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3.3.2Additional lessons

Separating Photosynthetic Pigments — IB Biology SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 57 of 142

📚 IB Biology SL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 6Method

1
Grind leaves with a little solvent such as propanone to extract the pigments.
2
Draw a pencil line near the bottom of the chromatography paper and apply a small, concentrated spot of extract on it.
3
Stand the paper in a container with solvent below the pencil line and cover it.
4
Remove the paper before the solvent reaches the top and mark the solvent front straight away.
5
Measure the distance moved by each pigment spot and by the solvent front, and calculate Rf.

Part 2 of 6Pigments

Carotene
  • Carotene Orange or yellow and the most soluble, so it travels furthest up the paper.
Xanthophyll
  • Xanthophyll Yellow pigment that travels a shorter distance than carotene.
Chlorophyll a
  • Chlorophyll a Blue-green pigment that travels less far than the xanthophylls.
Chlorophyll b
  • Chlorophyll b Yellow-green pigment which moves the least of the main pigments.
Why they separate
  • Why they separate Each pigment differs in solubility in the solvent and attraction to the paper, so they move at different rates.

Part 3 of 6Rf and Good Practice

Rf and Good Practice
Rf value
  • Rf value Rf equals distance moved by the pigment divided by distance moved by the solvent front.
Range
  • Range Rf values are always between 0 and 1 and have no units.
Pencil
  • Pencil Use pencil for the origin line, because ink would dissolve and move with the solvent.
Safety
  • Safety Solvents are flammable and harmful, so work in a ventilated area away from flames.

Part 4 of 6Interpreting a Chromatogram

Interpreting a Chromatogram
Order
  • Order The pigment with the highest Rf is the most soluble in the solvent and travels furthest.
Spots
  • Spots The number of spots shows how many different pigments are present in the extract.
Repeat
  • Repeat Rf values depend on the solvent used, so only compare results from the same solvent.

Part 5 of 6Worked Example

Question

  • A yellow pigment travels 6.3 cm while the solvent front travels 9.0 cm.
  • Calculate the Rf value.

  1. Rf = distance moved by pigment ÷ distance moved by solvent front.
  2. Rf = 6.3 ÷ 9.0 = 0.70.

Answer Rf = 0.70 (example data).

Part 6 of 6Exam Checklist & Tips

3.3.2 – Key Points for the Exam

  • Chromatography A technique for separating mixtures by different rates of movement in a solvent.
  • Rf value Ratio of the distance moved by a pigment to the distance moved by the solvent front.
  • Solvent front The furthest point reached by the solvent on the paper.
  • Origin The line on which the sample is first applied.

Common Mistakes

  • Drawing the origin line in ink, which moves up the paper.
  • Letting the solvent run off the top so the solvent front cannot be measured.
  • Placing the origin line below the surface of the solvent.
  • Giving Rf values with units or values greater than 1.

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