Paper Chromatography (CIE IGCSE Chemistry: Co-ordinated Sciences (Double Award))

• Chromatography is used to separate substances and provide information to help identify them
• The components have different solubilities in a given solvent
  • E.g. Different coloured inks that have been mixed to make black ink
• A pencil line is drawn on chromatography paper and spots of the sample are placed on it
  • A pencil is used for this as ink would run into the chromatogram along with the samples
• The paper is then lowered into the solvent container, making sure that the pencil line sits above the level of the solvent so the samples don’t wash into the solvent container
  • The solvent used is usually water but it can be other substances such as ethanol
• The solvent travels up the paper by capillary action, taking some of the coloured substances with it
• Different substances have different solubilities so they will travel at different rates, causing the substances to spread apart
  • Those substances with higher solubility will travel further than the others

How to carry out chromatography

The pigments in ink can be analysed using paper chromatography

• 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

Example chromatogram results

The brown ink has separated showing a spot of red ink, blue ink and yellow ink

• 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

Extended tier only

• 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:

R_f =  

• The R_f value:
  • Is a ratio
  • Has no units
  • Will always be less than 1