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PROFESSOR: If you
haven't already

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viewed Distillation Part
1, Simple and Fractional

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Distillations, then please do
so before watching this video.

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If you have, then you know
that simple distillation

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is an effective technique
for purifying liquids,

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if the liquids boil below
about 150 degrees Celsius.

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If you have to purify a
higher-boiling liquid,

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however, then you must perform
a distillation under vacuum.

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The third type of distillation
is a vacuum distillation.

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This technique must be
used if the liquid boils

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above about 150
degrees Celsius, where

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it is likely to decompose.

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As you know, the boiling
point of a liquid

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occurs when it's
vapor pressure is

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equal to the external pressure.

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If a system is under vacuum,
it has a reduced pressure

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and the liquid is able to
boil at a lower temperature.

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To set up the apparatus for
the vacuum distillation,

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insert a Claisen adapter
between the distillation flask

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and head.

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Liquids often boil violently
under reduced pressure,

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and the Claisen adapter is used
to decrease the probability

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that liquid will bump
into the condenser.

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Instead of using
a vacuum adapter,

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attach a cow or pig to
the end of the condenser

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and place a receiving
flask on each joint.

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Because the system will be under
vacuum during the distillation,

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you will not be able to
replace the receiving

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flask as you did during
atmospheric distillation.

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A cow or pig allows you to
keep the system under vacuum

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but still switch to
new receiving flasks.

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Next attach heavy-walled vacuum
tubing to the outlet on the cow

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or pig adapter.

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If the walls of the
tubing are too thin,

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then they will
collapse under vacuum.

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Attach the other end
of the vacuum tubing

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to a vacuum source, such as a
vacuum pump or water aspirator.

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The vacuum source
should also be connected

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to manometer, a device
that measures pressure.

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Finally, make sure that all
joints are greased well.

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If they're not, solvent
may get into the joints

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and freeze them
shut under vacuum.

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Apply a thin film of grease
to the top of the male adapter

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and twist the female
adapter onto it

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until this film of grease can
be seen all around the joint.

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Do not apply too much
grease to the joint

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and never apply grease close to
the bottom of the male adapter.

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These actions may
cause the grease

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to seep into the
distilling flask

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and contaminate your liquid.

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Before performing a
vacuum distillation,

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you must check for
leaks in your system.

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To do this, make sure that
all of stopcocks are closed

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and turn on the vacuum.

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Then open the
stopcocks one by one,

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starting from the one that is
closest to the vacuum source.

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As each stopcock
is opened, listen

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for a hissing noise, which
implies a leak in the system.

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If you find the leak,
turn off the vacuum,

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introduce air into the
system, regrease the joints,

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and try again.

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The system must be free from
leaks before you can proceed.

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Once you are convinced that
the system is free from leaks,

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fill the solvent trap
with liquid nitrogen.

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This step is
essential to prevent

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destruction of the vacuum
pump by vaporized solvent.

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Then transfer the liquid mixture
into the distilling flask

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and add a stir bar.

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Do not add boiling stones
for a vacuum distillation.

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Boiling stones are useless
because the vacuum removes

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the air in their pores.

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Move the heating mantle
up to the distilling flask

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and place a stir
plate underneath.

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Do not turn on the heat.

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The first step in conducting
a vacuum distillation

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is turning on the stir plate.

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If stirring is not occurring
during a vacuum distillation,

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the probability of bumping
increases dramatically.

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Next, slowly and carefully
introduce the system to vacuum.

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You will see the
solution start to boil

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as low boiling impurities and
air run through the apparatus

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without heat.

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After the liquid mixture
has stopped boiling,

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turn on the heat.

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When the distillation
has been started,

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you can determine
the temperature

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at which the solution will
boil under the reduced pressure

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conditions through the
use of a nomograph.

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First, find the boiling
point of the compound

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at atmospheric pressure on
line B of the nomograph.

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Note the current pressure of
your system from the manometer

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and mark its location on
line C of the nomograph.

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Using a ruler or straightedge,
line up these two points

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and find the point of
intersection on line A.

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This number should
be a good estimate

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of your expected boiling point.

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Carry out the vacuum
distillation as normal.

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When finished, remove the heat
source from the distillation

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flask, turn off the vacuum,
and allow the solution

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to cool completely.

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Then introduce air into the
system, stop the stirring,

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and remove the receiving flask.

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Other things to remember
when carrying out vacuum

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distillations are as follows.

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If you know that you have some
low-boiling liquid present

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in your mixture, first carry
out a simple distillation

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to collect it before
attaching the vacuum.

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If you must separate compounds
with similar boiling points,

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then you can insert a
fractionating column

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between the distillation
flask and Claisen adapter.

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Unlike distillations at
atmospheric pressure,

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where a narrow boiling
point range is observed,

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it is not unusual to see a 10
to 20 degree Celsius temperature

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range during a vacuum
distillation due to pressure

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changes.

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Vacuum distillation allows you
to purify high-boiling liquids

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in a reasonable time
period without having

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to worry about decomposition.

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