Elements, Compounds and Mixtures

Aim

• Use paper chromatography to separate the components of a mixed food colouring
• Identify the components by comparing R_f values and spot positions against known reference colourings

Apparatus

• 250 cm³ beaker (with a watch glass or piece of clingfilm as a lid)
• Strip of chromatography paper sized to fit inside the beaker
• Five fine capillary tubes (one fresh tube per colouring)
• Four reference food colourings labelled A–D
• One mixed food colouring of unknown composition labelled U
• Wooden spill (or any pencil long enough to span the beaker mouth)
• Paper clip
• Ruler and a sharp pencil

Method

1. Draw a horizontal baseline in pencil about 2 cm in from one short edge of the paper
2. Using a fresh capillary tube for each colouring, place a small spot of each of A, B, C, D and U on the baseline, evenly spaced
   • Keep every spot under about 3 mm wide
   • Label each spot beneath in pencil
3. Add water to the beaker so it forms a shallow pool no deeper than 1 cm
4. Clip the top of the paper to the wooden spill and lay the spill across the beaker mouth so the paper hangs vertically, spotted end down
5. Lower the paper into the beaker so its lower edge dips into the water, with the baseline staying above the water level
6. Cover the beaker (watch glass or clingfilm) to keep the air inside saturated with solvent vapour
7. Let the solvent climb the paper undisturbed until it has risen roughly three-quarters of the paper's length, and always before the front meets the top edge
8. Lift the paper out, immediately mark the solvent front in pencil, and let the paper dry flat
9. Measure (in mm) two distances: baseline → solvent front, and baseline → each spot's centre
10. Calculate R_f for every spot in lanes A–D and U
11. Identify the unknown. A spot in U with the same R_f, and at the same height, as a reference spot is the same substance

Results — suggested table

Sources of error

• Baseline drawn in pen, not pencil. Pen ink itself runs up the paper and contaminates every lane. Always use pencil for baseline and labels
• Sample spots too large. Bands smear during migration and the spot centre is hard to pin down. Use a fresh capillary tube per colouring and keep each spot small
• Solvent above the baseline. Samples dissolve straight into the solvent pool, so no migration occurs. Always check the solvent level before lowering the paper
• Solvent reaches the top edge of the paper. Once the front has gone off the paper, you cannot tell how far it travelled, so no R_f is calculable. Lift the paper out well before the front gets there
• Beaker left uncovered. Solvent evaporates from the paper, slowing the run and shifting the spot positions. Cover the beaker to keep the inside saturated with vapour