Chromatography is a technique commonly used to separate mixtures of compounds into their constituent components. Separation is always based on some physical characteristic of the compounds, like size or polarity. Techniques like this are used to analyze bodily fluids for drugs, e.g. for Olympic athletes. It is also used to determine the compounds that give characteristic flavors to food, so the food industry can determine how to add flavor compounds artificially.

In this lab we will separate artificial food dyes. The separation is based on polarity. These dyes are all polar, so they will be soluble in water. We absorb them onto paper, which is also polar. Then we use water, with a little salt, to move the dyes on the paper. Since some of the dyes are more polar than others, they are absorbed more tightly to the paper and are moved more slowly by the salt water.

To learn how mixtures of compounds can be separated, and to learn what food dyes are in the foods we eat.

Absorb

Polar

Non-polar

Chromatography

Soluble

Purchase the five FD&C food dyes: Blue 1, Blue 2, Red 3, Red 40, Yellow 5 and Yellow 6. These should be made into 0.5% solutions in water by dissolving 0.25 g of each dye in 50 ml of water. Probably 10 ml will be much more than you need. To make 10mL of dye solution requires the amount of dry dye approximately equal to the size of a small pea. Store in a screw top vial or bottle if possible.

You can purchase the dyes from:

For 500 ml of 0.1% NaCl solution dissolve 0.5 g NaCl in 500 ml of water.

For Chromatography paper you may purchase blotter paper from Staples Office Supply store or any other office supply store. That works just as well as real chromatography paper. The blotter paper should be white and cut into rectangles of about 5" x 3". This size will fit in the 150 ml beakers. If you have taller beakers, you can adjust the size of the paper so the paper folded into a cylinder (see below) will fit the beaker or wide-mouthed jar.

This project is easy enough that students could work alone or in groups of two or three.

1. Each group should take two rectangles of chromatography paper.
2. Using a pencil, a line should be drawn about 1 cm from the bottom of the card, and small cross lines drawn about 1 cm apart, as shown below. There should be room for at least 10 cross lines on each card.

3. Label each of the cross lines with the color that will be spotted there. e.g. B1, B2, R3, R40, Y5, Y6, MR, MB, MO, MY, MG, where the first five are the FD&C colors, and the "M" designations are M&M colors. The other card could be labeled for the FD&C colors and Kool Aid flavors, such as KG, KS for Kool Aid Grape or Kool Aid Strawberry.
4. The colors can now be applied to each cross line by taking a round toothpick for each sample, dipping it in the dye and touching the tip of the toothpick to the line. For the M&M's , a drop of water on the M&M will dissolve enough dye for the toothpick to pick some up. A small drop of water on a little bit of Kool Aid powder will also be sufficient to make a sample to spot the paper.
5. Once the paper is spotted, it should be folded into a cylinder and stapled at top and bottom as shown below.
6. The cylinder can now be placed ( with the spots at the bottom) in a 150 ml beaker containing about 1/2 cm of the 0.1% NaCl solution. The plastic wrap can be used to cover the top of the beaker. The paper should be removed from the beaker when the solvent gets to about 1 cm from the top of the paper.

By comparing the spots on the commercial items with the spots of the FD&C food dyes, it is possible to determine which dyes are used in which foods.

One can see that some of the vegetable colors are pure dyes (Blue and Yellow); green is not surprisingly blue and yellow. If one looks very carefully at the red vegetable dye, you can see that it is a mixture of red 40 and red 3. It is fun to look at all the colors in the brown M& M’s. Try it.

The students could hand in a report with the results from your chromatography investigation.

Students might want to explore different solvents or liquids in the beaker. Why do we use 0.1% salt water? Would regular water work just as well? Would rubbing alcohol work? These are questions the students can answer through experimentation.

SCIENCE CONTENT STANDARD #1: NATURE OF SCIENCE

This experiment fulfills Benchmark 1 for grades 5-8 which states that students should "design modify, and conduct an investigation through testing, revising and occasionally discarding ideas, all of which lead to a better understanding of how things work." It also satisfies Benchmark 3, which states that students should "collect and summarize date from an experiment and interpret the results in terms of the data."

This lesson is related to Fall Colors, which also looks at separation of pigments, using chromatography.