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[MUSIC PLAYING]

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PROFESSOR: Recrystallization.

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A large percentage
of a chemist's time

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in the laboratory is
spent purifying chemicals.

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Learning to use a variety
of different purification

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techniques efficiently
and effectively

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is key to becoming a
successful experimentalist.

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Recrystallization is
one powerful method

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for the purification
of crystalline solids.

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This video will outline the
purification of naphthalene

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by recrystallization.

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The technique of
recrystallization

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involves five main steps.

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First, an appropriate
solvent is determined.

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Recrystallizations
can be carried out

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using either one or a
mixture of two solvents,

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depending on the
solubility of the sample.

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Next, the sample is
dissolved in a minimal amount

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of the proper solvent.

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At this point, the
solution can be decolorized

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and insoluble contaminants
can be removed

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by performing a hot filtration.

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The resulting solution
is then cooled

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to induce crystallization.

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And finally, the
purified crystals

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are collected by filtration,
washed, and dried.

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This video will first
outline the general procedure

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for carrying out a one
solvent recrystallization

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of naphthalene followed
by instructions

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for adapting the procedure to a
two solvent recrystallization.

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Choosing an appropriate
solvent for a recrystallization

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is a very important step.

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You won't get acceptable results
if you don't pick carefully.

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It is helpful to have a variety
of solvents at your disposal.

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If solubility data for your
compounds has been published,

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this will help you to narrow the
range of potential candidates.

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Once you have decided on
a few prime contenders,

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you will perform solubility
tests to find the ideal choice.

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You will need a hot water bath
for the solubility tests as

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well as the recrystallization.

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[ALARM BLARING]

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MAN: Caution.

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Organic solvents are flammable.

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Never heat over an open flame.

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PROFESSOR: To set
up a water bath,

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place a large crystallizing
dish about half full

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with water on a hot plate.

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Add some boiling stones
and turn on the heat.

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Don't turn it up all the way.

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Start halfway and
adjust from there.

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To perform a
solubility test, place

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approximately 20 milligrams of
your compound in a test tube.

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Add a half of a milliliter
of the appropriate solvent

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and swirl.

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Wait a couple of
minutes and then make

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note of whether or not
the compound dissolves.

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For example, at
room temperature,

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naphthalene is insoluble
in ethanol and water

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but soluble in
toluene and acetone.

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If there is still solid
remaining, clamp the tube

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and heat it in the water
bath until the solvent boils.

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Swirl the tube and
make note of whether

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or not the compound dissolves.

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After heating, naphthalene
is still insoluble in water,

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but it is now soluble
in hot ethanol.

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Now that you have finished
the solubility tests,

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how do you decide
which solvent to use

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for your recrystallization?

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The ideal solvent is one
in which your compound

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is insoluble at
room temperature but

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soluble at the boiling point.

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Remember, at room
temperature, naphthalene

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was soluble in both toluene and
acetone, so neither of these

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will work.

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After heating, naphthalene
remained insoluble in water,

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even at the boiling point.

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This means that ethanol
is the solvent of choice.

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It dissolves the sample
at its boiling point

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but not at room temperature.

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Now that we have
decided on a solvent,

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it is time to dissolve the
sample in some hot ethanol.

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Get ready by heating some
ethanol in the water bath.

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Remember to add
some boiling stones

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and clamp the flask securely
so that it doesn't tip over.

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Place the sample in
an Erlenmeyer flask.

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But before adding
solvent, it is a good idea

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to set aside a small crystal.

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It may come in handy
later if you have trouble

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getting crystals to form.

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When dissolving your
sample, use a minimal amount

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of hot solvent.

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This will ensure a good
recovery of crystals in the end.

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Add hot solvent
in small amounts,

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swirling and heating in
between each addition

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until the sample just dissolves.

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[ALARM BLARING]

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MAN: Caution.

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If it won't dissolve,
it's probably an impurity.

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PROFESSOR: Now that the
sample is in solution,

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impurities can be removed
through decoloration

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and hot filtration.

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Take a close look
at the solution.

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Is it the right color?

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For example, if you're
expecting white crystals

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and your solution is
yellow, then there

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is a soluble impurity.

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One easy way to remove small
amounts of colored impurities

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is with activated charcoal.

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Before adding any charcoal,
cool your solution

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to slightly below
the boiling point.

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[ALARM BLARING]

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MAN: Caution.

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Adding charcoal to
a boiling solution

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will cause it to boil over.

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PROFESSOR: Add a small
amount of charcoal

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to absorb the impurity.

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Do not add too much
or the charcoal

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will also absorb your compound.

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Swirl the flask.

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Boil it in the water
bath for 2 to 3 minutes.

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Charcoal is very fine and can
be hard to remove by filtration.

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It is easiest to add a
small amount of a filtering

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agent, such as [INAUDIBLE],,
to absorb the charcoal

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and facilitate the filtration.

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Add only a small
amount, swirl the flask,

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and heat it once again
for 2 to 3 minutes

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before performing
a hot filtration.

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If the solution is colorless,
the decoloration step

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is unnecessary,
but you still need

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to look carefully for any
undissolved solid in the flask.

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If you find any,
it can be removed

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by performing a hot gravity
filtration with fluted filter

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paper.

