Chromatography

• Chromatography is a method for separating dissolved substances from one another. It works because some of the coloured substances dissolve in the solvent better than others, so they travel further up the paper. This technique is often used to separate out inks, or food colourings.
• Paper chromatography consists of two stages:

  • the stationary phase (a.k.a. the phase that doesn't move - this is the paper)
  • the mobile phase (a.k.a. the phase that moves - this is the solvent)
• Separation by chromatography produces a chromatogram, and can be used to distinguish between a pure substance (that produces one spot), and a mixture (impure) which produces multiple spots. Different substances will move at different rates through the paper.
• When setting up the experiment you must make sure to draw the baseline in pencil, otherwise the ink from a pen would also dissolve in the solvent. 
• The Rf value of a spot can be used to identify unknown chemicals if they can be compared to a range of reference substances. 
• The Rf value will always be the same for each substance (when the same solvent is used), and an Rf value will always have a value of less than 1. It can be calculated by using:

  Rf value = distance moved by substance ÷ distance moved by solvent