6 - Chromatography

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Cards (6)

  • What is the method for this practical? (part 1)
    1. Draw a horizontal pencil line 2 cm from the short edge of the chromatography paper. Mark pencil spots at equal intervals across the line but not too close to the end of the paper.
    2. Use a glass capillary tube to put a small spot of each colouring on the pencil spots. A small spot ensures that the colouring separates clearly. Label each spot in pencil.
    3. Pour about 1 cm^3 of water into the beaker.
    4. Suspend the paper in the beaker so that bottom edge of the paper dips into the water.
  • What is the method for this practical? (part 2)
    5. Wait for the water solvent to travel at least three quarters of the way up the paper. Remove the paper and draw another pencil line at on the dry part of the paper right next to the wet edge. This is the solvent front, the distance travelled by the solvent.
    6.Hang the paper up to dry thoroughly.
    7.Calculate the Rf values for each spot. Rf value is calculated by distance travelled by substance/solvent front.
  • What relationship between a sample and the mobile phase makes the sample move faster?
    More soluble components/components with more affinity to make the solvent move faster.
  • What is the relationship between a sample and the stationary phase that makes the sample move slower and what kind of bonding does this often involve?
    More affinity for the stationary phase means that a component moves slower, often attracted by hydrogen bonding.
  • How are substances separated by chromatography?
    If suitable stationary/mobile phases are chosen, the balance between affinity for the mobile phase and affinity for the stationary phase is different for each component of the mixture. Thus, they move at different rates and are separated over time.
  • Why will different substances show different Rf values?
    They are bonded differently and have different polarities - more polar bonds means longer retention time or a smaller Rf value since hydrogen bonding is attracted more strongly to the stationary phase.