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CHEMISTRY · Stage 8 · Chapter 8: Pure Substances and Solutions

Chemistry 8.3 Chromatography — Separating Dissolved Pigments and Chromatograms

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8.3 Chromatography — Separating Dissolved Pigments and Chromatograms — Alhussan International Academy — Chemistry (Stages 7–9) notes: Describe how paper chromatography separates and identifies substances in a mixture and interpret a chromatogram.Chromatography = a method used to …

  • Start line above the solvent
  • Pure substance
  • Same substance
  • Green felt-tip ink
  • Uses of paper chromatography
  • Other types

Part 1 of 5What chromatography is

1Chromatography1 point
= a method used to separate and identify the substances in a mixture, and to show whether a substance is pure.
  • It works if all the substances in the mixture dissolve in one solvent.
2Principle1 point
— the solvent (mobile phase) moves through the paper (stationary phase) and carries the substances at different speeds.
  • Substances that dissolve better in the solvent and stick less to the paper move further.
3Chromatogram
= the paper with the separated spots.

Part 2 of 5Method for paper chromatography

  1. Draw a pencil line about 2 cm from the bottom of a strip of chromatography paper.
  2. Place a small spot of each ink or dye on the line, keeping the spots small and well spaced, and include known reference substances if available.
  3. Stand the paper in a beaker with a shallow layer of solvent such as water, so the solvent level is below the pencil line.
  4. Cover the beaker to keep the air saturated with solvent vapour, and let the solvent rise up the paper by capillary action.
  5. Remove the paper just before the solvent reaches the top, mark the solvent front with a pencil and let it dry.
  • Pencil — pencil lead does not dissolve in the solvent, whereas ink would run.
  • Start line above the solvent — if the line is under the solvent, the spots would dissolve into it.

Part 3 of 5Reading a chromatogram

  • Pure substance — it gives one spot, whereas a mixture gives more than one.
  • Same substance — spots at the same height with the same colour are likely to be the same substance, so unknowns are compared with known substances.
  • Green felt-tip ink — it is a mixture of two dyes, yellow and blue, and the blue dye travels further.
  • Sweets — a chromatogram of five sweets showed the brown dye is a mixture of red, yellow and blue dyes, while the red dye makes one spot and is pure.
  • Rf value = distance moved by the substance ÷ distance moved by the solvent.
    • Rf values are between 0 and 1 and are the same for a given substance in the same solvent and paper.
    • A spot 3.0 cm from the start line with the solvent front at 6.0 cm has Rf = 0.50.

Part 4 of 5Uses and limitations

  • Uses of paper chromatography — separating substances in inks, plants and sweets.
  • Other types — they all separate mixtures and are used to identify substances, measure amounts and check purity.
    • Scientists and pharmaceutical companies use chromatography in making some COVID-19 vaccines.
    • Airport security officers use it to test clothes and luggage for explosives.
    • Scientists use it to identify food nutrients.
    • It is also used to analyse drugs in sport, identify pollutants in water and match inks in crime investigations.
  • Insoluble substances — they will not move, so another solvent such as ethanol may be needed.
  • Fair testing — use the same type of paper, the same solvent and similar spot sizes.
  • Modern methods — gas chromatography and mass spectrometry use the same basic principle.

Part 5 of 5Chromatography enquiry: vitamin A in cassava

  • Problem — in Africa and Asia up to half a million children go blind each year, and one cause is a lack of vitamin A.
  • Scientific question — Steve Adewusi (Nigeria) and Howard Bradbury (Australia) asked how much vitamin A is in the roots and leaves of different types of cassava.
  • Evidence — they used chromatography to measure vitamin A in yellow and white cassava roots and in dark green and light green leaves.
    • They also used secondary data on the vitamin A needed to be healthy from the World Health Organization (WHO), which they trusted because WHO is well respected.
  • Conclusion — dark green leaves have more vitamin A than light green leaves.
    • Eating yellow cassava root and cassava leaves can provide enough vitamin A, but white roots alone cannot.
  • Recommendation — to prevent blindness, children should eat yellow cassava, not white, and also eat cassava leaves.

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