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Name: Safwan Kamal Askender
Stage: 3
Solid-liquid extraction
In this process the components of a solid mixture are extracted
into a solvent. The 'batch process', analogous to liquid-liquid
extraction, involves grinding the solid to a fine powder, mixing it
with the appropriate solvent, and filtering off the solid by gravity
or under vacuum and then evaporating the solvent from the
extract solution. However, a more elegant 'continuous extraction
process', called Soxhlet extraction, is available when the most
appropriate solvent is known.
The apparatus for Soxhlet extraction is shown in Fig. 14.5 and
comprises a flask containing the solvent, a Soxhlet extractor and a
reflux condenser. The solid to be extracted is placed in a porous
thimble, made from hardened filter paper, and the solvent is
heated so that its vapour flows past the thimble, condenses and
fills the extractor with hot solvent to extract the solid. When the
extractor is full, the solvent (together with the extracted material)
siphons back into the solvent flask and the process is repeated
automatically. The advantage of this procedure is that fresh
solvent continually extracts the solid, which is concentrated in the
The disadvantage is that the compound extracted is kept at the
boiling point of the solvent for a prolonged period. Soxhlet
extractors come in sizes of 10 mL to 5000 mL, based on the
volume of solvent contained in the extractor.
A Soxhlet extractor is a piece of laboratory apparatus. invented in
1879 by Franz von Soxhlet. It was originally designed for the
extraction of a lipid from a solid material. Typically, Soxhlet
extraction is used when the desired compound has
a limited solubility in a solvent, and the impurity is insoluble in
that solvent. It allows for unmonitored and unmanaged operation
while efficiently recycling a small amount of solvent to dissolve a
larger amount of material.
The Soxhlet extractor setup, composed of a round-bottom flask,
condenser, and the Soxhlet apparatus, is frequently used
to extract solutes that are poorly soluble in solvents. The Soxhlet
extraction enables a small amount of solvent to be used with a
large solid