You can do paper chromatography using a coffee filter to separate the pigments in colored candies, like Skittles™ or M&M ™ candy. This is a safe home experiment, great for all ages.
Time Required: about an hour
- Coffee filters usually are round, but it's easier to compare your results if the paper is square. So, your first task is to cut the coffee filter into a square. Measure and cut a 3x3" (8x8 cm) square from a coffee filter.
- Using a pencil (ink from a pen would run, so pencil is better), draw a line 1/2" (1 cm) from the edge of one side of the paper.
- Make six pencil dots (or however many colors of candy you have) along this line, about 1/4" (0.5 cm) apart. Underneath each dot, label the color of the candy you will test on that spot. You won't have space to write the whole color name. Try B for blue, G for green, or something equally easy.
- Space 6 drops of water (or however many colors you are testing) equally distant on a plate or piece of foil. Position one candy of each color on the drops. Give the color about a minute to come off into the water. Pick up the candy and eat it or throw it away.
- Dip a toothpick into a color and dab the color onto the pencil dot for that color. Use a clean toothpick for each color. Try to keep each dot as small as possible. Allow the filter paper to dry, then go back and add more color to each dot, a total of three times, so you have lots of pigment in each sample.
- When the paper is dry, fold it in half with the color sample dots on the bottom. Ultimately, you are going to stand this paper up in a salt solution (with the liquid level lower than the dots) and capillary action is going to draw the liquid up the paper, through the dots, and toward the upper edge of the paper. The pigments will become separated as the liquid moves.
- Prepare the salt solution by mixing 1/8 teaspoon of salt and three cups of water (or 1 cm3 of salt and 1 liter of water) in a clean pitcher or 2-liter bottle. Stir or shake the solution until it is dissolved. This will produce a 1% salt solution.
- Pour the salt solution into a clean tall glass so that the liquid level is 1/4" (0.5 cm). You want the level to be below the sample dots. You can check this by holding the paper up against the outside of the glass. Pour out a little salt solution if the level is too high. Once the level is correct, stand the filter paper inside the glass, with the dot side down and the edge of the paper wetted by the salt solution.
- Capillary action will draw the salt solution up the paper. As it passes through the dots, it will begin to separate the dyes. You will notice some candy colors contain more than one dye. The dyes separate because some dyes are more likely to stick to the paper, while other dyes have a higher affinity for the salt water. In paper chromatography, the paper is called the 'stationary phase' and the liquid (salt water) is called the 'mobile phase'.
- When the salt water is 1/4" (0.5 cm) from the top edge of the paper, remove it from the glass and place it on a clean, flat surface to dry.
- When the coffee filter is dry, compare the results of chromatography for the different candy colors. Which candies contained the same dyes? These are the candies that have corresponding bands of color. Which candies contained multiple dyes? These are the candies that had more than one band of color. Can you match any of the colors with the names of the dyes listed on the ingredients for the candies?
- You can try this experiment with markers, food coloring, and powdered drink mixes. You can compare the same color of different candies, too. Do you think the pigments in green M&Ms and green Skittles are the same? How can you use paper chromatography to find the answer?
What You Need:
- Skittles or M&M candies
- coffee filter
- tall glass
- table salt
- plate or foil
- pitcher or empty 2-liter bottle
- measuring cups/spoons
by: Anne Marie Helmenstine, Ph.D.