Paper Chromatography

Introduction

Chromatography is a method for separating the substances in a mixture. The method was first applied to the separation of colored substances. (Greek: chromos = color). Although the effectiveness of the separation is most easily detected if the components of the mixture are colored, the method is applied today to all kinds of mixtures.

Filter paper is a good material for the stationary phase of a chromatographic experiment. Many substances that are soluble in water or other mixed solvents are more or less tightly adsorbed (bonded to the surface) onto the fibers of the paper. A spot of solution containing several components is placed at the bottom of a strip of paper. The paper strip is then placed into a solvent. As the solvent is drawn up into the paper by capillary action, those substances that are not adsorbed on the paper will move at the same rate as the solvent. Substances that are bound very tightly to the paper will not move very far. Substances that interact weakly with the paper will move, but more slowly than the solvent.

The difference in migration of the various compounds in the mixture results in their separation. In this experiment you will separate the various colored dyes in water soluble inks.

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Procedure

Cut a piece of chromatography paper 5 cm wide by 12 cm long. (Circles of 15 cm diameter # 1 filter paper work well.) Draw a pencil line (not a pen line) about 2 cm from one of the 5 cm edges and parallel to that edge. About 5 mm from a 12 cm edge, mark a point on the line with a sharp pencil. Mark additional points on that line every 1 cm starting from the first point.

Use different soluble marking pens to mark small spots at the pencil points. Place the paper in a dry 250-mL beaker. Place a rod (such as the stem removed from a Beral pipet) across the top of the beaker (supported at the lip). Fold the top of the paper over the supporting rod. Remove the paper. Place water in the 250-mL beaker to a height of 1.5 cm. Reinsert the paper hanging it over the rod. Check to see that the paper is not touching the sides of the beaker; adjust if necessary. See the figure.

Wait until capillary action draws the water up to a distance 5 mm from the rod. Remove the paper. Mark the boundary of the solvent with a pencil. Allow the paper to dry in the air.

Figure. Developed chromatogram.

For each pen, note the number and color of component dyes. Which dyes, if any, move at the same rate?

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Questions
  1. Do any of the inks appear to be composed of a single colored component? Explain your reasoning.
  2. Which inks appear to be composed of multiple dyes? What colors are in them?
  3. Which dyes appear to adhere to the paper the best?
  4. Which dyes appear to adhere to the solvent the best? What conclusion might you draw about the structure of the molecules of those dyes?
  5. What would happen if you took the individual colors (that is, dyes) of a multiple colored ink, dissolved them in water, and mixed them? (Try this if time permits.)
  6. Will ink from your writing pen separate into individual dyes? (Try it if time permits.)

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Safety

Wear eye protection at all times.

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TG Answers
  1. Usually one of the pens will contain a marking fluid composed of a solution of a single dye. This pen will lead to a single spot on the paper chromatogram.
  2. Some of the inks will contain two or more dyes.
  3. The dyes that move the smallest amount adhere best to the paper.
  4. The dyes that move the most "adhere" to the solvent best.
  5. It is possible to produce writing fluids by mixing the appropriate dyes.
  6. Few of the students will have pens with water soluble inks. For those that do, the darker colors usually separate into several dyes.

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TG Lab Hints

Allow 15 minutes for the water to rise up the paper. Check to make sure that the paper does not touch the walls of the beaker.

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TG Materials

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