CHROMATOGRAPHY OF ARTIFICIAL FOOD DYES
Knight foundation summer Institute
Terry Newirth, Haverford College
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.
|FD&C Food Dyes*||150 ml beakers or wide-mouthed jars|
|Commercial Vegetable Colors||plastic wrap|
|M & M's||pencils|
|Kool Aid||round toothpicks|
|0.1% NaCl solution||rulers|
|chromatography paper or blotter paper||small stapler|
Preparation Before Class:
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:
286 Baxter St. Tolland,
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.
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& Ms. 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.
Philadelphia Science Content Standards:
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.