1.2.5Principles of Chemistry
Practical- Investigating Paper Chromatography — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 13 of 50
Part 1 of 5Method
Paper chromatography separates the dyes in a sample as the solvent carries them up the paper.
1
Draw a pencil line about 2 cm from the bottom of the paper, then place small, separate spots of each known colouring and the unknown mixture along it.
2
Pour a shallow layer of water, no deeper than about 1 cm, into the beaker so that the spots stay above the solvent level.
3
Clip the top of the paper to a wooden splint so that its bottom edge dips into the solvent, then cover the beaker with a watch glass.
4
Allow the solvent to travel about three quarters of the way up the paper, then remove the paper and mark the solvent front in pencil straight away.
5
Measure the distance from the baseline to the solvent front and to the centre of each spot, then calculate the Rf value of each colouring.
Part 2 of 5Apparatus and Controls
Pencil baseline
- Drawn in pencil because ink is a mixture of dyes that would move with the solvent and confuse the results.
Capillary tubes
- A different tube is used for each sample so that colourings do not contaminate one another and each spot stays small.
Covered beaker
- A lid keeps the air above the solvent saturated with its vapour, so the solvent moves evenly up the paper.
Solvent level
- The solvent must stay below the baseline, so the samples do not dissolve straight into the solvent in the beaker.
Part 3 of 5Sources of Error
Sources of Error
Large spots
- Large sample spots spread out and merge, which makes the spot centres hard to measure and reduces the accuracy of the Rf values.
Solvent runs off
- If the solvent reaches the top of the paper, the distance it travelled cannot be measured, so the Rf values cannot be calculated.
Ink baseline
- An ink line would dissolve and run up with the solvent, producing extra spots that look like components of the sample.
Evaporation
- Solvent evaporating from an uncovered beaker slows the movement of the spots and changes their final positions.
Part 4 of 5Worked Example
Question
- In this experiment the solvent front moved 60 mm from the baseline and food colouring A moved 24 mm.
- Calculate the Rf value of colouring A.
- Rf = distance moved by spot ÷ distance moved by solvent.What the symbols mean: Rf has no unit and is always between 0 and 1 · both distances are measured from the baseline
- Rf = 24 ÷ 60.
- Rf = 0.40.
Answer
- The Rf value of colouring A is 0.40.
- An unknown spot with the same Rf in the same solvent is probably colouring A.
Part 5 of 5Exam Checklist & Tips
1.2.5 – Key Points for the Exam
- Baseline The pencil line on which the samples are placed before the chromatography run.
- Solvent front The furthest point reached by the solvent on the paper, marked after the run.
- Capillary action The movement of a liquid up a porous material such as paper, which carries the solvent and dissolved dyes upward.
- Rf value The distance moved by a spot divided by the distance moved by the solvent.
- Chromatography A separation technique in which components of a mixture travel at different rates through a medium.
Common Mistakes
- Drawing the starting line in ink, which runs with the solvent and produces false spots.
- Letting the solvent level rise above the starting line, so the samples dissolve directly into the beaker.
- Leaving the paper in the solvent until the front runs off the top, which prevents calculation of the Rf values.
- Using a large sample spot, which causes smearing and an inaccurate measurement of the spot position.
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