Chromatography (AQA GCSE Chemistry: Combined Science)

What is chromatography used for?

• 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) and different adhesion to the supporting medium - usually paper
• A pencil line is drawn on chromatography paper and spots of the sample are placed on it
• Pencil is used for this as ink would run into the chromatogram along with the sample
  
  • 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 travels up the paper by capillary action, taking some of the coloured substances with it
• Different substances have different solubilities so will travel at different rates, causing the substances to spread apart
• Those substances with higher solubility will travel further than the others

Chromatography diagram

The pigments in ink can be analysed using paper chromatography

The mobile and stationary phase

• All chromatography techniques use two phases called the mobile phase and the stationary phase
• In paper chromatography:
  • The mobile phase is the solvent in which the sample molecules can move, which in paper chromatography is liquid e.g. water or ethanol
  • The stationary phase in paper chromatography is the actual chromatography paper itself

• The substances which are more soluble in the solvent will travel further up the paper because they spend more time in the mobile phase and are thus carried further up the paper than the less soluble components

• 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 spots
• An impure substance therefore will produce a chromatogram with more than one spot

Chromatogram

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

Calculating R_f values

• These values are used to identify the components of mixtures
• The R_f value of a particular compound is always the same but it is dependent, however, on the solvent used
• If the solvent is changed then the value changes
• 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
• These values are known as reference values
• Retention factor, R_f, is calculated by the equation:

 

• The R_f value is a ratio and therefore has no units and will be less that 1

Calculating Rf value

Using R_f values to identify components of a mixture

• The R_f value of the substances in the chromatogram above can be calculated by: