Introduction to Soil Chemistry

142 extraction
taining one to several percent water. The soil is weighed and extracted as if it
were dry (see Chapter 4, Section 4.13.1). It must be kept in mind that under
these conditions the extractant will pick up and in some cases become saturated
with water that will need to be removed before proceeding with the
analysis.
Another approach is to add an inorganic drying agent such as anhydrous
sodium sulfate to the moist or wet soil. This latter approach is limited by the
contact between the drying agent and the soil.Although adding a drying agent
to soil will remove some water, it cannot be expected to remove all water.
Thus, even in this case the extractant will contain some water after extraction
and may need to be dried before analysis can be undertaken.
Because even air dry soil will have a variable amount of water, analytical
results are reported on the bases of the dry weight of soil taken. This procedure
is described in Chapter 4 and expressed in equations (4.3) and (4.4).
After extraction organic solvents can be dried using a number of common
drying agents. Usually these are inorganic salts that react with water in a
solvent and become hydrated. Not all drying agents are suitable for all solvents
or solvents containing certain functional groups; thus care must be taken
in selecting the drying agent. Common drying agents and their characteristics
are listed in Table 7.2 [20].
Drying conditions are specific for specific extractions procedures, and thus
the extraction method used will determine the drying required for the method
used.
7.6. EXTRACT ANALYSIS
Extracts may be analyzed using by colorimetry, spectroscopy (Chapter 8),
chromatography, or a combination of chromatography and spectroscopy
(Chapter 9). Spectroscopic, chromatographic, and chromatographic/mass spectroscopic
analysis will most often be carried out directly on the clean, dry
extract.
7.7. CONCLUSIONS
A variety of methods, both physical and chemical, are used to separate components
of interest from the soil matrix.This is usually accomplished using one
of a number of different procedures, solvents, and solutions. Once isolated, the
extracted component will be cleaned by one of several purification procedures
that may also result in concentration. In other cases the extractant will be concentrated
after the cleanup process is completed. In many instances one of the
most important contaminants to remove will be water. Once cleaned, the
extract may be analyzed by a wide variety of spectrophotometric and chromatographic
methods.