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Thin-Layer Chromatography (A2 Only)

Chemistry: AQA A Level, topic 6.10.2

Thin-Layer Chromatography

Thin-layer chromatography (TLC) is a type of chromatography used to identify and separate chemicals in a mixture.

1) Setup the apparatus

1) Setup the apparatus

  • Use an aluminium TLC plate coated in silica or alumina.
  • The substances to be separated or identified must be dissolved in organic solvent and dotted along the bottom of the TLC plate.
  • The TLC plate is placed in a closed jar with a small amount of organic solvent inside.
2) Run the experiment

2) Run the experiment

  • The jar is left and the organic solvent in the jar rises up the TLC plate and over the dots.
  • The TLC plate is removed when the solvent front (top of the rising solvent on the plate) nears the top edge of the plate.
  • We can now use this TLC plate to either identify or separate the components in the mixture.
3) Evaluate the results

3) Evaluate the results

  • The mobile phase is the organic solvent (typically apolar).
  • The stationary phase is the TLC plate (typically polar).
  • The more polar substances will interact more with the stationary phase and so will be found lower down the plate.
  • The less polar substances will interact more with the mobile phase and so will rise with the solvent more quickly and will be found higher up the plate.

Separation and Identification

TLC can be used to separate and identify substances in mixtures.

Separation

Separation

  • When separation is required, substances from the mixture can be isolated by cutting up regions with just one spot from the TLC plate.
Rf values

Rf values

  • We use Rf values to identify substances. These values are calculated as:
  • Rf = distance travelled by substance ÷ distance travelled by solvent
  • We can calculate the Rf value for reference substances and the tested substances.
Identification

Identification

  • Rf values depend on the solvent that they are measured in.
  • The identification process is comparing the Rf value of the reference and tested substance.
Calculation of Rf

Calculation of Rf

  • We have a mixture and we suspect that it contains a particular component e.g. propanol.
  • Since Rf values are relative to the conditions, we must only compare values measured under the same conditions.
  • The best way to do this is to add a spot of the suspected component, e.g. propanol, to the same TLC plate as the mixture and run them together, then we can see if the Rf values align.
  • If the Rf values do align, there's significant evidence that the suspected component is in the mixture.

Colourless Chemicals

Some chemicals used in TLC can't be identified because they aren't visible on the TLC plate. The following are methods for locating these colourless chemicals:

Fluorescent dyes

Fluorescent dyes

  • Fluorescent dyes are visible when UV light is shone on them.
  • This allows us to see where the spot of the colourless chemical is and so we can calculate its Rf value.
Iodine

Iodine

  • Iodine turns the colourless spots purple.
  • This is another method that allows us to see where the spot of the colourless chemical is so that we can calculate its Rf value.

Test yourself on Thin-Layer Chromatography (A2 Only)

  • When do you stop the TLC?

  • A component has a great affinity for the mobile phase.
    Where will it be found on the TLC plate?

  • What does TLC stand for?

  • What are Rf values dependent on?

  • Adding fluorescent dyes alone is not enough to locate chemicals, what else is required?

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