Paper Chromatography

Specification Point 2.9:
  • Describe paper chromatography as the separation of mixtures of soluble substances by running a solvent (mobile phase) through the mixture on the paper (the paper contains the stationary phase), which causes the substances to move at different rates over the paper.
  • All chromatography techniques use two phases called the mobile phase and the stationary phase.
  • In paper chromatography:
    • The mobile phase is the solvent in which the sample molecules can move, which in paper chromatography is liquid e.g. water or ethanol.
    • The stationary phase in paper chromatography is the actual chromatography paper itself.
  • The components in the mixture separate as the mobile phase travels over the stationary phase.
  • This occurs due to differences in the solubility of each component in the mobile phase which affects how far each component can travel.
  • Those components with higher solubility will travel further than the others. 
  • This is because they spend more time in the mobile phase and are thus carried further up the paper than the less soluble components.

Interpreting Chromatograms

Specification Point 2.10:
  • Interpret a paper chromatogram:
    a) to distinguish between pure and impure substances
    b) to identify substances by comparison with known substances
    c) to identify substances by calculation and use of Rf values

Pure and Impure Substances

  • Pure substances will produce only one spot on the chromatogram.
  • If two or more substances are the same, they will produce identical chromatograms.
  • If the substance is a mixture, it will separate on the paper to show all the different components as separate
  • An impure substance therefore will product a chromatogram with more than one spot.

Comparison with Known Substances

  • Unknown substances can be identified by comparison with chromatograms of known substances.
  • Paper chromatography is carried out on the unknown sample and a sample of a known compound.
  • This is done simultaneously on the same paper using the same solvent.
  • If the resulting spot(s) are at the same height then the substance being tested is the same as the known substance.
  • The known substances used to compare are called reference materials.

Chromatography Reference Material, Edexcel GCSE Chemistry

Comparison of an unknown sample using known reference materials 

Retention factor (Rf) values

  • These values are used to identify the components of mixtures.
  • The Rf value of a particular compound is always the same.
  • Calculating the Rf value allows chemists to identify unknown substances because it can be compared with Rf values of known substances under the same conditions.

Calculation

  • Retention factor = distance moved by compound  ÷ distance moved by solvent.
  • The Rf value is a ratio and therefore has no units.

Rf Values Chromatography, Edexcel GCSE Chemistry

Using Rf values to identify components of a mixture

Core Practical: Investigating Inks

Specification Point 2.11:
  • Investigate the composition of inks using simple distillation and paper chromatography.

Aim: 

  • To investigate the composition of ink using simple distillation and paper chromatography

Materials:

  • Full distillation apparatus
  • Ink samples
  • Beaker with lid
  • Suitable solvents
  • Pencil, ruler and chromatography paper
Apparatus Simple Distillation, Edexcel GCSE Chemistry

Set-up simple distillation

Apparatus Chromatography, Edexcel GCSE Chemistry

Set-up for chromatography

Simple Distillation

  • Ink consists of a solvent which has different dyes dissolved in it.
  • The investigation must thus include analysis of both the solvent and the dyes used.
  • The solvent must first be separated which can be done by simple distillation.
  • Solvents tend to have low boiling points than the dyes which tend to be more viscous so it will evaporate first.
  • Add anti-bumping granules and heat gently so as not to go past the boiling point. 
  • Record the temperature of boiling point.

Paper Chromatography

  • This will separate the different colours in the ink so the number of dyes used can be determined.
  • Make sure the base line is drawn in pencil and place a lid on the apparatus if the mobile phase being used is volatile.
  • Rf values can then be calculated and compared with reference values.
  • Alternatively further chromatography runs can be done with known dyes and compared to the sample.

Analysis of results:

  • The boiling point of the solvent can be used this to determine its composition by comparing it to reference tablesg. ethanol boils at 78ºC so if the solvent evaporates at 78ºC it is likely to be ethanol.
  • Rf values can be calculated and compared with reference values.
  • Alternatively further chromatography runs can be done with known dyes and compared to the sample.

Conclusion:

  • Both techniques are useful for separation and identification of compounds.

Edexcel GCSE Chemistry Notes

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.