Instructor Side
JCE Classroom Activity: #24
The Write Stuff: Using Paper Chromatography to Separate an Ink Mixture by the Journal’s Editorial Staff
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Integrating the Activity into Your Curriculum Many articles and activities have been written concerning chromatography, in JCE and elsewhere. Several references are provided (1–3). Paper chromatography is a simple activity that requires minimal equipment. The separation of ink via paper chromatography is a standard experiment in many courses. This activity is a qualitative version of the standard experiment. It also introduces students to methods of selecting the best solvent for a separation and the effects of adding acid and base, extensions that may not normally be in a standard experiment. The activity may be used to introduce a more quantitative experiment or to suggest ideas for additional experimentation or student projects. This activity can lead to a discussion of mixtures and ways to separate their components. Discussion of how separation is achieved through chromatography is left to the instructor. Capillary action, solute–solvent interaction, and differences in polarity and solubility may be more meaningful to students after completion of the activity. The activity is designed to be fun, colorful, and open-ended. It might be used to interest students in chemistry at the beginning of the year, to suggest ideas for projects, or as a year-end activity. It could be incorporated into a crime lab simulation, where students match separated ink components from known pens with those from a sample of “evidence”.
About the Activity Many thanks to Doris Kolb, whose observations and suggestions formed the basis of this activity. The activity can be done at home or in the lab. All materials needed are commonly found in the home. Solvents investigated are water, rubbing alcohol (70% isopropyl alcohol), vinegar, household ammonia solution (blue sudsy ammonia can be used), and various mixtures of these. Black Flair markers are recommended. Any water-soluble marker could be used, but the colors observed and the order in which they appear varies. Pint jars are a good match for the length of coffee filter strips and small plastic cups work well with shorter filter paper strips. If the activity is done in a school setting, it may be necessary to spread it over several days. Other course work can be done while the chromatograms develop. Several optional activities are suggested on the student side. Instructors may wish to have students design their own experiments to test a hypothesis of their choosing. Students are instructed to mark chromatograms so that Rf values can be determined, but the instructor will need to provide the necessary directions. The separation achieved by this procedure will generally be sufficient to identify the colors of dye present, but may not give well-separated spots. The December 1992 issue of JCE (1) features a collection of chromatography activities. They include instructions for chromatography on T-shirts, radial chromatography with coffee filters (the results of which are colorfully displayed on the cover), and the separation of dyes in candy.
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Answers to Questions __1. The paper strip and solvent molecules adsorbed by the cellulose form the stationary phase. The mobile phase is a liquid solvent. There are several way to increase the separation of components. One way is to use a longer paper strip. __2. The minimum number of components is the maximum number of colors observed using any solvent. It is possible that not all components are separated. Ink from the black Flair pen we tested contains at least four components. __3. Results will vary. Typical results for steps 2 and 6 using black Flair pens and coffee filters are summarized by solvent in the table. Color changes may be due to reactions of some dyes with acid or base or to hydration states of organic dyes. Order of appearance of color is deammonia rubbing alcohol + alcohol + termined by attraction of the dye to the solwater vinegar solution alcohol vinegar ammonia vent. In some cases, different molecules may have about the same attraction and appear yellow green green dark blue green green as a single color spot. Reaction with acid green orange pale pink orange purple yellow or base may change the polarity of molecules. In step 7, the degree of separation pale pink dark purple purple dark pink orange purple and the order in which colors appear can — — — purple pink pink vary with the concentration of the solvent.
Additional Activities and Demonstrations 1. J. Chem. Educ. 1992, 69, 977–980, 987–998; multiple activities on chromatography for the precollege student. 2. Sarquis, M.; Sarquis J. Fun with Chemistry, Vol. 2; Institute for Chemical Education: Madison, WI, 1993; pp 1–35; multiple activities from section “Colorful Separations”. 3. Borgford, C. L.; Summerlin, L. R. Chemical Activities, Teacher Edition; American Chemical Society: Washington, DC, 1988; pp 257–263; multiple chromatography activities.
