Locating Agents & Rf Values (Cambridge O Level Chemistry)

• For chromatography to be useful the chemist needs to be able to see the components move up the paper, which is not the case for colourless substances such as amino acids or sugars
• Locating agents are substances which react with the sample and produce a coloured product which is then visible
• The chromatogram is treated with the agent after the chromatography run has been carried out, making the sample runs visible to the naked eye

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

Calculation

• The formula to calculate the retention factor is:

                       

• The R_f value is a ratio and therefore has no units

Using R_f values to identify components of a mixture