C18.6 Paper Chromatography (Part 2, Colourless)

• paper chromatography can also be used to find substances which are colourless, or invisible
• Colourless paper chromatography method

  1. Place a spot of each solution along a pencil line (the base line) on slotted chromatography paper

  2. Place a suitable solvent (ethanoic acid and water for amino acids) at the bottom of a beaker

  3. Roll the paper into a cylinder and place it into the beaker, cover it

  4. The solvent will rise up the paper, remove the paper before it reaches the top

  5. Mark a pencil line on it to show where the solvent has reached (the solvent front)

  6. Put the paper in the oven to dry out

  7. Spray it with a locating agent to make the amino acids show, then warm the paper in the oven for around 10 minutes

  8. Mark a pencil dot at the center of each spot and measure from the origin to the dot and from the origin to the solvent front
• Rf = distance from origin to dot / distance from origin to solvent front
• Rf stands for retention factor. retention means to hold back
• the Rf value is the distance travelled by the substance in the mobile phase per distance travelled by the solvent in the stationary phase
• the Rf tables are used to calculate the relative molecular mass of a compound
• the Rf value can help you find which amino acid you have found
• substances will travel along the paper at different speeds based on their solubilities and attraction to the paper, which makes them separate. when they are separated they are sprayed with a chemical to make it visible. the distance compared to the solvent is used to identify them
• the base line is the pencil line used to mark the bottom of the chromatography paper
• a suitable locating agent for chromatography is ninhydrine
• a suitable solvent is a mixture of butanol, ethanoic acid and water
• solvent front


  the line where the solvent has reached