Radial Paper Chromatography

Aim
(1) To illustrate how a mixture of coloured compounds can be separated.
(2) To see if due to the nature of their molecules, different substances may have different affinities towards materials such as water and paper.
(3) That materials which might appear to be pure substances are actually mixtures.
(4) That compounds retain their unique physical and chemical properties even when mixed.
(5) To show that when water is soaked up into a piece of filter paper on which water-soluble markers of various colours have been spotted, the separation of the components of the markers makes a beautiful coloured pattern.

Apparatus
(1) Petri dish (preferably glass )
(2) Large filter paper sheets (30 cm diameter if possible but any size will do )
(3) Small twists of filter paper to act as wicks.
(4) Variety of coloured water-soluble markers.
(5) Cover (preferably see-through), large enough to enclose the filter paper. An upturned basin or pneumatic trough will do.
(6) Water

Diagram


 


Procedure
(1) Bore a small hole in the centre of a number of the large sheets of filter paper. Use the point of a pencil.
(2) Test fit a twist of filter paper into the hole in one of the large sheets of filter paper, to create a short wick which will reach the water in the Petri dish. Do not get the paper wet.
(3) Remove the wick.
(4) Use various water-soluble markers to make a circle of spots around the hole in the centre of the filter paper with a diameter of a 10p coin. To get a bright chromatogram, let the spots dry and then re-apply the marker a number of times.
(5) Black markers are the most interesting. If you can get two different makes of marker you can alternate the spots on the paper around the circle and this will develop into a beautiful pattern.
(6) Re-attach the wick making sure it is in firm contact with the sheet of filter paper.
(7) Half fill the Petri dish with water and dry the rim thoroughly.
(8) Gingerly place the filter paper on the Petri dish and check that the water is beginning to soak out along the paper. If necessary bend the bottom of the wick to allow it to fit into the Petri dish.
(9) Place the cover over the filter paper and observe every now and then for 20 to 30 minutes or as long as the class will allow.
(10) Do not let the water soak beyond the rim of the filter paper.
(11) When the water has soaked close to the edge of the paper, remove the cover and the filter paper and allow it to dry.

Question related to the Practical
(1) Why is the cover needed?
The water would evaporate too rapidly and would fail to properly draw the coloured molecules along the paper.
(2) Why do the coloured components of the inks separate?
The water and the paper attract the different coloured molecules in an ink to a different extent. The colour molecules that travels furthest are more attracted to the water and the ones that travels least are attracted most to the filter paper.

Extension
(1) Similar patterns can be produced on a t-shirt using permanent markers and isopropyl alcohol as the solvent.

(2) A forensic slant can be given to the pupil work if the teacher pre-spots the papers as follows:- Take two black markers one each from different manufacturers. Put one spot on with the first marker and all the rest of the spots in the circle with the other marker. Challenge the pupils to find which of the black spots is different from the others. Use this exercise to introduce a discussion on the use of chromatography by chemists.

Related Course Topics
(1) Separation of Mixtures.
(2) Solubility of compounds
(3) Capillary Attraction

Comment
Even though chromatography is not on the Junior Certificate Science syllabus, it would be unsatisfactory to have pupils leaving school without some brief mention of the topic as it is at the centre of so much modern chemical analysis. It is now on the new (2000) Leaving Certificate syllabus.

References
Bob Becker, Kirkwood High School, Kirkwood, Missouri
20 Demonstrations Guaranteed to Knock Your Socks Off. ISTA AGM, Dundalk