Required Practical 06: Chromatography

Question 1

What is the method used when identifying substances in paper chromatography? (7)

Answer 1

1. Draw pencil line ~2cm away from bottom of chromatography paper.
2. Plot five pencils spots at equal distances on the pencil line - at-least 1cm clear at each side.
3. Mark, and label underneath, each spot with the different food colourings given.
4. Pour water at a depth of 1cm into a beaker.
5. Attach paper to a glass/wood rod and lower paper into solvent - ink should not be submerged, paper should not touch sides, beaker should not be moved
6. Remove paper when solvent has moved 3/4 up, mark the solvent line and hang the paper dry.
7. The dots in the unknown substance that lines up with the dots in the other colours are the same.

Question 2

How do you identify the chemicals in the colours on a chromatography paper? (4)

Answer 2

1. Measure distance from the pencil line to the centre of each spot.
2. Measure distance moved from water to pencil line.
3. Use the equation: Rf = distance moved by chemical / distance moved by solvent
4. Use database to identify chemical based on Rf value

Question 3

What are the drawbacks to using Rf values to identify chemicals in paper chromatography? (2)

Answer 3

• Some different substances have the same Rf value so experiment may need to be repeated with different solvents to narrow down possibilities
• If substance never analysed before, Rf value will not be on database - further analysis needed.