Required Practical: Investigating Chromatography

Aim

To investigate the use of paper chromatography to separate a mixture of coloured substances and identify the different substances in the mixture.

You’ll carry out the following in this practical:

  • Separate a mixture of food colourings using paper chromatography.
  • Calculate the Rf value for each food colouring.
  • Use the calculated Rf values to identify the different coloured substances in the mixture.

Method

1. Use a pencil and a ruler to draw a horizontal line across the chromatography paper, approximately 2 cm above the bottom of the paper.

  • This is the pencil line

2. Use a capillary tube (a very thin glass tube) to add small spots of each ink (A, B, C, and D) and an unknown ink (U) to the line on the paper.

  • Ensure that the spots are evenly spaced across the paper, maintaining at least a 1 cm gap between each ink. Also, label each spot with a pencil.

3. Pour water into a beaker to a depth of 1 cm.

  • This water will act as the solvent

4. Use a piece of tape to attach the paper to a glass rod. Then lower the paper into the solvent so that the bottom of the paper dips into the solvent.

  • Make sure the pencil line is above the waterline

5. Allow the solvent to pass three quarters of the way up the paper, and then remove the paper from the beaker.

6. Draw another pencil line on the dry part of the paper, as close to the wet solvent as possible.

  • This line is called the solvent front line

7. Allow the chromatogram to dry before measuring the distance travelled by each spot and the solvent.

Results

Make a table to keep track of your findings. For example:

Food colouring (FC)ColourDistance travelled by FC (mm)Distance travelled by solvent (mm) Rf Value
Ablue3.250.64

Repeat this process for each food colouring tested.

Analysis

1. Measure the distance travelled by the water solvent. To do this, measure the distance (in mm) between the two pencil lines.

2. For each food colouring spot (A, B, C, D, and U), measure the distance travelled (in mm) from the pencil line to the centre of the spot.

3. Calculate the Rf value for each spot by dividing the distance travelled by the food colouring spot by the distance travelled by the solvent.

Rf= distance travelled by the substance divided by the distance travelled by the solvent.

4. Compare the Rf values of food colourings A, B, C and D to that of the unknown substance.

We can also look up the Rf value in a database to identify the unknown substance. However, many substances can have that Rf value, so it may be necessary to use different solvents to narrow down the possibilities.

Hazards, Risks and Safety Measures

It is important to use appropriate apparatus and methods in this practical activity, including the proper usage and handling of chemicals. Eye protection is required, and risks should be assessed to minimize the risk of injury.

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