Distillation
Guide
What's
distillation used for?
Distillation
is a laboratory technique used for separating and purifying
liquids.
How does
distillation work?
Distillation
works by exploiting the different boiling temperatures of
liquids. To separate two or more liquids by distillation,
you first heat them in a flask. The more volatile liquid (the
liquid with the lower boiling point) will typically evaporate
first and the vapor will pass into a condensing column, where
it can revert into a liquid (condense) on the cool glass where
it trickles into a collection flask. Heating further will
cause the less volatile liquids to evaporate and distill at
higher temperatures. The two main kinds of distillation are
simple distillation and fractional distillation,
and both are used widely.
What is
simple distillation?
The setup
for a simple distillation is shown in Figure 1. A simple distillation
apparatus consists of a boiling flask (round-bottom flask)
attached to an adapter holding a thermometer (to determine
the boiling temperature of the liquid). The adapter connects
to a condenser into which cold water is constantly passed
through. The condenser leads into a collection flask for the
purified liquid.
Figure
1. Simple distillation. |
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What is
fractional distillation?
Fractional
distillation is essentially the same as simple distillation
except that a fractionating column is placed between the boiling
flask and the condenser. The fractionating column is usually
filled with glass or plastic beads. These beads improve the
separation between the liquids being distilled. The reason
that fractional distillation gives better separation between
the liquids is because the glass beads in the fractionating
column provide "theoretical plates" on which the
refluxing liquid can condense, re-evaporate, and condense
again, essentially distilling the compound over and over.
The more volatile liquids will tend to push towards the top
of the fractionating column, while lower boiling liquids will
stay towards the bottom, giving a better separation between
the liquids. Of course, the more theoretical plates that you
add to a column (the more surfaces or beads), the longer the
distillation will take (typically), and the more energy required
to keep reevaporating liquid in the fractionating column (this
is more of a concern in industrial distillations than in an
academic lab where energy cost is not a major cause for worry).
Figure 2. Fractional distillation. |
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So, simple
or fractional?
The choice
of whether to use fractional distillation or simple distillation
depends on the two liquids being separated. Typically, using
simple distillation is preferrable because the apparatus is,
well, simpler, and a simple distillation typically goes faster
than a fractional distillation (and requires less energy).
On the other hand, fractional distillation gives better separation
between the liquids. The choice of whether to use simple or
fractional distillation, then, depends usually on the difference
in boiling temperatures between the two liquids. If there
is a large difference in the boiling points (>70oC)between
the two liquids then simple distillation is probably the best
option. On the other hand, if there is only a small temperature
difference between the two liquids a fractional distillation
is the preferrable option.
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Simple
distillation |
Fractional
distillation |
Advantages |
- simpler
setup than fractional
- faster
distillation times
- consumes
less energy than fractional distillation
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- much
better separation between liquids than simple distillation
- can
more readily purify complex mixtures than simple distillation
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Disadvantages |
- requires
the liquids to have large boiling point differences
(>70oC)
- gives
poorer separation than fractional distillation
- only
works well with relatively pure liquids
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- more
complicated setup than simple distillation
- takes
longer for liquids to distill
- consumes
more energy than simple distillation
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Best
used for: |
separating
relatively pure liquids with large boiling differences
or liquids with solid impurities |
separating
complex mixtures of liquids with smaller boiling point
separations. |
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