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### Course: Class 11 Chemistry (India)>Unit 12

Lesson 11: Methods of purification of organic compounds

# Calculating retention factors for TLC

In thin-layer chromatography, the retention factor (Rf) is used to compare and help identify compounds. The Rf value of a compound is equal to the distance traveled by the compound divided by the distance traveled by the solvent front (both measured from the origin). For example, if a particular compound travels 1.5 cm and the solvent front travels 6.0 cm, then the compound's Rf value is 0.25. . Created by Angela Guerrero.

## Want to join the conversation?

• Is it possible for one of the spots to travel "faster" than the mobile phase? As in just remain on the pushing edge of the mobile phase? Or is it only able to travel BECAUSE of the mobile phase and the farthest spot will always be behind the mobile phase?
• The furthest spot can only go as high as the solvent front of the mobile phase. Note that the spots have to be somewhat soluble in the mobile phase. And the only reason the spots move is because they are dissolved and are pulled along as the mobile phase climbs. So no, the spots cannot travel faster than the mobile phase.
• Is the stationary phase always polar and the mobile phase always unpolar since she says that the lower Rf the more polar, and the higher Rf the less polar, or is it possible to do it the other way around?
• It's possible to do the other way around, though the standard TLC does use a non-polar mobile phase. and a stationary polar phase.
• Rf,c should be 0.67, not 0.66
• True
• yo what do different rf values indicate?
• If the RF value is large, then that means the solute was attracted to the solvent (which was moving). This is because as the solvent moves up, the solute follows, since their polarities are similar. This would mean that the solute has a relatively similar polarity to the solvent and a lower polarity than the silica gel.
On the other hand, if the RF value is small, then the solute is more polar than the mobile solvent and is thus attracted to the stationary silica gel. Therefore, the solute is more polar.

Hope that helped!
• Are the retention factors supposed to have two significant figures? The ruler on the left of the plates for both experiments has one significant figure and therefore wouldn't the Rfs be round to have one significant figure?
• The number of significant figures is limited by the precision of your instrument. So here the Rf values will have a low number of significant figures because the measuring device (the ruler) isn't very precise. A more graduated ruler will allow us to use more significant figures.

With a ruler like this our measured Rf values will have two significant figures. We'll be able to tell for certain that an Rf value is between a certain integer (giving us our first sig fig), and then exactly where in between those integers we would estimate (giving us our second sig fig). Scientific measurements are reported so that every digit is certain except the last, which is estimated.

Hope that helps.
• How does size affect the retention (or retardation) factor of an amino acid when testing amino acids on a chromatography paper?
• what should be the purity of carrier gases be
• If mobile phase is considered non-polar then why the less polar compound is attracted to non-polar phase. Is less polar polar materials attracted to non-polar?
• that's the idea. "like attracts like" (the less polar a compound, the more attracted it will be to a non-polar molecule, and vice versa).
• How would I calculate an retention factor (Rf) value of a point located along the origin? For example, using the example points given at , would the value of a point located on the origin be recognized as 4/4 for a total value of 1 or 0/4 for a total value of 0? Thanks in advance for your assistance.
• Do you mean the starting line? If so then it would be zero. Because measurements of the distant traveled by the spots are the lengths between the spot and the starting point. You're Rf is essentially the distance that the spot has traveled over the distance of the solvent front/the distance traveled by the solvent/mobile phase. Thus if your spot did not move from where it was spotted, it's travelled distance measured would effectively be zero. Hence the Rf value, which is a fraction, will also be zero, giving you an Rf factor of zero.
• Hi,
I have noticed two approaches to calculate the rf. The more common one as used here is to measure to the centre of the spot divided by the distance of the solvent front. The problem with this approach is that it is impossible to get an Rf of 1 (but possible to get an Rf of 0). That is the centre of the spot can never be equal to the distance travelled by the solvent front.

The other approach is to measure to the edge of the spot closest to the solvent front. The issue with this approach is that is impossible to get an Rf of 0 (but possible to get an Rf of 1). Here the compound spreads pass the origin when it is being spotted.

Is either one of these approaches better than the other?

## Video transcript

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.