12.2.1Experimental Techniques and Chemical Analysis
Paper Chromatography
5 min read 6 parts Lesson 5 of 9 2 quizzes
Section 1 of 6How chromatography separates colours
1Solvent rises
- Solvent rises The solvent soaks up the paper by capillary action and carries the coloured substances with it.
2Different solubilities
- Different solubilities Each substance dissolves in the solvent to a different extent and is held on the paper to a different extent.
3Different distances
- Different distances The more soluble a substance, the further it travels up the paper, so the mixture spreads into separate spots.
Section 2 of 6Running a chromatogram
1
Draw a start line in pencil near the bottom of the chromatography paper. Pencil is used because ink would dissolve and run up the paper with the samples.
2
Place small spots of each sample, and of any known reference substances, on the line, keeping them well apart.
3
Stand the paper in a container with a shallow layer of solvent, usually water but sometimes ethanol. The start line must be above the solvent level so the spots do not wash off.
4
Leave the solvent to rise up the paper, then remove the paper before the solvent reaches the top and mark the solvent front in pencil.
5
Let the paper dry and compare the positions of the spots.
Section 3 of 6Interpreting a chromatogram
Pure or mixture?
- Pure or mixture? A pure substance gives a single spot.
- A mixture gives two or more spots.
Same substance?
- Same substance? Identical substances give identical chromatograms.
- A spot from an unknown that lines up horizontally with a reference spot shows the same substance is present.
Example
- Example A brown ink that separates into red, yellow and blue spots is a mixture of three dyes, while each of those dyes alone gives one spot.
Section 4 of 6Retention factor, Rf (extended / HT only)
| Point | Detail |
|---|---|
| Equation | Rf = distance moved by substance ÷ distance moved by solvent |
| Both distances | Measured from the start line; the substance distance is taken to the centre of the spot |
| Value | A ratio with no units, always less than 1 |
| Use | Compare with known Rf values under the same conditions to identify a substance |
| Solvent | Changing the solvent changes the Rf value |
| Worked example | A spot moves 3.0 cm while the solvent moves 6.0 cm, so Rf = 3.0 ÷ 6.0 = 0.50 |
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Chromatography | A technique that separates soluble coloured substances by how far they are carried by a solvent. |
| Solvent front | The highest point reached by the solvent on the paper. |
| Capillary action | The rise of a liquid through narrow gaps in the paper. |
| Reference spot | A known substance placed on the chromatogram for comparison. |
| Rf value | The ratio of the distance moved by a substance to the distance moved by the solvent. |
Section 6 of 6Exam technique
1
Drawing the start line in ink or biro. It must be pencil so that it does not move up the paper.
2
Placing the start line below the solvent level, which washes the samples into the solvent.
3
Measuring Rf from the bottom of the paper instead of from the start line.
4
Giving Rf units or a value greater than 1. It is a ratio and is always below 1.
5
Assuming two spots at different heights are the same substance. Only spots at the same height in the same solvent match.