1.2.4Principles of Chemistry
Interpreting Chromatograms — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 11 parts Lesson 11 of 50
Part 1 of 4Reading a chromatogram
Number of spots
- Number of spots A pure substance gives one spot only.
- A mixture separates into two or more spots, one for each component.
Matching spots
- Matching spots If two substances are the same, they produce spots at the same height.
- A known reference sample is often run alongside an unknown one to make comparison easy.
Aligned spots
- Aligned spots Spots in a horizontal line across the paper are probably the same substance, even if the colours cannot be seen in black and white diagrams.
Part 2 of 4Example conclusions
| Observation | Conclusion |
|---|---|
| Brown ink gives three spots | The ink is a mixture of three dyes |
| Red, yellow and blue references each give one spot | Each reference is a pure substance |
| Brown ink spots line up with red, yellow and blue spots | The brown ink contains all three of those dyes |
| A spot that does not line up with any reference | It is a different substance not among the references |
| A spot that stays on the start line | That substance is insoluble in the solvent used |
Part 3 of 4Calculating Rf
1
Use a ruler to measure from the start line to the centre of the spot: this is the distance moved by the substance.
2
Measure from the same start line to the solvent front: the distance moved by the solvent.
3
Use Rf = distance moved by substance ÷ distance moved by solvent.
4
Give the answer as a number between 0 and 1 with no units.
5
Example: a spot 3 cm up the paper when the solvent has moved 6 cm gives Rf = 3 ÷ 6 = 0.5.
Part 4 of 4Using Rf to identify substances
1Measure Rf
- Measure Rf Calculate the Rf for each unknown spot.
2Compare with reference
- Compare with reference Look up the Rf values of known substances measured with the same solvent.
3Match
- Match A match in Rf means the substances are probably the same.
4Same conditions only
- Same conditions only Rf changes if the solvent changes, so values are only comparable under the same conditions.
Key terms
| Term | Meaning |
|---|---|
| Chromatogram | The paper showing the separated substances after chromatography. |
| Rf value | The retention factor: distance moved by substance divided by distance moved by solvent. |
| Solvent front | The highest point reached by the solvent on the paper. |
| Reference substance | A known sample used for comparison. |
Avoid these mistakes
1
Measuring the substance distance to the top of the spot instead of its centre.
2
Measuring distances from the bottom edge of the paper rather than from the start line.
3
Giving Rf a unit such as cm, or a value greater than 1.
4
Comparing Rf values from two chromatograms run in different solvents.
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