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- [Instructor] So let's
say that I have a vial
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of some mystery liquid right over here,
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and I want to start figuring
out what's going on there.
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And the first step is to think about,
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is it just one substance
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or is it a mixture of multiple substances?
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And the focus of this video is a technique
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to separate out the substances
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to understand at least how many there are,
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and this technique generally
is called chromatography,
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but we'll focus on thin
layer chromatography
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which is the most common
that you might see,
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but other variations of chromatography
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like paper chromatography
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operate on very similar principles.
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So what we're going to do is set up
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on top of something like glass or plastic,
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we're going to put a thin layer
of a solid polar substance.
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Now, what you typically do
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is put a thin layer of silica gel,
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that's the most common solid
polar substance that folks use.
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And it's also porous.
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And the fact that it's
porous is really important
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because we're going to want
liquid to have capillary action
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and travel up through it.
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Now, the silica gel, as I mentioned,
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this thing is very polar.
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Now, what we're going to do
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is take some of our mystery substance,
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let's say it's this color right over here,
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and we're going to place a
dot of it on that silica gel.
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You then want to take this plate
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that has the silica gel on it
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and that little dot of
our mystery substance,
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and then you want to dip just
one end of it in a solution.
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And what's really important is that
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the solution is less
polar than the silica gel.
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Less polar here.
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And we'll talk a little
bit about what happens
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depending on how polar this is.
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Now, usually this is going
to be a very shallow amount
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of this solution, which, as we'll see,
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will be something of a solvent.
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And you usually want to put
it in a closed container
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like this
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so that this fluid down
here doesn't evaporate out.
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And then what do you
think is going to happen?
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Well, as I mentioned, this
is a porous substance here.
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And so you're going to
have capillary action.
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This fluid at the bottom
is going to move upwards
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through the silica gel,
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through those little
pores in the silica gel.
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This is the stationary phase.
Why do we call it that?
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Well, 'cause it's not moving.
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And you can imagine we would
call this less polar solvent
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the mobile phase,
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because that is traveling
through the silica gel
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and it's picking up some
of this mystery substance
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and it's transporting it.
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And let's say this mystery substance
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is made up of two different things.
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If something is more polar,
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that means it's going to be more attracted
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to the stationary phase
which is very polar.
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And so it's not going to travel that far,
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while the parts of our mystery substance
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that are less polar,
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they're not going to be attracted
to the silica gel as much.
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So they're going to travel
further with the solvent.
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So maybe it might go like that.
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And you would run this
until your mobile phase
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makes a good way to the
top of your silica gel
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right over here.
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Now, just looking at this,
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and the reason why it was
called chromatography is
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when they originally did this,
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they were actually separating
out various tissues
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in vegetation that had different colors.
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The chroma is referring
to the various colors,
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but it doesn't necessarily
even have to refer to things
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that have different colors
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or sometimes you might need
a UV light to see them.
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But when you run thin
layer chromatography,
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you will see that your
original dot will have traveled
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to various degrees with your solvent
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and then will now be multiple dots
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depending on how many things
were in your original mixture.
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And as I just mentioned,
this thing right over here,
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this is the less polar thing
is going to travel further
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than the more polar thing, more
polar constituent substance,
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because the more polar
thing is more attractive
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to the silica gel, which is stationary,
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and there is a way to quantify
how far these things traveled
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relative to your solvent.
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And that's called a retention factor.
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Retention factor.
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Which the shorthand is R subscript f.
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And it's just defined
as the distance traveled
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by the solute divided
by the distance traveled
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by the solvent.
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And we need to be clear.
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It's not the distance traveled
by the solvent in total,
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it's the distance traveled by
the solvent from this origin,
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from where we applied
this dot right over here.
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So, past the origin.
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And let me label that as the origin.
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So what would it be in this situation?
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Well, to help us there, we
would have to get out a ruler.
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So the retention factor for
substance A right over here,
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so I'll put that dot there, label that A,
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would be equal to the distance
traveled by the solute,
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which we can see, it
traveled one centimeter,
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one centimeter,
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over the distance traveled by
the solvent past the origin.
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And so that is going to be,
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we see it traveled five
centimeters past the origin.
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So one centimeter over five centimeters,
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which is the same thing as 0.2.
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And then the retention
factor for substance B
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is going to be equal to,
how far did it travel?
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Well, it traveled three centimeters
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out of a total of five
centimeters for the solvent,
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past this origin,
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past where we put the
sample right over there.
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Five centimeters, which is equal to 0.6.
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So notice, in this situation,
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the more polar substance
had a lower retention factor
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than the less polar substance,
and that makes sense.
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Because our stationary phase
is more polar than our solvent,
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and so the things that are
more polar were harder to move
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by the less polar solvent.