Section 1 of 5Mobile and Stationary Phases
Every chromatography technique has two phasesone stays fixed and one carries the sample along.
Stationary phase
- The chromatography paper itself, which holds the sample spots in place and does not move with the solvent.
Mobile phase
- The solvent, usually water or ethanol, which carries dissolved substances up the paper by capillary action.
Solubility
- Substances that dissolve more readily in the solvent spend more time moving with it and therefore travel further up the paper.
Pencil baseline
- The starting line is drawn in pencil because ink contains coloured dyes that would run and mix with the sample spots.
| Chromatogram result | What it suggests |
|---|---|
| One spot only | A pure substance |
| Two or more spots | A mixture of substances |
| Spot at the same height as a known sample | Probably the same substance, if the same solvent was used |
Section 2 of 5Running a Chromatogram
Careful setup keeps the spots separate and gives results that can be compared.
1
Draw a pencil baseline near the bottom of the chromatography paper and place a small spot of each sample on it.
2
Lower the paper into the solvent so that the solvent level sits below the pencil line, not above it.
3
Allow the solvent to rise up the paper by capillary action until it is close to the top of the paper.
4
Remove the paper and mark the solvent front straight away, because the solvent evaporates and the front becomes hard to see.
5
Measure the distance from the baseline to each spot and to the solvent front, then calculate the Rf values.
Section 3 of 5Identifying Substances with Rf
Identifying Substances with Rf
Rf formula
- Rf equals the distance moved by the substance divided by the distance moved by the solvent, both measured from the baseline.
Units
- Rf is a ratio with no units, and its value always lies between 0 and 1 because no substance can travel past the solvent front.
Conditions
- An Rf value can only be compared with a known value when the same solvent and the same type of paper are used.
Identification
- An unknown spot with the same Rf as a known substance, in the same solvent, suggests that the two substances are the same.
Section 4 of 5Worked Example
Question
- The solvent front rises 8.0 cm above the baseline and one dye spot moves 5.2 cm from the baseline.
- Calculate the Rf value of this dye.
- Write the formulaRf = distance moved by substance ÷ distance moved by solvent.What the symbols mean: Rf has no unit and is always between 0 and 1 · both distances are measured from the baseline
- Substitute the valuesRf = 5.2 cm ÷ 8.0 cm.
- Divide and give the answer without units: Rf = 0.65.
Answer Rf = 0.65
Section 5 of 5Exam Checklist & Tips
4.1.3 – Key Points for the Exam
- Chromatography A technique that separates substances by their different movement between a stationary phase and a mobile phase.
- Stationary phase
- The phase that does not move;
- in paper chromatography it is the paper itself.
- The phase that does not move;
- Mobile phase
- The phase that moves and carries the sample;
- in paper chromatography it is the solvent.
- The phase that moves and carries the sample;
- Rf value The ratio of the distance moved by a substance to the distance moved by the solvent front.
- Solvent front The furthest level reached by the solvent on the chromatography paper.
Common Mistakes
- Drawing the baseline in ink, which runs into the chromatogram and mixes with the sample spots.
- Letting the solvent level rise above the baseline, so that the samples wash straight into the solvent.
- Measuring the distance to each spot from the bottom of the paper instead of from the baseline.
- Giving an Rf value with units such as cm, or quoting an Rf value larger than 1.