GCSE Combined Science (AQA)

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Rf Values

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Chemical analysis Purity, formulations and chromatography

Rf Values

7:50 Separating Mixtures by Chromatography
Spec 4.8.1.3
Previous Formulations Next Tests for Gases
  • Two phases in paper chromatography: stationary phase and mobile phase.
  • Stationary phase: chromatography paper.
  • Mobile phase: solvent.
  • Chromatography paper with a dot of mixture is the stationary phase.
  • Solvent moves during the process, carrying substances up the paper.
  • Separation depends on distribution between mobile and stationary phases.
  • Chromatogram result shows substances separated into spots.
  • Substances more attracted to the mobile phase move further up the paper.
  • Substances more attracted to the stationary phase move less up the paper.
  • RF value: ratio of distance moved by a compound to the distance moved by the solvent.
  • Measure distances from the pencil line (origin) to the center of the spot.
  • RF value calculation: distance traveled by substance divided by distance traveled by solvent.
  • RF values are unitless and range between 0 and 1.
  • Different compounds have different RF values in different solvents.
  • RF values help identify compounds as they are consistent for a given compound in a specific solvent.
  • Example calculation: RF value of 0.39 indicates glucose in the given solvent.

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