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Chromatography: A Separating Process (page 2)

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Author: Janice VanCleave

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Yellow streaks of color form and move with the water as it rises up the paper strip. A band of yellow forms on the upper portion of the paper strip below the top line made by the water. The R value for the yellow candy used by the author was 0.65. Note: The flow rate (R) will depend on the yellow dye in the brand of candy used.

Why?

Chromatography is a process of separating mixtures by encouraging different parts of the mixture to move through an absorbing material at different rates. Each separated part of the mixture can be identified by its color and flow rate (R). The separation is due to capillary action (the rising of the surface of a liquid in small tubes) and solubility.

Chromatography: A Separating Process

In this experiment, the tiny fibers in the paper have space between them and form tube-like structures throughout the paper. The solvent, water in this case, is drawn into the fiber tubes and rises due to adhesive and cohesive forces. Adhesion is the attraction that the water molecules have for the paper. Cohesion is the attraction that the water molecules have for one another. The liquid surface within the spaces is not flat but slightly crescent-shaped, because the adhesion of the water to the paper is greater than the cohesion between the water molecules.

The adhesive attraction of the water to the paper is strong enough to move the water up the sides of the fiber tubes against the downward pull of gravity. The water molecules clinging to the fiber tubes then pull the lower water molecules up the center of the tubes. As the water pulls itself upward, it carries the yellow food dye with it The distance the dye travels is determined by the attraction it has for water versus its attraction for the paper. The pull of gravity and the attraction to the paper pull the dye molecules out of the water and deposit them on the paper. The color pattern produced on the paper strip is called a chromatogram.

Try New Approaches

  1. Is the flow rate the same for other colors of food dye?
    1. Repeat the experiment using other candy colors. Some of the colors are a combination of colors. For example, green is made of the primary colors of blue and yellow, and orange is a combination of red and yellow. Determine the flow rate (R) for each color.
    2. Repeat the original experiment using a mixture of the different colors. Calculate the flow rate (R) for each color band on the chromatogram and compare with those calculated for each individual candy color. Science Fair Hint: Display the chromatogram and calculated flow rate for each color band.
  2. Does the absorbency of the filter paper affect the flow rate? Repeat the original experiment using different types of paper, such as the white edge on newspaper, chromatography paper obtained from your science teacher, and paper towels.

Design Your Own Experiment

  1. Is the order of the separating solutes different if the solvent flows down instead of up? Cut a 2-inch (5-cm) strip from a paper towel. Using black water-soluble ink, draw a line across the strip about 4 inches (10 cm) from one end. Set a bowl full of water on a counter next to a sink. Place the end of the paper strip nearest the black line in the bowl of water. Do not let the black line touch the water in the bowl. Allow the strip to hang over the edge of the bowl into the sink (see Figure 16.3). Compare the chromatogram produced in this manner with those previously formed. Display the chromatograms and indicate the procedure by which they were produced.
  2. Prepare a leaf chromatogram by laying a geranium leaf, face down, on a round coffee filter. Rub the edge of a coin back and forth over the leaf to produce a green spot about 1 inch (2.5 cm) from the rounded edge of the filter paper. Allow the green spot on the paper to dry before repeating the process. Note: You want to collect enough pigment from the leaf to make a dark green spot on the paper. Fold the paper in half twice and secure it with a paper clip. Pour a small amount of rubbing alcohol into a saucer. CAUTION: Keep the alcohol away from your nose and mouth. Place the rounded edge of the paper cone in the alcohol. To prevent the alcohol from evaporating too quickly, cover the paper and saucer with a box (see Figure 16.4). Allow the paper to sit undisturbed for 30 minutes. Display the dry chromatogram.

Chromatography: A Separating Process

Get the Facts

  1. Plants have a green color because of a pigment called chlorophyll. Use a biology text to find out more about plant color pigments. Use this information to identify the colors on the leaf chromatogram.
  2. Chromatography is used in industrial plants to separate and measure solutes. Use chemistry texts to find out more about the process of chromatography. What types of absorbents are used other than filter paper? Explain the process of gas chromatography.
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