4.9.5Unit 4. Rates, Equilibria and Further Organic Chemistry
Chromatography – TLC — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 55 of 56 Unit quiz
Part 1 of 5How TLC Separates
Every chromatography method relies on repeated distribution of each component between a fixed phase and a moving phase.
Stationary phase
- Stationary phase A thin layer of silica or alumina on a glass or plastic plate holds polar groups that can adsorb the substances being separated.
Mobile phase
- Mobile phase A solvent travels up the plate by capillary action and carries each component along at a rate that depends on how strongly it is attracted to each phase.
Competition
- Competition A component that sticks strongly to the plate is held back, while one that dissolves well in the solvent is carried further up.
Result
- Result Different substances end up at different heights, giving a series of separate spots that can be compared with known samples.
Part 2 of 5Running a Plate
1
Draw a baseline lightly in pencil about 1 cm from the bottom, because pencil graphite will not dissolve in the solvent and spread up the plate.
2
Place a small, concentrated spot of each sample and of any reference compounds on the baseline, letting each dry before adding more.
3
Stand the plate in a covered container with solvent below the baseline, so that the atmosphere is saturated with vapour and the solvent front rises evenly.
4
Remove the plate before the solvent reaches the top and immediately mark the position of the solvent front in pencil.
5
Dry the plate and make colourless spots visible using ultraviolet light on a fluorescent plate, iodine vapour, or ninhydrin spray for amino acids.
Part 3 of 5Interpreting the Chromatogram
Interpreting the Chromatogram
Rf value
- Rf value Rf equals the distance moved by the spot divided by the distance moved by the solvent front, so it always lies between 0 and 1.
Identification
- Identification A spot is matched to a known compound when its Rf is the same as the reference run on the same plate with the same solvent.
Purity
- Purity A pure compound gives a single spot, whereas two or more spots show that the sample is a mixture.
Conditions
- Conditions Rf depends on the solvent and plate used, so comparisons are only valid when these are identical.
| Feature | Strongly adsorbed compound | Weakly adsorbed compound |
|---|---|---|
| Attraction to plate | High | Low |
| Distance moved | Short | Long |
| Rf value | Small | Large |
Part 4 of 5Worked Example
Question
- On a TLC plate the baseline is 1.0 cm above the bottom edge.
- A spot has its centre 4.2 cm above the bottom edge and the solvent front is 9.0 cm above the bottom edge.
- Calculate the Rf value.
- Distance moved by the spot = 4.2 - 1.0 = 3.2 cm.
- Distance moved by the solvent front = 9.0 - 1.0 = 8.0 cm.
- Rf = 3.2 / 8.0 = 0.40.
Answer Rf = 0.40
Part 5 of 5Exam Checklist & Tips
4.9.5 – Key Points for the Exam
- Stationary phase The solid layer on the plate that adsorbs the components.
- Mobile phase The solvent that moves up the plate carrying the components.
- Rf value Distance moved by spot divided by distance moved by solvent front.
- Locating agent A reagent or light used to make colourless spots visible.
- Solvent front The furthest point reached by the solvent when the plate is removed.
Common Mistakes
- Drawing the baseline in pen, whose ink dissolves and moves with the solvent.
- Letting the solvent run above the top of the plate so the front cannot be measured.
- Measuring the spot distance from the bottom edge of the plate instead of from the baseline.
- Quoting an Rf greater than 1, which is impossible because a spot cannot outrun the solvent.
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