Introduction to Soil Chemistry

extraction 131
Table 7.1. Common Extraction Solvents and Their Structure and Characteristics
Name Structure Characteristics
Water
H
O
H
Dissolves many compounds and is present in
many other extractants; can cause
interference in analytical and instrumental
methods
Methanol CH3 O
H
Miscible with water; can dissolve some
hydrophobic or sparingly soluble materials;
can cause interference in analytical and
instrumental methods
Acetone O Miscible with water; dissolves many
H3C C
CH3 hydrophobic compounds; low boiling point
Diethyl ether C2H5 O C2H5 Very low solubility in water; low boiling point;
dissolves most hydrophobic compounds
Hexane CH3(CH2) 4CH3 Hydrophobic—insoluble in water; dissolves
hydrophobic compounds; has higher boiling
point than diethyl ether or acetone
Methylene
chloride
CH2Cl2 Hydrophobic—insoluble in water; dissolves
hydrophobic compounds; has low boiling point
fore essential. Some toxic elements, such as arsenic and selenium, are commonly
present in soil and it is essential to differentiate naturally occurring
levels from contamination. Organic contaminants, both industrial chemicals
such as poly(chlorinated biphenyl)s (PCBs) and agricultural chemicals such as
insecticides and herbicides, must be determined at low concentrations. In all
these cases this is best done when there are no interferences from contaminants
in the solvents, including water.
There are a limited number of solvents used in common soil extracting procedures,
ranging from water to the nonaqueous and hydrophobic. Each has its
own uses and limitations. The most common solvents and their chemical structure
and characteristics are listed in Table 7.1. Solvents are available in a
variety of purities designed for various uses, and thus it is important to carefully
read the description of the solvent and its grade before use. High purity
does not guarantee compatibility with the extraction, analysis, or instrumentation
to be used.
Many organic solvents are sold containing small amounts of additional compounds,
generally used to increase their stability and shelf life. Diethyl ether
is available in several different “stabilized” forms. In some cases these stabilizing
agents, such as alcohols, may interfere with subsequent analysis. In addition
to stabilizing agents, ethanol is often denatured, which means that it is not
drinkable. Depending on the manufacturer, the denaturing agents may be
other alcohols or other compounds. Thus, not only must the purity but also
any additives present must be known.