Separation Techniques (OCR Gateway GCSE Chemistry: Combined Science)

Investigating a Mixture of Dyes

Practical Skills Development

• Safe use of a range of equipment to purify and/or separate chemical mixtures including chromatography
• Safe use and careful handling of liquids under controlled conditions
• Safe use of appropriate qualitative reagents and techniques to analyse and identify unknown samples or products 
• Presenting observations and other data using appropriate formats
• The example given below focuses on chromatography, however you should be able to explain how other methods of separation including filtration, crystallisation, and distillation are used in a practical context
  • Information on these methods can be located in 2.1 Separation Techniques

Aim

• To investigate how paper chromatography can be used to separate and identify a mixture of food colourings

Health & Safety Aspects

• No particular hazards unless organic solvents are used

Materials:

• A 250 cm³ beaker
• A wooden spill/splint
• A rectangle of chromatography paper
• Four known food colourings labelled A–D
• An unknown mixture of food colourings labelled U
• Five glass capillary tubes
• Paper clip
• Ruler & pencil

Diagram of the apparatus needed

Practical Tip:

The pencil line must never be below the level of the solvent as the samples will be washed away

Method:

1. Use a ruler to draw a horizontal pencil line 2 cm from the end of the chromatography paper
2. Use a different capillary tube to put a tiny spot of each colouring A, B, C and D on the line
3. Use the fifth tube to put a small spot of the unknown mixture U on the line
4. Make sure each spot is no more than 2-3 mm in diameter and label each spot in pencil
5. Pour water into the beaker to a depth of no more than 1 cm and clip the top of the chromatography paper to the wooden spill. The top end is the furthest from the spots
6. Carefully rest the wooden spill on the top edge of the beaker. The bottom edge of the paper should dip into the solvent
7. Allow the solvent to travel undisturbed at least three quarters of the way up the paper
8. Remove the paper and draw another pencil line on the dry part of the paper as close to the wet edge as possible. This is called the solvent front line
9. Measure the distance in mm between the two pencil lines. This is the distance travelled by the water solvent
10. For each of food colour A, B, C and D measure the distance in mm from the start line to the middle of the spot

Results

Conclusions

• The R_f values of food colours A, B, C and D should be compared to that for the unknown sample as well as a visual comparison being made
• The use of chromatography and R_f values is a viable method of identifying unknown mixtures given reference material