Chromatography

    Method

1. Students collect the necessary equipment and draw a pencil line 2cm away from the bottom of the chromatography paper. This line will be their starting point when taking measurements.
2. They must mark five small crosses in pencil along the starting line, staying 5mm away from the outer edges of the paper. They should label them, in pencil, as A-D and ‘X’ for the unknown.
3. The open end of a capillary tube should be dipped into the first food dye sample so that a small amount is drawn up into the tube.
4. Students then touch the chromatography paper with the open end of the capillary tube so that a small spot of food dye is applied to the cross labelled A. They then wait for this to dry and repeat to make the spot more concentrated.
5. Using a fresh capillary tube for each food dye, they should repeat the process for the other three known samples, and then repeat using the unknown mixture applying this to the cross labelled ‘X’.
6. They must then wrap the top end of the chromatography paper around a wooden splint and secure it with a paper clip.
7. Water is added to a 250ml beaker so there is 1cm in depth.
8. Once the dye spots are dry, students should lower the strip of chromatography paper into the beaker, making sure that the pencil line they have drawn is just above the water surface, adjusting the paperclip at the top if necessary, resting the wooden spill on the top of the beaker. The sides of the paper should not be touching the beaker.
9. They should watch as the water travels up the chromatography paper so that it nears the top of the paper. Students should see the food dyes moving up the paper and the mixture on spot ‘X’ should start to separate.  They then remove the paper from the water and draw another pencil line on the dry part of the chromatogram just above the line which the water reached and hang it somewhere to dry thoroughly.
10. Once the chromatograms are dry, students must measure the distance between the two pencil lines they have made at the top and bottom of the chromatography paper in millimeters. This is the distance travelled by the solvent.
11. They then make a spot in pencil in the middle of each of the food colouring spots.
12. Students measure the distance of each of the spots in millimeters from the starting point pencil line and record their results in a suitable table.
13. To calculate the Rf value for each of the spots, the distance moved by each spot is divided by the distance moved by the solvent. Students must work these out for each of the spots and add them to their table.
14. Can they identify the individual components of the dye mixture ‘X’?