Calculating retention factors for TLC | Chemical processes | MCAT | Khan Academy
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Calculating retention factors for TLC | Chemical processes | MCAT | Khan Academy

December 31, 2019


Remember that when you
run a TLC plating lab you have twp phases, the
stationary phase shown as this blue silica gel on
the plate and a mobile phase. The mobile space
is a solvent that’s less polar than the
solid stationary phase. Silica gel is very, very polar. Let’s say that you had a
plate that looked something like this. You had initially
spotted two compounds. We’ll call them A and
compound B. And then what you saw on the plate was
that your mobile phase had traveled up to about here, A
had traveled to about here, and B had traveled this far. But what does that really mean? How can we even
report these values? The way we’d report them if we
were writing up a lab report or writing a
manuscript, you’d need something known as the
retardation factor, also known as the retention
factor or RF for short. RF is equal to the
distance traveled by solute over the distance
traveled by the solvent. So the first step
you need to do is measure these distances
for the different compounds and also for the solvent, also
known as the mobile phase. So let’s put a ruler
next to our TLC plate, much like you would if
you were sitting in lab. We’ll say that this is 1 unit,
2 units, 3 units, and 4 units. So we can measure the
distance that A has traveled, and that’s from the starting
line to the center of the spot. That’s two units. And for compound B, again
from the starting line to the center of the
spot, that’s 3 units. And for the solvent, the
starting line to this finish line, that is 4 units. So let’s plug that
into our equation. If we wanted to
solve RF of A, you need the distance
traveled by compound A over the distance
traveled by the solvent, so let’s say A over S.
Here, that would be equal to 2 over 4, and
the convention is to report these values
as decimal points, so we’ll say that this is 0.5. Now, we’ll do the same
for compound B. RF of B is equal to distance traveled by
B over distance traveled by S. In this case, that’s equal
to 3 over 4, or 0.75. So what can we tell about
these two compounds? If we remember from talking
about the mobile phase and stationary phase, compounds
that travel really far must be more attracted
to the mobile phase, and therefore are less polar. So we can say that compound B is
less polar and travels faster. The opposite is
true for compound A. Since this doesn’t
move as much, it’s more attracted to
the polar silica gel, and hence it’s more polar than
compound B and travels slower. Think about it like it’s getting
stuck in the stationary phase and doesn’t really want
to move away from it. So there we’ve done
our first example. Let’s do another one. In this example, we can see that
our initial reaction mixture separated into four
different compounds. Let’s label these
as A through D, with A being the orange
spot, B as the yellow one, C as the green one, and
D as the purple one. Again, we’ll use the same
process that we used earlier. So the first step
is to take a ruler and put it next your TLC plate. This is 1 unit, 2
units, 3, 4, 5, and 6. So let’s calculate
the RF of A. This is equal to the
distance traveled by A over the distance
traveled by the solvent, so we need to measure these. First, we can see that A has
traveled 1 unit, equal to 1, and the solvent has
traveled about 6 units. So we’ll say that’s
1 over 6 then. Let’s convert that to
decimals and you have 0.17. We can do the same for
each these compounds. Next, we’ll take
B. This is again equal to B over S, which equals
this distance is about 3 units. So we have 3 over 6,
which is equal to 0.50. Next, we’ll measure
this for C. The RF of C is equal to the
distance traveled by C over the distance traveled
by S, which equals– distance traveled by C is 4– so
that’s going to be 4 over 6, which is equal to 0.66. And lastly for D,
again we’ll have to measure the distance traveled
by D over distance traveled by S. In this case,
this distance is 5, so this would be 5 over
6, which is equal to 0.83. Now what can we say about
these overall trends? Again, we said that compounds
that travel really, really far are pretty nonpolar,
and compounds that don’t travel
very far at all are more attracted to
the stationary phase and hence are more polar. So if we look at
these RFs, we can show that there really
is a trend here. Compounds with a smaller
RF are more polar, since they’re more attracted
to the stationary phase. And compounds with a
bigger RF are less polar, since they’re more attracted
to the mobile phase. Let’s review quickly
what we’ve learned today. We learned how to calculate
the RF value, also known as the retention factor
or retardation factor, and how you would report that
when presenting in a lab report or in the literature. We showed that compounds
with big RFs are less polar, and compounds with pretty
small RFs are more polar.

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  1. This is great, makes so much sense, only problem for me is that my prelab doesn't show the solvent line, only the starting line. I guess I'll have to draw one and tell my instructor about it.

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