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2.2.11Additional lessons

Chromatography — IB Chemistry HL Revision Notes & Topical Questions

5 min read 6 parts Lesson 35 of 172

📚 IB Chemistry HL 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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Section 1 of 6Principle of Separation

Chromatography separates substances by their different attractions to a stationary and a mobile phase.

Stationary phase
  • Stationary phase The fixed material, such as paper or a silica layer, onto which substances can adsorb or dissolve.
Mobile phase
  • Mobile phase The solvent that moves through the stationary phase and carries the mixture with it.
Partition
  • Partition Each component divides between the two phases according to its attractions to each.
Result
  • Result Components with a greater attraction for the mobile phase travel further.

Section 2 of 6Carrying Out the Experiment

1
Draw a pencil baseline near the bottom of the plate or paper and place small spots of each sample on it.
2
Stand the plate in a container with a shallow layer of solvent below the baseline.
3
The solvent rises by capillary action, carrying the components upwards at different rates.
4
Remove the plate before the solvent reaches the top and mark the solvent front at once.
5
Make colourless spots visible with ultraviolet light or a locating agent, such as ninhydrin for amino acids.

Section 3 of 6Polarity and Movement

Polarity and Movement
Polar stationary phase
  • Polar stationary phase Paper and silica are polar, so polar components stick strongly and move shorter distances.
Non-polar components
  • Non-polar components These have little attraction to the stationary phase and travel further with the solvent.
Changing the solvent
  • Changing the solvent A more polar solvent carries polar components further up the plate.
Identification
  • Identification Matching Rf values or spot positions with known samples identifies the components.

Section 4 of 6The Rf Value

Definition
  • Definition Rf = distance moved by the component ÷ distance moved by the solvent front.
Range
  • Range Rf is a ratio, so it has no units and always lies between 0 and 1.
Reproducibility
  • Reproducibility Rf is constant for a substance only in the same solvent, stationary phase and temperature.
Pure substance
  • Pure substance A pure sample gives one spot, while a mixture gives several.

Section 5 of 6Worked Example

Question

  • A spot travels 3.6 cm while the solvent front travels 8.0 cm.
  • Calculate its Rf.

  1. Rf = distance moved by spot ÷ distance moved by solvent front.
  2. Rf = 3.6 cm ÷ 8.0 cm.

Answer Rf = 0.45, with no units.

Section 6 of 6Exam Checklist & Tips

2.2.11 – Key Points for the Exam

  • Stationary phase The solid or supported liquid that does not move.
  • Mobile phase The solvent that moves through the stationary phase.
  • Rf Retention factor: distance moved by the spot divided by distance moved by the solvent front.
  • Locating agent A chemical sprayed on a chromatogram to make colourless spots visible.

Common Mistakes

  • Drawing the baseline in ink, which itself separates into dyes.
  • Measuring the solvent front after it has dried, so its position is lost.
  • Giving Rf units or a value greater than 1.
  • Placing the starting spots below the solvent level.

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