Introduction to Soil Chemistry

134 extraction
Table 7.2. Characteristics of Drying Agents
Characteristics:
Drying Agent Capacity/Speed/Intensity a
Restrictions
Calcium chloride High/medium/high Reacts with many functional
groups
Calcium sulfate Low/fast/high Applicable in most situations
Magnesium sulfate High/fast/moderate Applicable in most situations
Potassium carbonate Medium/medium/moderate Reacts with acids and phenols
Sodium sulfate High/low/low Hydrate decomposed on
heating
Molecular sieve 5Å High/high/high Suitable for most solvents
a Capacity = amount of water absorbed, speed = rate of water removal, intensity = level of water
remaining after drying (e.g., “high” means that smallest amount of free water remains after
drying).
case including water. Thus these agents must always be kept tightly sealed and
checked for purity from time to time. The purchase and use of small, individually
packaged amounts of drying agents is another way to limit the possibility
of introduction of contaminants.
7.4. EXTRACTION PROCEDURES
The actual extraction procedure may be as simple as adding the extractant to
the soil sample in an Erlenmeyer flask and mixing. It might be more complex,
involving Soxhlet, ultrasonic, microwave-assisted, accelerated solvent or
supercritical fluid extraction. In extracting soil using hydrophobic extractants
the procedure may call for drying the soil before extraction. Air drying may
be sufficient; however, mixing the soil with a drying agent; often anhydrous
calcium sulfate may be required. A specific amount of drying agent will be
called for, or the procedure may call for adding an amount of drying agent
equal to the weight of soil or the addition of drying agent until the soil is dried
to a prescribed consistency. Common drying agents are listed in Table 7.2.
7.4.1. Simple Mixing
Mixing is commonly accomplished using a reciprocal shaker equipped with
clips for holding the flask to the shaker platform. The shaker can then be set
to the prescribed orbits per minute and time for the extraction. This type of
mixing works well when a large number of samples need to be extracted at
one time. Alternatively, the sample could be stirred using a mechanical or
magnetic stir bar stirrer.