Materials Included In Kit

Additional Materials Required

Prelab Preparation

Make a small hole in the center of each plastic rotating disk. The hole should not be too large or else the disk will not spin with the motor axle. To make a small hole—heat up the tip of a dissecting needle or an outstretched paper clip in a candle flame. Puncture the plastic disk in the center with the heated end of the needle or clip. Caution: Perform this simple procedure before class and in an well-ventilated hood, as there may be a melted plastic odor. Wash hands thoroughly with soap and water before leaving the laboratory.

Safety Precautions

Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Do not touch the motor axle while the rotor is spinning. Remove the battery from the centrifuge device when not in use and during storage.

Disposal

Dispose of the water down the drain. The filter paper disks can be saved or discarded in the trash. Save all other materials for future use.

Teacher Tips

• Two chromatography centrifuge devices are provided in this kit, along with enough filter paper and markers to make at least 90 radial chromotograms. If students work in groups, the two devices should be enough for a class of 30 students to share since each separation takes no longer than 30 seconds. If possible, the ideal situation is to purchase one device for each pair of students. This laboratory activity can reasonably be completed in one 50-minute class period.
• Use water-soluble markers or felt tip pens. Many different brands are available at local stores. Have students bring in their own markers to try. A set of eight water-soluble marking pens is available through Flinn Scientific, Catalog No. AP8466. Experiment with a variety of different black water-soluble markers to determine the composition of each. Also try different- colored markers other than black to see the pigments in each.
• The radial chromatogram is complete when the water line is near the edge of the filter paper (about 1–2 cm away). Lay the disks flat to dry.
• Coffee filters can be used as a suitable substitute for the filter paper. The “ruffled” sides of the coffee filter should be removed with scissors. The water is quickly absorbed by the coffee filter, which reduces the separation quality of the pigments.
• To demonstrate the advantages and disadvantages of centrifugal chromatography, have your students conduct a standard paper chromatography experiment with water-soluble markers and compare the results. The separations are better with the prolonged developing time. The chromatography centrifuge provides faster results, but the resolution is slightly lower.
• Teachers who have access to a video projection system, such as a FlexCam^®, may wish to demonstrate the results to the entire class. Teachers may want to count aloud the drops of water added to show students the small quantity of water needed to achieve excellent results.
• Students are often tempted to add the water to the filter paper too quickly, causing the water to fly off the spinning platform. The filter paper may also tear if the water is added too rapidly.

Further Extensions

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 strips.

References

Flinn Scientific would like to thank Jeff Bracken, chemistry teacher at Westerville North High School, Westerville, OH, for sharing this idea with us. Jeff would like to thank David Katz for introducing him to the idea of spinning chromatography at Chem Ed ’97. His incredible creativity laid the foundation for designing this apparatus. Jeff would also like to thank Ryan Lykens, his student assistant, for helping to develop the Chromatography Centrifuge.

Introduction

Use spinning (radial) paper chromatography to separate the components of an ink mixture from a water-soluble, felt-tip pen. The chromatography centrifuge device allows separations to be performed faster and easier than traditional methods and results in amazingly beautiful 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 absorbance. The two important components of chromatography are the absorbent and the eluent. A good absorbent is usually a solid material that will attract and absorb the materials to be separated. Paper, silica gel, or alumina are all very good absorbents. The eluent is the solvent which carries the materials to be separated through the absorbent.

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 absorbent. 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 absorbent is what separates the molecules.

Paper chromatography is commonly used as a simple separation technique. In paper chromatography, the absorbent 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” onto 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 bethe 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 characteristic physical properties.

The typical paper chromatography method requires a full 20–30 minutes to perform because the developing solvent (usually water) must migrate along the length of the piece of paper through capillary action. While the results are impressive, the length of time required to complete a lab of this type can be a major drawback.

If, however, the paper chromatography system is spinning, the length of time needed for the solvent to migrate along the paper can be greatly reduced. Just as in the spin cycle of a washing machine, the water will spread out quickly when dropped onto a spinning piece of filter paper. The spinning action (or centrifugal force) of the chromatography centrifuge accelerates the radial flow of water through the adsorbent (filter paper), forcing the mobile phase (water) outward through the filter paper. As a result, the water migration is faster than it would be via simple capillary action. This effect is similar to that observed when pressure is applied in a chromatography column. Impressive results are obtained in less than 30 seconds!

The laboratory technique, known as centrifugal chromatography, has been used for over 30 years in research laboratories. The benefits of using a chromatography centrifuge device are now appreciated in the high school science laboratory.

Materials

Safety Precautions

Do not touch the motor axle while the rotor is spinning. Remove the battery from the centrifuge device when not in use. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

1. Obtain a piece of filter paper. Determine the center of the piece of filter paper. To do this, set the plastic disk onto the motor axle of the centrifuge device. Set the piece of filter paper onto the plastic disk and mark the center hole. Use a sharp pencil tip or push pin to puncture a small hole into the center of the piece of filter paper.
2. Using a black (or dark-colored) water-soluble marker, draw four to six small dots in a circular or random pattern around the center hole in the filter paper. (Note: You may wish to use two different markers and alternate the dots in a circle around the center hole.) See Figure 1.
   {11935_Procedure_Figure_1}
3. Place the plastic rotating disk (with a center hole) onto the exposed motor axle of the chromatography centrifuge. Set the filter paper on the disk. The axle should penetrate the center hole of the filter paper to secure the paper to the rotating disk (see Figure 2). Note: This avoids the use of tape to hold the filter paper, although that is an option if necessary.
   {11935_Procedure_Figure_2}
4. Start the motor running by completing the circuit on the centrifuge using the attached alligator clips. Watch the disk rotate around in a circle. Using a Beral-type pipet, add a few drops of water on the center of the rotating piece of filter paper. Add additional (yet minimal) drops of water if necessary (see Figure 3). Note: If water is added too quickly, water and ink can be thrown from the disk.
   {11935_Procedure_Figure_3}
5. Watch carefully and make observations as the filter paper rotates on the centrifuge. Disconnect the clips when the chromatography separation is complete.
6. Dispose of the water down the drain. The filter paper disks can be saved or discarded in the trash. Save all other materials for future use.

Post-Lab Questions
(Answer the following questions on a separate piece of paper.)

1. What colored pigments make up each black ink?
2. Why do different black inks separate into different pigment patterns?
3. Which inks appear to be made up of more than one pigment? Which inks appear to be a single pigment?
4. Knowing that water is a very polar solvent, what can you infer about the relative polarities of the various pigments in each ink?
5. What are the benefits of centrifugal chromatography? What are the disadvantages?
6. Optional: If an ultraviolet (UV or “black”) light is available, shine it on each of the strips in a darkened room. What do you see? Can you make any additional inferences about the pigments in the various inks?