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Vacuum filtrations cool
the solution too much

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and crystals are lost on
the filter paper and funnel.

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Before filtering your solution,
add some excess solvents

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and heat it to the
boiling point to ensure

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that the compound
stays dissolved

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during the filtration.

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Wet the filter paper
with some hot solvent

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and filter the solution.

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Rinse the flask two or three
times with hot solvent,

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filtering each rinse.

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Finally, rinse the filter
paper with hot solvent

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to minimize product loss.

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Crystals frequently form in the
filtrate during the filtration,

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but they easily
dissolve with heating.

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[ALARM BLARING]

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MAN: Caution.

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Your compound is
no longer dissolved

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in a minimum amount of solvent.

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PROFESSOR: Before
proceeding to the next step,

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you need to boil away
the excess solvent.

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Remember to add a few boiling
stones to the solution and boil

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until solid begins to form.

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At this point, add a small
amount of hot solvent

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to redissolve the solid.

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When your sample is
once again dissolved

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in the minimum amount of
solvent, set it aside to cool.

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Cover the flask to
prevent contamination

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with airborne
particles and allow

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it to cool to room
temperature undisturbed.

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The flask can also be placed in
a beaker stuffed with a paper

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towel to slow down the cooling
and promote slow crystal

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growth.

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This is especially helpful
in a cold laboratory.

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Ideally, crystals will begin
to form as the solution cools.

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You may want to watch.

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It can be very beautiful.

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[MUSIC PLAYING]

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If the solution reaches
room temperature

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without crystal
growth, you may want

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to try a trick or two to
jumpstart the process.

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First, try scratching
the side or bottom

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of the flask with a glass rod.

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Don't press too hard
or you'll break it.

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If that doesn't work, try
adding the seed crystal

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that you set aside
at the beginning.

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If you forgot to set
aside seed crystal,

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you can also try dipping a
glass rod in the solution,

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letting it dry so that
the minute crystals form

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on the surface, and
then dipping it back in.

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[GONG CLASH]

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When crystallization
has begun and your flask

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is at room temperature,
place it in an ice bath

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to drive the crystallization
to completion.

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Leave it in the ice bath
for at least 15 minutes

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before collecting the crystals.

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The final step in
a recrystallization

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is collecting and washing
the purified crystals.

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To do this, you will need a
flask of ice cold solvent,

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ethanol in this case, and
a vacuum filtration setup.

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Always use a clean, dry flask
to collect the filtrate.

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In many cases, a second
batch of crystals

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can be obtained by
concentrating the mother liquor

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and crystalizing a second time.

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Turn the vacuum on and
wet the filter paper

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with a small amount
of cold solvent.

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Pour the crystals
and the mother liquor

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onto the funnel
using cold solvent

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to transfer all of the
crystals from the flask.

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[VACUUMING]

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Use cold solvent to
wash the crystals

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and pull air through them
to begin the drying process.

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After pulling air through the
crystals for a few minutes,

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scrape the crystals
onto a weighing dish

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and spread them out
to facilitate drying.

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Depending on the
compound, the crystals

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can be dried in a desiccator,
an oven, or on a vacuum line.

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When one ideal solvent cannot
be found for a crystallization,

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it is possible to use a
mixture of two solvents.

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Once again, you must choose
the solvents very carefully.

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In the first
solvent, solvent one,

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your sample should be soluble
even at room temperature.

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And in the second
solvent, solvent two,

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your sample must
remain insoluble even

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at the boiling point.

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The two solvents must also
be miscible at all ratios

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so that they don't separate
out into different layers

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during the recrystallization.

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We know from our
solubility tests

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that naphthalene is
soluble in both toluene

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and acetone even at
room temperature, so

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which one should we choose
to be our solvent one?

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That depends entirely on
our choice for solvent two.

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We know from our
solubility tests

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that naphthalene is
insoluble in water

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even at the boiling point.

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That makes water a very good
candidate for solvent two.

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While acetone and
water are miscible,

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toluene and water are not.

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The recrystallization
of naphthalene

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will be carried out using
acetone as solvent one

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and water as solvent two.

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As before, dissolve your
sample in the minimal amount

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of hot solvent.

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In this case,
solvent one, acetone.

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Add just enough hot solvent
to obtain a clear solution.

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Then add solvent two, in
this case water, dropwise

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until the solution turns cloudy.

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Swirl and heat the
solution to make sure

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that the cloudiness persists.

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And do not confuse
cloudiness with the turbidity

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that is observed upon mixing
two solvents together.

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Once you have obtained a
persistently cloudy solution,

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add hot solvent one dropwise
until the solution just

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turns clear.

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Now you're ready to set
the solution aside to cool.

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Once the recrystallization
is complete,

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00:17:22,030 --> 00:17:25,810
cool down a mixture of solvents
in a ratio similar to that

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which you recrystallized from.

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This cold mixture will be
used to collect and wash

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your crystals.

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[MUSIC PLAYING]

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Remember this video is
intended to help you prepare

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for lab by providing
a demonstration

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of the proper
experimental technique.

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It is not intended
as a replacement

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for reading your lab manual
or the supplementary material.

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In order to become a
great experimentalist,

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it is important that you
understand both theory

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and technique.

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Now it's your turn.

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Good luck.

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