A spot of ink on paper, a little solvent, and ten minutes later the mixture has spread into its separate components. Paper chromatography tells you whether a substance is pure, what a mixture contains and, with Rf values, what each component is.
In paper chromatography spots of the samples are placed on a pencil baseline, and the paper stands in a solvent whose level is below the baseline. The solvent moves up the paper and carries each substance a different distance, depending on its solubility in the solvent and its attraction to the paper. A pure substance gives one spot; a mixture gives several. An unknown is identified when its spots match those of known substances at the same heights. Extended: colourless substances are made visible with a locating agent, and each substance has an Rf value = distance moved by the substance ÷ distance moved by the solvent front.
Every detail of the set-up has a reason. Examiners ask for the reason as often as for the step.
| Step | Reason |
|---|---|
| draw the baseline in pencil | pencil (graphite) does not dissolve in the solvent and run up the paper; ink would |
| put small, concentrated spots on the baseline | small spots give sharp, separate spots that do not merge |
| solvent level below the baseline | otherwise the samples dissolve into the solvent in the beaker |
| cover the beaker with a lid | keeps the air saturated with solvent vapour so the solvent does not evaporate from the paper |
| remove the paper before the solvent reaches the top, mark the solvent front | the solvent front is needed to compare and calculate Rf values |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Why is the baseline on a chromatogram drawn in pencil?
A substance gives a single spot on a chromatogram. What does this suggest?
Why must the solvent level be below the baseline?
An unknown dye gives spots at the same heights as dyes P and R. What does the dye contain?
Extended. Rf values let you compare chromatograms run for different times, as long as the solvent and paper are the same.
Measure both distances from the baseline, not from the bottom of the paper, and measure to the centre of the spot. Rf is spot ÷ solvent front, so it can never be bigger than 1: if you get 2.5, you divided the wrong way round. A substance that gives one spot in one solvent may still be a mixture; a second solvent can split it.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
(Extended) A spot moves 3.0 cm and the solvent front moves 7.5 cm from the baseline. What is the Rf value?
(Extended) Why is a locating agent sprayed onto some chromatograms?
(Extended) A substance has Rf = 0.65. The solvent front moves 8.0 cm. How far does the substance move?
One spot from a sample stays on the baseline. What does this show?
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