Chemistry A2 · Analytical techniques. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
In thin-layer chromatography a spot of mixture is placed on a baseline near the bottom of a plate coated with a solid, such as aluminium oxide. This solid is the stationary phase. A solvent, the mobile phase, rises up the plate and carries the substances with it.
Each substance is shared between the two phases. A substance that is strongly attracted (adsorbed) to the stationary phase moves slowly. A substance that is more soluble in the solvent moves further. So the substances separate into spots.
Aluminium oxide is polar, so polar molecules stick to it more strongly and travel less far than non-polar molecules.
Students measure the distances from the bottom edge of the plate. Both distances must be measured from the baseline, and the spot is measured to its centre.
Answer: Rf = 0.60
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