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.
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.
- Retention factor = distance moved by compound ÷ distance moved by solvent.
- The Rf value is a ratio and therefore has no units.
Using Rf values to identify components of a mixture