This Activity Sheet may be reproduced for use in the subscriber’s classroom. JChemEd.chem.wisc.edu • Vol. 77 No. 2 February 2000 • Journal of Chemical Education
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JCE Classroom Activity: #24
Student Side
The Write Stuff: Using Paper Chromatography to Separate an Ink Mixture by the Journal’s Editorial Staff The word chromatography comes from two Greek words meaning “color” and “to write.” Chromatography is one of the most important analytical techniques used by chemists. Several forms of chromatography have been developed that can be used to separate many kinds of mixtures, not just those containing colored components. All chromatographic methods separate the components of mixtures by distributing the components between stationary and mobile phases. You can use a simple chromatographic technique, paper chromatography, to find out if ordinary black marking pens contain only black ink, or a mixture of colored dyes.
Try This You will need: four containers (about the same height as strips prepared in step 1), water, rubbing alcohol (70% isopropyl alcohol), household ammonia, vinegar, coffee filters or filter paper, black Flair marker (or other nonpermanent black marker), scissors, ruler, four pencils, tape, paper towels, and a graduated cylinder or measuring spoons. __1. Cut several strips from coffee filters or pieces of filter paper. Make the strips 1–2 cm wide and as long as possible. Cut off the rounded edges to form rectangular strips. Use a pencil to draw a straight line across the narrow end of each strip about 2 cm from the end. The end with the line is now the bottom of the strip. __2. Label the four containers with numbers 1–4. Add the following solvents to the containers to a depth of about 1 cm: 1, water; 2, rubbing alcohol; 3, vinegar; 4, household ammonia solution. __3. Touch the tip of the black marker to the center of the pencil line on one of the strips to make a small spot 1–2 mm in diameter. Allow the black spot to dry, then touch it again with the marker tip. Repeat for three more strips. __4. Balance a pencil across the top of container 1. Dip one of the spotted strips into the container so that the bottom of the strip is beneath the level of the solvent but the pencil line and spot are above it, and the strip does not touch the Pencil sides of the container. Tape the strip to the pencil to hold it in place. Repeat with Paper the remaining strips and containers. strip __5. The solvent will move up the paper strip owing to capillary action. What happens to the ink spot when the solvent reaches it? When the solvent is near the top of a strip, Ink spot remove that strip from its container and place it on a labeled paper towel. Quickly Solvent mark the highest level reached by the solvent with a pencil, then allow the strip to dry. Do not continue to the next step until all four strips have been removed and marked. __6. Empty the containers into a sink and rinse them with water. To containers 1 and 2, add the following solvent mixtures to a depth of about 1 cm: 1, 50% ammonia/50% rubbing alcohol; 2, 50% vinegar/50% rubbing alcohol. Repeat steps 3–5 using these two containers. __7. Repeat step 6 using different concentrations of ammonia and vinegar (25%, 75%; etc.) in rubbing alcohol. Does changing the composition of the mixture affect the results? Optional (Consult your instructor before trying any additional experiments.) __1. Design a procedure to determine if different brands of black markers contain the same components, if different colors of the same brand of marker contain the same components, or to separate dyes in Crayola’s Changeables Color Change Markers both before and after using the color changer marker. __2. Design a procedure to determine the effect of using different types of paper (coffee filters, filter paper, white paper towels, paper napkins, tissue, etc.) __3. Design a procedure to separate components of other water soluble mixtures such as artificial food colors, or natural products such as red cabbage extract. __4. Determine Rf values for each component of a mixture.
Questions __1. In paper chromatography, what is used as the stationary phase? The mobile phase? How could you improve the separation of components in this experiment? __2. How many different colored dyes are in the black ink? __3. Compare and contrast the results obtained with the solvents in steps 2, 6, and 7. Provide a possible explanation for any differences you observe.
Information from the World Wide Web
(accessed December 1999)
Chromatography. http://antoine.fsu.umd.edu/chem/senese/101/matter/chromatography.shtml Chromatography. http://www.mtnimage.com/~toad/chromo/chroma.htm Annotated List of Chromatography Web Resources. http://www.mtnimage.com/~toad/chromo/resource.htm This Activity Sheet may be reproduced for use in the subscriber’s classroom. 176B
Journal of Chemical Education • Vol. 77 No. 2 February 2000 • JChemEd.chem.wisc.edu
