Interpreting Chromatograms (Edexcel IGCSE Chemistry (Modular))

Interpreting chromatograms

• We can use a chromatogram to compare the substances present in a mixture to known substances and make assumptions

  • Pure substances will produce only one spot on the chromatogram

  • Impure substances will produce more than 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 spots

• It is common practice to include a known compound as a reference spot

  • This can help match up to an unknown spot or set of spots in order to identify it

Interpreting chromatograms

Diagram showing the analysis of a mixture and pure substances using chromatography

• We can draw several conclusions from this chromatogram:

  • The brown ink is a mixture as there are three dots

  • Red, yellow and blue are pure as there is only one dot for each 

  • The brown ink contains red, blue and yellow as the dots are in line with one another horizontally

Rf Values

• R_f values are used to identify the components of mixtures

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

  • However, it does depend on the solvent used

  • If the solvent is changed then the R_f value changes

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

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

• The R_f value:

  • Is a ratio

  • Has no units

  • Will always be less than