WEBVTT

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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.