Locating agents

• For chromatography to be useful, the chemist needs to be able to see the components move up the paper

  • This is not the case for colourless substances such as amino acids or sugars

• Locating agents can be used to see the spots

  • These are substances which react with the sample and produce a visible / coloured spot for the product(s) 

• The chromatogram is treated with the agent after the chromatography run has been carried out, making the sample runs visible to the naked eye

Retention factor (Rf) values

• Rf values are used to identify the components of mixtures

• The Rf value of a particular compound is always the same

  • However, it does depend on the solvent used

  • If the solvent is changed then the Rf value changes

• Calculating the Rf value allows chemists to identify unknown substances because it can be compared with the Rf values of known substances under the same conditions

• The retention factor, Rf, is calculated by the equation:

Rf =  distance moved by substance/distance moved by solvent

• The Rf value:

  • Is a ratio

  • Has no units

  • Will always be less than 1