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2.1.6Elements, compounds and mixtures

Chromatography — OCR Gateway GCSE Combined Science A J250 - Chemistry Revision Notes & Topical Questions

5 min read 12 parts Lesson 7 of 18

📚 OCR Gateway GCSE Combined Science A J250 - Chemistry Hub / Topical Revision
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Part 1 of 10Principle of paper chromatography

Use
  • Use separates substances with different solubilities in a solvent, such as the dyes in black ink.
Spots
  • Spots a pencil line is drawn and spots of sample placed on it.
  • Pencil
    • used because ink would run with the samples.
  • Solvent
    • the paper is lowered into solvent with the pencil line above the surface.
  • Movement
    • the solvent rises by capillary action, carrying substances with it.
  • Separation
    • more soluble substances travel further.

Part 2 of 10Method

1
Draw a pencil line near the bottom of the chromatography paper.
2
Place small spots of each sample on the line, with a known reference alongside.
3
Lower the paper into a container with solvent below the line.
4
Let the solvent rise through the paper.
5
Remove the paper before the solvent reaches the top and mark the solvent front.
6
Dry the paper and examine the chromatogram.

Part 3 of 10Interpreting a chromatogram

Terms
  • Terms paper chromatography is the technique and a chromatogram is the paper showing the result.
Same substance
  • Same substance gives the same pattern and identical spots.
  • Reference spot
    • a known compound used for comparison with unknown spots.
  • Mixture
    • separates into several spots, one for each component.
  • Pure substance
    • gives one spot.
  • Impure sample
    • shows more than one spot.

Part 4 of 10Locating agents

Problem
  • Problem colourless substances such as amino acids and sugars cannot be seen on the paper.
Locating agent
  • Locating agent a chemical that reacts with the sample to give a coloured product.
  • Use
    • apply after the run so the spots become visible to the naked eye.

Part 5 of 10Retention factor (Rf)

Definition
  • Definition the distance moved by a substance divided by the distance moved by the solvent.
Equation
  • Rf = distance travelled by substance ÷ distance travelled by solvent.
    What the symbols mean: Rf has no unit and is always between 0 and 1 · both distances are measured from the baseline
  • Units
    • none, as it is a ratio.
  • Always between 0 and 1
    • a substance cannot move further than the solvent.
  • Constant
    • the Rf of a compound is the same under the same conditions.
  • Identification
    • compare the Rf of an unknown with known values.

Part 6 of 10Worked example of Rf

1
Measure the distance from the baseline to the centre of the spot, e.g. 3.6 cm.
2
Measure the distance from the baseline to the solvent front, e.g. 9.0 cm.
3
Divide: 3.6 ÷ 9.0 = 0.40.
4
Compare 0.40 with the table of known Rf values.
  • Measurement tip
    • always measure from the pencil line, not from the bottom of the paper.

Part 7 of 10Reasons for a good chromatogram

Small spots
  • Small spots large spots overlap and separate poorly.
Solvent level
  • Solvent level keep it below the baseline.
  • Lid
    • cover the container so solvent does not evaporate from the paper.
  • Stop
    • remove the paper before the solvent front reaches the top so the distance can be measured.
  • Drying
    • dry in air before using a locating agent.

Part 8 of 10Identifying components from a chromatogram

ObservationConclusion
Spot level with a reference spotSample contains the reference substance
One spot onlySample is probably pure
Several spotsSample is a mixture
Spot not matching any referenceUnknown substance present
  • Rf use
    • compare calculated Rf values with a data table for the same solvent.

Part 9 of 10Uses of chromatography

Food
  • Food testing whether a colouring is permitted by comparing with known dyes.
Forensics
  • Forensics matching an ink or dye from a sample to a suspect source.
  • Medicine
    • detecting drugs and checking that a product is pure.
  • Colourless samples
    • amino acids from a protein can be separated and sprayed with a locating agent.

Part 10 of 10Worked examples

  1. Explain why the starting line is drawn in pencil. [2]2 marks

  2. A spot travels 4.2 cm while the solvent travels 6.0 cm. Calculate the Rf. [2]2 marks

  3. A chromatogram of a food dye shows two spots. What does this show? [1]1 mark

  4. Why must a locating agent be used for amino acids? [2]2 marks

Key terms

TermMeaning
ChromatographyA method to separate substances based on their different solubilities.
ChromatogramThe paper showing the separated spots.
Solvent frontThe furthest point reached by the solvent.
Locating agentA reagent that makes colourless spots visible.
Rf valueDistance moved by a substance divided by distance moved by solvent.

Avoid these mistakes

1
Having the baseline below the solvent level — samples would wash away.
2
Calculating Rf with the wrong distances.
3
Giving Rf units — it has none.
4
Saying more soluble substances move less far.

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