Experiment 2. chromatography of colored inks1
Instructor's Guide - Experiment 2
is it pure or a mixture?
Chromatographic study of DYES and inks
Brief Description and Objectives
This is a very simple laboratory exercise that can be done in the same lab period with another experiment, either at the beginning of the course or sometime later. It is colorful and entertaining, as well as teaching some basic concepts about pure substances, mixtures, and a way to separate & identify the components of mixtures. (For this reason it relates naturally to the discussion of pure substances and mixtures in chapter 1 of Chemistry in Context.) It also provides an opportunity for students to revisit what they had learned in earlier years about combinations of colors.
Pens designed for use with overhead projectors contain water-soluble inks that work well for this exercise, and for this edition an alternative procedure using food dyes is also provided, allowing the experiment to be done in conjunction with chapter 11 as well. Students spot different colors onto a rectangular piece of filter paper and use water to elute the colored spots. Examination of the resulting chromatogram shows clearly which inks are single colored substances, which are mixtures, and which colored components match between the different colored pens.
Estimated Time Required:30 min
Materials Needed for 10 Pairs of Students
Although most experiments in this lab manual are designed to be done by pairs of students working together, this exercise is so simple that each student should prepare a chromatogram rather than two students sharing one.
20 beakers, 400 mL or larger recommended. (Alternative: one beaker per pair, with students taking turns using it)
20 glass stirring rods, long enough to rest on top of the beakers.
0 rectangular pieces of filter paper ca. 7 cm x 12 cm, depending on size of beakers used. Sized so that they fit in the beakers and are long enough to bend over a stirring rod resting horizontally on top of the beaker. They can be precut or 15 cm filter paper circles can be provided along with scissors.
10 metric rulers. (whatever length is conveniently available)
Pencils for labeling the chromatographic sheets.
One or both of the following samples:
• Set of colored felt-tip marking pens. Minimum of 4 colors; more if possible. Highly recommended:Sanford Vis-à-Vis transparency marking pens. Alternatively, any water-soluble felt-tip pens with vivid colors can be used, such as Liqui-Mark “Washables” ( that are sold with art supplies for children.
• Kool-aid or similar drink mix, dissolved in the minimum amount of water to make a concentrated solution.
Notes
1. With Vis-à-Vis Transparency Marking Pens, student observations will likely include the following. (Some students may need gentle prodding from the instructor to see the relationships.) The blue and red inks are actually mixtures of two similarly-colored dyes. Green, orange and purple are mixtures as expected, with spots matching those in other inks. Black is a mixture of at least three components. In water, the elution order (from greatest movement to least) is blue, red, yellow
2. An interesting variant is to use a circle of filter paper with a narrow strip cut to the middle and bent down to form a wick. The paper can then be placed on top of a small beaker or a Petri dish containing water.. Spots of different inks can be placed in a small circle near the middle. Alternatively, one colored spot can be placed right at the center of the filter paper, which should produce concentric rings of color!. In some classes this has spawned a whole bevy of creative applications, with students trying different configurations. Some students see the analogy to tie-dying.
3. The experiment can be made more quantitative by showing students how to calculate Rf values for each colored spot, then asking students to tabulate the Rf results. (Rf is discussed in Experiment 25.)
4.Additonal studies are possible. (a) Shift to alcohols or water/alcohol mixtures. (b) The water can be made acidic (e.g. 1 M HCl) resulting in a dramatic change in elution of at least one dye. Depending on their chemical knowledge, students may be able to develop an explanation for the change in elution order. (Protonation makes some ink components less soluble in the polar water solvent.)
5. Other brands of transparency marking pens are likely to contain somewhat different dyes.
6. For a recent reference on the history of paper chromatography see J. Chem. Educ., 79, 922 (August 2002)
Suggested Answers to Questions
1. Which of the dyes or inks appear to contain a single colored substance, and which are mixtures? Describe your evidence
Answers will vary with the samples chosen. Students should base their answers on the number of spots seen after running the chromatography.
2. Which samples have colored components in common with other samples? Explain your reasoning.
Answers will vary with the samples chosen. Students should look for spots that travel the same distance and have the same colors.
3. From these results, predict which colored components are most soluble in water and which are least soluble in water. Explain your reasoning.
Answers vary with the samples chosen. The further a dye travels, the more soluble it is in water.
4. Describe an experiment you could do in your kitchen to convince a friend that the hard coating on an M&M contains a mixture of several food colorings. Explain what you would do and what you would see as results.
Answers will vary but should include similar components. Coffee filters, paper towels, and similar products can take the place of the filter paper. The colored dyes must be transferred to one end of the paper. This could be done by rinsing the candy in some water or even by wetting the candy and rubbing it on the paper. A drinking glass can take the place of our beaker. Students should expect results similar to those done in lab. A coating made from several food colorings should result in several separate spots after water is allowed to draw up the paper.
5. (If you did optional step 1) An interesting household problem is that of removing ballpoint pen stains from clothing. In light of your observations, can you suggest what will and will not likely work? What additional tests could you do to investigate this problem further?
Ballpoint ink was not soluble in water sine the spot didn’t move. We could try running our chromatography with various solvents. The solvent that moves the ink furthest will be best for removing ink stains.
6. (If you did optional step 2) Suggest a possible explanation for why the results using alcohol are different from those in water.
Some dyes are more soluble in water; some are more soluble in alcohol.
laboratory Instructor’s Guide chemistry in context, 7th edition
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