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Chemistry Chromatography

Chromatography is technique that is used to separate out the components of a mixture. In the real world it has many applications; it is used in forensic science to identify substances. In biology for separating molecule such as amino acid. In the petroleum industry for separating products of reactions. These industries all use complicated expensive machines that use chromatography. However they all are based on the same basic principles that will be explained today.

The main principle of chromatography is a solvent is passed through a medium. The solvent carries the mixture and depending on the components interactions with the medium and solvent will determine the speed that they travel through the medium. If the components are travelling at different speeds then the components over time will separate.

Demo 1

  1. Cut round filter paper (the medium) so that a tongue is formed and fold this down, as in picture.
  2. Rule a straight line (with pencil) a 3rd the way up on the tongue.
  3. Mix a few food dyes together.
  4. Put a few drops on to the pencil line
  5. Fill a beaker with water (solvent). Make sure that the level of the water is lower than the level that of the food dyes when the filter paper is placed on the beaker.
  6. Place the filter on the beaker. Allow to stand. The longer you leave it the greater the separation.
  7. To stop the movement of the food dye take filter out of water.

Note: You may notice that a colour you used hasn’t appeared when the dyes separated it may have separated into different parts itself. This is common for non primary colour like green and purple. 

Demo 2

  1. Take some square filter paper and rule a pencil line about 20mm from the bottom.
  2. Along this line put dots of different colour water based felt tip pens.
  3. Place the filter paper in a beaker that has 10mm of water in the bottom
  4. Allow the filter paper to stand.
  5. The different colours in the pens dye will separate. Similar colours can be found in different pens. These can be identified as they will appear at the same level.

Extensions

The effect of different solvents – e.g. alcohol and NaCl

Rf values – allow you to compare different experiments