Chromatography: Food Coloring

This experiment illustrates a chromatographic separation as a good example of a physical change.

 

Materials

1 bottle of 5% isopropyl alcohol

1 bottle of 20% isopropyl alcohol

1 bottle of 70% isopropyl alcohol

1 bottle water

1 10 mL plastic syringe

1 C-18 "Sep- Pak" chromatographic bed (obtained from Waters Associates, Milford, MA)(1mL volume tube is OK)

1 piece of white paper

1 clear plastic cup with 10 drops of red food coloring + 10 mL water

1 clear plastic cup with 10 drops of blue food coloring + 10 mL water

 

What To Do

Place the two cups of food dye on a white sheet of paper to see the two different colored solutions. Next predict what will happen when the two solutions are combined. Combine the two solutions. A purple solution will result. Draw 10 mL of water into the syringe. Slowly force the water through the column. This will wet the column before the separation process. Next pull 1 mL of the purple solution into the syringe. Connect the Sep-Pak column to the syringe. Inject the purple solution onto the column. Collect the solution that emits from the column. The solution should be nearly colorless. Flush 10 ml of 5 % isopropyl alcohol slowly onto the column. Collect the emitted solution in a plastic cup. (This solution should be red.) Next flush the column with 10 ml of 20% isopropyl alcohol. Collect the emitted solution in the plastic cup (This solution should be blue). The column will still be holding a red layer. This is because the red dye was actually produced by combining two different red food dyes. Next flush the column with 70% isopropyl alcohol. The emitted solution will have a slight red tint to it. Take 1 mL of the original mixed food color solution and dilute it with 35 mL of water.Ask class what they expect to see when the red and blue solutions are combined (hopefully, many will say they should get the same color as the solution just prepared). The final two solutions should look identical in color. This demonstrates that chromatographic separation is a physical change.

 

Source

WOW Staff, 2002

© S. Olesik, WOW Project, Ohio State University, 2002.