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## AP®︎/College Chemistry

### Unit 3: Lesson 9

Separation of solutions and mixtures chromatography

# 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? • 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? • 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!
• Rf,c should be 0.67, not 0.66 • 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 • 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?
(1 vote) • 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.
(1 vote)
• 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?
(1 vote) • 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?
(1 vote) • 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.
(1 vote) • 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.
(1 vote)