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.

Thank you. That was very useful!!!

this is great ! thank you

Super useful thnx😊

Super useful thnx😊

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.

And the units? cm?

THANK YOU SO MUCH helped so much!

Thank you very much. Best video on the internet on this topic.

i love this, thank you

In the second chromatography is B more polar or less polar??

My teacher made it look so complicated