Chromatography

• What is the solvent?

  A liquid that the solute dissolves in to form a solution.
• What is the solute?

  The substance that is dissolved in a solvent to form a solution.
• What is the solvent front?

  The furthest point reached by the solvent as it moves up the chromatography paper.
• What is the Rf value?

  The ratio of the distance travelled by the solute to the distance travelled by the solvent.
• What is the calculation for the Rf value?

  Distance travelled by the solute (spot)/distance travelled by the solvent (solvent front)
• What does it mean if two solutes have travelled the same distance?

  They are the same molecule.
• What is the mobile phase?

  Where the molecules can move. In both paper and thin-layer chromatography, this is the liquid solvent i.e ethanol or water.
• What is the stationary phase?

  Where the molecules can't move.
• What is the stationary phase in paper chromatography?

  The chromatography paper.
• What are the basic principles of chromatography?

  -The mobile phase moves through or over the stationary phase.

  -Components in the mixture spend different amounts of time in the mobile phase and the stationary phase.

  -The components that spend longer in the mobile phase travel faster or further.

  -The time spent in the different phases is what separates out the components of the mixture.
• What is chromatography used for?

  Separating molecules of a mixture and identifying the components of that mixture.
• What is the stationary phase in thin layer chromatography?

  A solid i.e glass or plate with a thin layer of gel i.e. silica gel on top. This called a chromatography plate.
• Describe how to carry out thin layer chromatography to separate photosynthetic pigments

  1. Grind up leaves with anhydrous sodium sulfate and propanone.
  2. Transfer liquid to a test tube. Add some petroleum ether and gently shake. Two distinct layers will form. The top layer is the pigments mixed with the petroleum ether.
  3. Transfer some of the liquid from the top layer into a second test tube with some anhydrous sodium sulfate.
  4. Draw a horizontal pencil line near the bottom of a chromatography plate. Apply several drops of some of the liquid on the line to form point of origin.
  5. Once dry, put plate into a glass beaker with solvent so point of origin is a little bit above solvent. Put lid on beaker and leave plate to develop. As the solvent spreads up the plate, the different pigments move with it but at different rates and so separate.
  6. When solvent has nearly reached the top, take out and mark solvent front with pencil and leave to dry.
• What should you do when forming the point of origin?

  When applying each drop, make sure they're dry before adding next.
• In TLC, why do you need to hold the TLC plate carefully by the edges and avoid damaging the surface of the plate?

  So the movement of spots aren't affected by damage or the plates aren't contaminated i.e. by fingerprints.
• In TLC, why must you make sure the plate doesn't touch the sides of the jar?

  Because condensation of solvent on walls of the jar may affect movement of spots.