Aims of Experiment
Investigate how paper chromatography can be used to separate and identify a mixture of food colourings.
In this experiment you will:
- use paper chromatography to separate a mixture of food colourings
- calculate the Rf value for each pure food colouring in the solvent water
- use your Rf values to identify which colourings are in an unknown mixture
Risk Asessment
As a general rule, eye protection (goggles) must be worn for all practicals.
| hazard | possible harm | precaution |
|---|---|---|
| harmful solvents |
skin irritation, breathing problems | avoid skin contact (wear gloves), and ensure adequate ventilation/use a fume cupboard |
This risk assessment is provided as an example only, and you must perform your own risk assessment before doing this experiment.
Apparatus
Each group will need:
250 ml beaker
chromatography paper
a splint or pencil or glass rod/paper clips
selection of different coloured marker pens or felt‐tip pens
solvent (water)
Experiment Set-up
Method
- draw a pencil line across chromatography paper (about 1 cm above the bottom)
- use a pipette, or capillary tube, to add small spots of each ink (A,B, C and D) to the line on the paper, as well as unknown ink (U) (equally spread across the paper)
- place the paper into a container with a suitable solvent (e.g. water) in the bottom - making sure the pencil line is above the water line
- allow the solvent to move through the paper, but remove the chromatogram before it reaches the top
- allow the chromatogram to dry, then measure the distance travelled by each spot and by the solvent
Results and Analysis
Paper Chromatography
| ink | spot colour(s) | distance travelled by spot(s) (mm) | Rf value of spot(s) |
|---|---|---|---|
| A | |||
| B | |||
| C | |||
| D | |||
| U |
Calculate how far the solvent moved, then calculate the Rf value of each spot.
Compare the Rf values/colours of each spots in the different inks - what are the similarities and differences?
Are there any other colourings in mixture U which do not match A–D?
How do you know that a chemical is pure from chromatography results? Are there any other ways that chemists use to ensure that a substance is a pure substance?
Exam Question and Model Answer
A new type of waterproof pen has just been released, and it contains ink that is insoluble in water. A company suspects it may be a copy of their pen's ink. Describe how they should use paper chromatography to identify whether the inks are the same.
[6 marks]
Level 1 (1-2 marks)
Place a spot of the inks on the paper, and place the paper in a beaker with water (not covering the pencil line)
Hang the piece of paper in a beaker and let the water rise through the paper
Level 2 (3-4 marks)
Place a spot of the new pen's ink from the pen on chromatogaphy paper, and place a spot of the original ink on there too.
Hang the piece of paper in a beaker, and add a small amount of solvent (not water, e.g. ethanol) as to not cover the pencil line.
Allow the solvent to rise up the paper.
Level 3 (5-6 marks)
Draw a line (in pencil) across a piece of chromatography paper, about 1 cm above the bottom.
Place a spot of the new pen's ink from the pen on the pencil line, and place a spot of the original ink on the line.
Hang the piece of paper in a beaker, and add a small amount of solvent (not water, e.g. ethanol) as to not cover the pencil line.
Allow the solvent to rise up the paper.
Measure how far the solvent moved, as well as each spot on the chromatogram, and calculate the Rf values of each spot.
If the Rf values match, then the ink is a copy.