extremely important set of procedures.
Other environmental analytical procedures using titration can be found in
the United States Environmental Protection Agency’s (USEPA) Web site compilation
of methods, particularly the 9000 series methods [see Bibliography].