Identifying food colouring using paper chromatography  

You can also try this technique at home using food dye.

 

 

Cut a 10 cm long and 2 cm wide strip of paper. Newspaper is good for this but you can also use other types of paper.

 

 

 

 
Using a pencil and a ruler draw a straight line 2 cm from the end of the paper as shown on the right.

 
On the line you have drawn place a very small sample of the food dye you wish to examine. You may place more than one sample on the line as long as they are at least 1 cm apart.
Place the paper with the spotted dye samples into a glass containing water to a depth of 1 cm, as shown on the right. Make sure the spots are not submerged. The water will now move up the paper carrying the dye particles with it. This will cause the components of each dye to separate. It may take a few minutes or a few hours depending on the type of paper used. Be patient.

 

Remove the paper from the water when the water has stopped at its maximum height or is 2 cm from the top of the paper.

Now time to take a few measurements using a ruler.
Step 1 - measure the height the water reached.

 

Step 2 - measure the height each dye particle reached.

Step 3 - divide the height of the dye particle by the height of the water. This will give a number called the Rf.
In this case the yellow dye particles reached a height of 5 cm while the water reached a height of 8 cm.
Rf = 5 / 8 = 0.625.

Since both the yellow particles have the same Rf we can conclude they are the same particle.

 

 

Now consider the dyes B and C. Both contain blue particles.
a) Are the blue particles the same type? Explain

 

 

b) Is dye B a mixture or a pure substance? Explain

 

 

c) Explain why the red particles in dye B travel less up the paper than the blue particles of dye B?

 

Another dye was analysed using paper chromatography. The paper is shown on the right after it was taken out of the water. Are the blue particles used in this dye the same as the blue particle used in dye B above? Explain.

What are primary colours?

This dye appears to be , the original colour of the dye must have been

 
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