Required Practical 6: Chromatography

Question 1

What is the method in which paper chromatography can be used to identify substances?

Answer 1

1) Get a sample of food colouring which is a mixture of chemicals and call it U for unknown and also get four known food colourings that it could contain and label these A - D.
2) First we use a ruler to draw a horizontal pencil line on the chromatography paper make sure the line is 2 cm from the bottom of the paper.
3) Mark five pencil spots at equal spaces across them line at equal spaces (1 cm).
4) Next use a capillary tube to put a small spot of each of the known food colours and the unknown colour onto the pencil spots.
5) Pour water into a beaker to a depth of 1 cm.
6) Attach the paper to a glass rod using tape and lower the paper into the beaker and the bottom of the paper should dip into the water.
7) Put a lid on the beaker to reduce evaporation of the solvent.
8) At this stage, the water will move up the paper and the colours will be carried up and make sure not to move the beaker.
9) Remove the paper when the water has travelled around three quarters up.
10) Use a pencil to mark the point where the water reached.
11) Hang the paper up to dry.
12) Now see which colours matched up with sample U.

Question 2

How do we wanted to identity the chemicals in the colours?

Answer 2

We calculate the Rf values:

1) Measure the distance from the pencil line to the centre of each spot.
2) Measure the distance moved by the water from the pencil line.
3) Then we use the equation:
Distance moved by chemical —————————————– = Rf
Distance moved by solvent

4) Now look this Rf value up in a database to identify the chemical