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3.16.2Organic Chemistry

Thin-Layer Chromatography — Oxford AQA International A Level Chemistry Revision Notes & Topical Questions

5 min read 5 parts Lesson 53 of 55

📚 Oxford AQA International A Level Chemistry Hub / Topical Revision
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Part 1 of 5Phases and Retention

3.16.2 Thin-Layer Chromatography
Stationary phase
  • A thin layer of solid silica or alumina on a glass, plastic or foil plate, onto which the components can adsorb.
Mobile phase
  • A liquid solvent, polar or non-polar, that carries the mixture up the plate by capillary action.
Adsorption
  • Components that stick strongly to the stationary phase move slowly, while those that dissolve well in the solvent move further.
Rf value
  • Rf = distance travelled by the component ÷ distance travelled by the solvent front, always between 0 and 1.
Identification
  • Under the same conditions, a given compound always has the same Rf value, so it can be matched against known standards.

Part 2 of 5Running a TLC Plate

1
Draw a baseline in pencil about 1 cm from the bottom of the plate, since pencil does not dissolve in the solvent.
2
Place small spots of the sample and of reference compounds on the baseline, and let each spot dry.
3
Stand the plate in a covered beaker containing a shallow layer of solvent that is below the baseline.
4
Remove the plate when the solvent is near the top, and mark the solvent front immediately with a pencil.
5
Locate colourless spots using UV light, iodine vapour, or a developing agent such as ninhydrin for amino acids.
6
Measure the distances and calculate Rf values, or compare spots with the references on the same plate.

Part 3 of 5Interpreting Chromatograms

Interpreting Chromatograms
Polar solvent
  • If the mobile phase is polar, polar components dissolve in it and travel further than on a plate with a non-polar solvent.
Non-polar solvent
  • Non-polar components move further in non-polar solvents,
    • while polar components stay near the baseline, attracted to the polar stationary phase.
Purity
  • A pure compound gives one spot; extra spots show impurities.
Matching
  • If a sample spot has the same Rf and colour as a reference spot, the two may be the same compound.

Part 4 of 5Worked Example

Question

  • On a TLC plate the solvent front travelled 8.0 cm.
  • Spot A travelled 3.2 cm and spot B travelled 5.6 cm.
  • Calculate the Rf values and say which compound is more strongly adsorbed.

  1. Rf of A = 3.2 ÷ 8.0 = 0.40.
  2. Rf of B = 5.6 ÷ 8.0 = 0.70.
  3. Compound A moved less far, so it is held more strongly by the stationary phase.

Answer

  • Rf (A) = 0.40 and Rf (B) = 0.70;
    • A is more strongly adsorbed.

Part 5 of 5Exam Checklist & Tips

3.16.2 – Key Points for the Exam

  • Stationary phase
    • The phase that does not move;
      • in TLC, silica on a plate.
  • Mobile phase The solvent that moves through the stationary phase.
  • Rf value Ratio of distance moved by a component to the distance moved by the solvent front.
  • Adsorption Sticking of a substance to the surface of a solid.

Common Mistakes

  • Drawing the baseline in ink, which dissolves and runs with the solvent.
  • Letting the solvent level be above the baseline, so the spots wash away.
  • Measuring the distance to the top of the spot rather than the centre.
  • Not marking the solvent front before it dries, so the Rf value cannot be calculated.

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