Chromatography (DP IB Chemistry): Revision Note

Chromatography

What is chromatography?

• Chromatography is a separation technique used to analyse the components of mixtures

• It works by passing a liquid mobile phase over a solid stationary phase

• The components in the mixture move at different rates depending on:

  • Their solubility in the mobile phase

  • Their attraction to the stationary phase

• Separation occurs due to differences in intermolecular forces between the sample and each phase

What is paper chromatography?

• In paper chromatography, a drop of the mixture is placed on chromatography paper

  • The paper acts as the stationary phase

  • The mobile phase is a solvent (e.g. water or ethanol)

• The components of the mixture move at different speeds up the paper, depending on:

  • Their solubility in the solvent

  • Their attraction to the paper

• The paper is made of cellulose, which contains polar –OH groups

  • These form intermolecular forces (e.g. hydrogen bonds) with polar molecules in the mixture

• Polar substances:

  • Interact more strongly with the paper

  • Travel slower

  • Do not travel as far

• Nonpolar substances:

  • Interact less with the paper

    • They are more soluble in the solvent

  • Travel quicker

  • Travel further

Steps in paper chromatography

1. Draw a pencil baseline near the bottom of the chromatography paper

   • Pencil is used because ink would run during the experiment

2. Place small spots of sample on the baseline

3. Suspend the paper in a solvent with the baseline above the solvent level

4. Allow the solvent to rise by capillary action, carrying components at different rates

5. Remove the paper before the solvent reaches the top and mark the solvent front

6. Let the paper dry

   • The chromatogram will show separated spots for each component

What is thin layer chromatography (TLC)?

• TLC works on the same principle as paper chromatography

• The difference is in the stationary phase:

• Instead of paper, TLC uses:

  • A thin layer of an inert solid, such as silica or alumina

  • On a flat surface, such as glass or plastic

• The mobile phase is still a solvent

• The surface of the stationary phase contains –OH groups

  • These can form intermolecular forces (such as hydrogen bonds) with sample components

• As the solvent moves up the plate, components of the mixture are adsorbed to the surface to different extents

  • This affects how far each component travels

  • The result is a separation based on relative attractions to the stationary and mobile phases

What are retardation factors (R_f) values?

• The Rf (retardation factor) value describes how far a substance travels relative to the solvent front

• It reflects the balance between:

  • The substance’s solubility in the mobile phase

  • Its attraction to the stationary phase

• Substances that are more soluble in the solvent and interact less with the stationary phase:

  • Travel further

    • Have a higher R_f value

• R_f is calculated using the formula:

• R_f values can be used alongside other analytical data to deduce the composition of mixtures