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### Course: Chemistry library>Unit 9

Lesson 4: Separating mixtures and solutions

# 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.
• 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
• True
• 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 do we know that the solvent is going to be the compound that travels the furthest on the TLC?