BSI Education

Practice Assignment 05

Is your green sweet really green?

Are coloured sweets really just the colour they appear to be? Your task is to find out.

Food colourings must be safe to eat. Only a few dyes are allowed to be used in foods. However, just like paints, dyes can be mixed to give a wide range of colours. So, a green sweet may contain a green dye, or a mixture of blue and yellow.

Working in pairs, carry out the following test to see if your coloured sweet is just one colour or a mixture of colours. You will be using a process called chromatography to separate the dyes used to colour sweets. The process should also help you to identify some or all of the dyes. When you have finished, compare your results with others to see which dyes are mixtures.

What you have to do

  1. Get a copy of Standard Procedure SP0005:2005 Method for identifying food dyes in sweets. Read it carefully.
  2. Collect all the equipment you need.
  3. Follow the procedure carefully.
  4. While you are waiting, begin to write up your report and copy the table below.
  5. Calculate the Rf value(s) for the dye(s) on your chromatogram.
  6. Use your results to identify the dye(s) from your sweet. Note: Depending on the sweets, not all dyes may be identifiable.
  7. Compare your results with others to complete your table.
Sweet colour Dye colour(s) Rf values (s) E-number(s)
Blue      
Brown      
Green      
Mauve      
Orange      
Pink      
Red      
Yellow      

 

Some hints

  • To save time, fill your beaker for the water bath with hot water.
  • The smaller and more concentrated your spots, the better your chromatogram will be.

Some questions

Don’t forget to write up your results in your laboratory notebook.

  1. Which sweet colours contain a single dye?
  2. a) Which sweet colours are mixtures of dyes?
    b) Why do you think this is?
  3. Which colour sweets, if any, contain the same dye(s)? Note: Colours may look similar, but they are not the same dye unless their Rf values are the same within 0.05.
  4. How could you change your procedure to improve your results?
  5. What is chromatography?
  6. Suggest at least two other uses of chromatography.
  7. How would you explain to somebody how chromatography works?