Chemguide: Support for CIE A level Chemistry
Learning outcome 37: Analytical techniques
37.1 Thin-layer chromatography
Learning outcomes 37.1.1, 37.1.2 and 37.1.3
Make sure that you read the statements as we go through them below.
You need to start by reading the page about thin layer chromatography. We will look more precisely at the syllabus requirements when you have seen the whole thing.
This is just like the paper chromatography you may be familiar with, except that you use something other than paper.
This just wants you to know what the term stationary phase means. This is just the chemical part of the chromatography process which doesn't move. The syllabus specifically mentions aluminium oxide on a solid support, but it could equally well be silica gel on a solid support.
This is about the mobile phase - the liquid used to separate the components of a mixture. It can be polar or non-polar, and the polarity matters. We will talk some more about that in statement 37.1.3.
The most non-polar solvent would be something like a liquid hydrocarbon where the only intermolecular forces involved are van der Waals dispersion forces.
The most polar would be a solvent like ethanol or ethanoic acid where you have the full range of intermolecular attractions including hydrogen bonding.
Chromatography solvents are frequently a trial-and-error mixture of solvents.
The Rf value is discussed in detail on the page you have read.
The solvent front is the point reached by the solvent as it moves up the plate. The baseline is the pencil line where a spot of the initial mixture is placed.
Make sure that you know how to work out and use Rf values. There has been at least one question where you were expected to work these out from a diagram of a thin-layer chromatogram. See, for example, May/June 2019 paper 42 Q7.
You should also know their relevance to chromatograms where a mixture is run against possible known components. All of this is covered in the page you have read.
This is an important statement, and is covered on the Chemguide page you have read under the title "What separates the compounds as a chromatogram develops?"
You need to consider the intermolecular attractions between the substances in the solution with the solvent (the mobile phase) and with the stationary phase.
For example, if the attractions between the solute and the solvent molecules are strong, and between the solute molecules and the stationary phase are weak, the Rf will be high. The molecule you are investigating will spend more time in the solvent than stuck to the stationary phase.
If you change the solvent so that there is less attraction to the solute molecules, then the solute will spend less time in solution and the Rf value will be less.
If you don't have a really good understanding of the various sorts of intermolecular attractions possible between organic molecules, you aren't going to make a success of this.
A question on this is unlikely to be worth much more than 1 mark, but 1 mark is all you need to lose to possibly miss the grade you need.
© Jim Clark 2020