Paper chromatography (Required Practical)
Paper chromatography separates substances dissolved in a solvent, based on how strongly each is attracted to the stationary phase (paper) compared with the mobile phase (solvent).
How chromatography works
- Two phases:
- Stationary phase: chromatography paper (cellulose).
- Mobile phase: solvent (water, ethanol, etc.) that moves up the paper.
- A spot of mixture is placed near the bottom of the paper.
- The paper is placed in solvent (below the spot line).
- Solvent rises through the paper by capillary action.
- Each component distributes between the paper and the solvent. Components with stronger attraction to solvent rise further; those with stronger attraction to paper stay lower.
Method (required practical)
- Draw a horizontal pencil line ~1 cm from the bottom (pencil doesn't dissolve).
- Place small spots of mixture (and known reference compounds, if identifying) on the line.
- Pour solvent into a beaker so the level is below the spots.
- Place the paper in the beaker; cover with lid (so vapour saturates the air).
- Allow solvent to rise close to the top.
- Remove paper, mark the solvent front in pencil before drying.
- Allow paper to dry, then measure distances.
Calculating Rf values
Rf = distance moved by spot ÷ distance moved by solvent front
Rf is always between 0 and 1.
- Rf is constant for a given solvent / stationary phase / temperature.
- Compare Rf of unknown spots with reference Rfs to identify components.
✦Worked example
A spot moves 4.5 cm; solvent front 6.0 cm. Rf = 4.5/6.0 = 0.75.
If a database lists pure caffeine with Rf = 0.75 in the same solvent, the unknown likely contains caffeine.
Pure vs impure substances
- Pure substance: shows ONE spot.
- Impure substance / mixture: shows multiple spots.
This is a quick visual test of purity for water-soluble dyes (food colourings, ink).
⚠Common mistakes
- Drawing baseline in pen — ink dissolves in solvent and contaminates the experiment.
- Spots below solvent level at start — they dissolve into the solvent rather than running up the paper.
- Letting solvent run off the top — can't measure solvent front. Mark it before drying.
- Not covering the beaker — solvent evaporates from the paper, distorting Rf.
- Mixing up which phase is stationary — stationary = paper; mobile = solvent.
Links
Builds on C1.1 (separation techniques). Used in C8.1 (purity), forensic and food testing.
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