Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Prepare filter paper “wicks” for the students by cutting 12.5-cm pieces of filter paper into eight pie-shaped wedges. Prepare enough wedges so each student has a fresh wick for each chromatogram.
2. Label the pens as A, B, C, etc. for easy identification and place them in a central area of the lab so students can share.

Safety Precautions

While this activity is considered nonhazardous, follow all appropriate laboratory safety rules.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Water may be flushed down the drain according to Flinn Suggested Disposal Method #26b. The filter paper disks may be saved or disposed of in the trash according to Flinn Suggested Disposal Method #26a. Save all other materials for future use.

Teacher Tips

• Enough materials are provided in this kit for 30 students, working individually, to each make six radial chromatograms. Two hundred filter paper circles are provided—20 to make wicks and 180 to make chromatograms.
• It is suggested that the markers be numbered (coded) and placed in a central area so that the students can share them.
• Pens such as Vis-à-vis, Expresso, Le Pen, Papermate Flair, Boldliner, Mr. Sketch, Prang and Vis-Aid work well. Note that the specific brands of pens included in the kit may vary depending on availability.
• Experiment with a variety of different water-soluble markers or felt tip pens to determine the composition of each. Many different brands are available at local stores. Have students bring in their own markers to try. Use colors other than black.
• Coffee filters can be used as a suitable substitution for the filter paper. The “ruffled” sides of the coffee filter should be removed with scissors. Water, however, is more quickly absorbed by coffee filters than by the filter paper. This has a tendency to lower the separation and resolution of the ink pigments.
• If a chromatogram is running too slowly or not at all, check to make sure that the wick has been inserted snugly into the hole and that there is good contact between the wick and the inside edge of the hole.
• Avoid excessive handling of the filter paper circles. Oils from the skin can interfere with the capillary action that draws the water through the paper.
• Make the spots as concentrated as possible. To maintain the same intensity of color for the large-scale chromatograms, be sure to make the spots proportionately darker and larger. This is best done by holding the pen to the paper for a longer period of time or, even better, by re-spotting the same point several times, allowing the ink to dry between each spotting.

Further Extensions

• Before discussing chromatography, spot the filter papers ahead of time for the students. Make one spot with one black marker, and the remaining five spots with a different black marker. Hand out a paper to each student and have them notice that all six spots look identical. Challenge them to come up with a method to determine which spot is a different ink than the others. Lead them to the idea of chromatography and its usefulness in identification of unknowns.
• A terrific extension of this activity is to adjust the polarity of the solvent to try to achieve different separations. In fact, when a 50/50 mixture of water and acetone was tried, very interesting results were obtained. A yellow ink, which was previously thought to only contain one pigment, separated into two yellow pigments, one of which was fluorescent. The yellow and pink pigments in red and brown inks actually reversed their order on the paper.

Discussion

Demonstration of Large-Scale Chromatography

Introduction
Demonstrate radial chromatography to students by starting this large-scale version at the beginning of the lab period. Allow it to run throughout the lab, and observe it at the end. The large-scale radial chromatogram can be laminated and makes for a beautiful wall display.

Procedure

1. Follow the student Procedure for preparing the filter paper, except use the 24-cm diameter circles and make the spots proportionately darker and larger (see Tips section).
2. Place one of the 8" clear plastic saucers on the table. Place a 1-oz plastic polypropylene cup into the center of the large saucer.
3. Fill the small cup to near the top with tap water.
4. Set the large prepared filter paper circle on top of the dish with the wick extending down into the water. The small center cup should help to support the center of the circle; the rim of the dish should help to support the outer edge (see Figure 2).
   {14133_Discussion_Figure_2}
5. Cover the circle with the second plastic saucer. Allow the chromatogram to run to within 1–2 cm of the outer edge of the circle. This may take 60–80 minutes. When it is done, carefully lift the chromatogram and set it on a paper towel to dry.

References

Special thanks to Bob Becker, Kirkwood High School, Kirkwood, MO, for providing us with this activity.

Introduction

Use radial chromatography to separate the components in various black ink mixtures and watch as brilliant patterns of pigments form on the radial chromatograms.

Concepts

• Chromatography
• Separation of mixtures
• Physical properties

Background

Chromatography is probably the most useful method of separating organic compounds for identification or purification. There are many different types of chromatography, but most work on the principle of adsorbtion. The two important components of chromatography are the adsorbent and the eluent. A good adsorbent is usually a solid material that will attract and bind the components in a mixture. Paper, silica gel or alumina are all very good adsorbents. The eluent is the solvent that carries the materials to be separated through the adsorbent via capillary action.

Chromatography works on the principle that the compounds to be separated are slightly soluble in the eluent and will spend some of the time in the eluent (or solvent) and some of the time on the adsorbent. When the components of a mixture have varying solubilities in the eluent, they can then be separated from one another. The polarity of the molecules to be separated and the polarity of the eluent are very important. This affinity for the eluent versus the adsorbent is what separates the molecules.

Paper chromatography is commonly used as a simple analytical separation technique. In paper chromatography, the adsorbent is the paper itself. The eluent can be any number of solvents; in this lab, the eluent is water. Water is a very polar molecule. The polarity of the eluent is very important in paper chromatography since a small change in polarity can dramatically increase or decrease the solubility of some organic molecules. The organic pigments in the inks, which will be “spotted” on the filter paper, separate out as they are carried with the water at different rates. Those molecules that have a polarity closest to the polarity of the water will be the most soluble and will move outward on the radial chromatogram the fastest.

Many materials, such as the ink in felt-tip pens, are actually mixtures made up of several different organic compounds, or pigments. Each of these pigments has a different molecular structure and, usually, a different polarity. Many of these pigments can be easily separated using paper chromatography, because even when mixed together, they tend to maintain their own characteristic physical properties.

Materials

Safety Precautions

While this activity is considered nonhazardous, follow all appropriate laboratory safety rules.

Procedure

1. Obtain a piece of filter paper. Use a sharp pencil tip or pushpin to puncture a small hole into the center of the filter paper.
2. Using a black (or dark color) water-soluble marker, draw four to six small but concentrated dots in a circular or random pattern around the center hole in the filter paper, about 1 cm out from the hole. (Note: You may wish to try two different markers, A and B, and alternate the dots, A-B-A-B, in a circle around the center hole. The flowerlike pattern can be quite impressive.) See Figure 1.
   {14133_Procedure_Figure_1}
3. Fill a plastic cup to within about one centimeter from the top with tap water. Use a paper towel to dry off any drops of water that are on the rim.
4. Roll up a wedge-shaped filter paper “wick” into a tight cone and insert it into the center hole (see Figure 1).
5. Set the prepared filter paper circle on top of the water-filled cup and observe as the water is soaked up through the wick and then outward radially across the paper. Observe how the advancing water front acts on the spots of black ink.
6. When the water has advanced to within 1–2 cm of the outer edge of the circle (should take 10–12 minutes), carefully lift the chromatogram up and set it on a paper towel to dry.
7. Dispose of the water down the drain. The filter paper disks may be saved or discarded in the trash. Save all other materials for future use.