🔧 Apparatus
A beaker, a strip of chromatography paper, a pencil, a ruler, capillary tubes, the coloured samples, a solvent such as water, and something to hold the paper up, like a glass rod or a clip.
Purity and Chromatography » Required Practical: Chromatography
What you'll learn this session
Study time: 30 minutes
AQA spec: Required practical 6 (8.2.6)
- How to set up and run paper chromatography step by step
- Why the start line is drawn in pencil and the solvent must stay below it
- How to measure distances and calculate Rf values
- The mistakes examiners see most often
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Unlock This CourseThe aim of the practical
In this practical you use paper chromatography to separate coloured substances and to tell the difference between them. You then calculate Rf values from your results. The ideas behind the method are in the lesson on Chromatography. Here we focus on doing it properly, getting good results and answering exam questions on it.
A typical task is to test several unknown food colourings, or inks, and decide which ones are made of the same dyes as a known sample.
Key terms:
- Baseline: the line you draw near the bottom of the paper, where the spots are placed.
- Solvent front: the furthest point the solvent has reached up the paper.
The method
Pencil for the start line, solvent below it - get the setup right and the dyes separate cleanly up the paper
⚠ Safety
Wear eye protection. If the solvent is flammable, keep it away from flames. Clear up spills quickly. Do not taste any of the samples, even if they are food colourings.
Follow these steps in order:
- Pour a small amount of solvent into the beaker. It should be about 1 cm deep.
- Use a pencil and ruler to draw a straight baseline about 2 cm from the bottom of the paper.
- Mark a small cross on the baseline for each sample, spaced evenly apart. Write the name of each sample in pencil at the top of the paper, above its cross.
- Use a clean capillary tube to place a tiny, concentrated spot of each sample on its cross. Use a different tube for each sample.
- Let the spots dry. If you want a stronger spot, add another drop on top of the first once it is dry.
- Hang the paper in the beaker so the bottom edge dips into the solvent. The baseline and the spots must be above the solvent level.
- Cover the beaker with a lid or cling film so the solvent does not evaporate.
- Leave it alone while the solvent soaks up the paper. Do not move the beaker.
- Take the paper out just before the solvent reaches the top. Straight away, draw a pencil line across the paper at the solvent front.
- Leave the paper to dry. This is your chromatogram.
Why each step matters
✏ Pencil, not pen
Pencil is made of graphite, which does not dissolve in the solvent. Pen ink is often a mixture of dyes. It would run up the paper with your samples and give extra spots that are not part of your results.
💧 Baseline above the solvent
If the spots start under the solvent, the dyes dissolve into the beaker instead of travelling up the paper. You would see nothing on the paper.
🔵 Small spots
Large spots spread out and overlap. Tiny spots give clear, separate dots that are easy to measure.
The lid matters too. It keeps the air in the beaker full of solvent vapour, so the solvent moves up the paper evenly. Marking the solvent front at once matters because the solvent keeps evaporating from the paper and the line soon disappears.
Variables
- Independent variable: the sample being tested.
- Dependent variable: the distance each substance moves up the paper, which gives the Rf value.
- Control variables: the type of paper, the solvent, the starting position of the spots, the size of the spots, and the time the paper is left in the beaker.
Changing the solvent changes the Rf values. So you can only compare results that used the same solvent.
Measuring and calculating Rf
Measure from the pencil baseline, not the paper edge, then Rf = distance moved by spot ÷ distance moved by solvent
Use a ruler and measure in millimetres or centimetres. Always measure from the baseline, not from the bottom edge of the paper.
- Measure from the baseline to the solvent front. This is the distance moved by the solvent.
- For each spot, measure from the baseline to the centre of the spot. This is the distance moved by that substance.
Then use the equation you met earlier: Rf = distance moved by substance ÷ distance moved by solvent. Both distances must be in the same units. Rf has no units and is always less than 1.
Worked example
A student tests a purple sweet colouring. The solvent front is 8.0 cm from the baseline. The pink spot is 5.6 cm from the baseline and the blue spot is 2.0 cm from the baseline.
Pink: Rf = 5.6 ÷ 8.0 = 0.70
Blue: Rf = 2.0 ÷ 8.0 = 0.25
So the purple colouring is a mixture of two dyes.
Telling the difference between substances
To compare samples, run them side by side on the same paper with the same solvent. If two spots travel the same distance, they could be the same substance. If they end up at different heights, they are different substances. If you run a known dye next to your unknown sample, you can check by eye whether the unknown contains it, then support this with the Rf values.
Worked example
Three sweets, A, B and C, are tested next to a known orange dye. The solvent front is 10.0 cm. The orange dye has a spot at 6.5 cm. Sweet A has a spot at 6.5 cm. Sweet B has spots at 3.0 cm and 7.5 cm. Sweet C has spots at 6.5 cm and 2.0 cm.
The Rf of the orange dye is 6.5 ÷ 10.0 = 0.65. Sweets A and C both have a spot at 0.65, so they contain the orange dye. Sweet B has no spot at 0.65, so it does not.
Improving your results
- Make the spots as small as you can, and use a clean tube each time.
- Use a ruler for the baseline so the line is straight.
- Do not touch the paper where the substances run, as grease from fingers can affect the result.
- Stop the experiment before the solvent reaches the top edge. You need to mark the solvent front.
- Repeat the experiment and compare the Rf values to check they are close.
Common mistakes
Drawing the baseline in pen, so the ink runs. Putting the spots below the solvent level. Measuring to the top of a spot instead of the centre. Measuring from the edge of the paper instead of the baseline. Forgetting to mark the solvent front before it dries. Using different solvents and then comparing Rf values.
Exam-style question
A student uses paper chromatography to find out whether a green ink contains the dyes in a known sample, dye P. (a) Explain why the start line is drawn in pencil. (b) The student sets up the beaker so that the start line is under the solvent. Explain what would go wrong. (c) The solvent front moves 12.0 cm and a spot moves 9.0 cm. Calculate the Rf value.
Model answer
(a) Pencil does not dissolve in the solvent, so it does not run up the paper and make extra spots. Pen ink would.
(b) The dyes would dissolve in the solvent in the beaker instead of travelling up the paper, so there would be no separation on the paper.
(c) Rf = 9.0 ÷ 12.0 = 0.75
Exam tip
Show your working for Rf: write the equation, put the numbers in, then give the answer. Marks are often given for the method even if the final number is wrong.