Chemistry for Everyone edited by
JCE DigiDemos: Tested Demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
A Clock Reaction Sympathetic Ink from Consumer Chemicals submitted by:
Stephen W. Wright* Pfizer Global Research and Development, Groton, CT 06340; *
[email protected] Marsha R. Folger and Matthew A. Rice† Lyme-Old Lyme High School, Old Lyme, CT 06371
checked by:
Michael D. Roadruck Ottawa Hills High School, Toledo, OH 43606 Kyle B. Yeakel Department of Chemistry, Kutztown University, Kutztown, PA 19530
Chemical reactions that produce color changes are popular with many audiences. They are especially effective when presented as “chemical magic” demonstrations to arouse interest in science with students or with public audiences of all ages. In connection with our ongoing effort to develop chemical demonstrations using consumer chemicals we have developed a new clock reaction sympathetic ink that produces a black color that appears at a predictable length of time following development. The impetus for this demonstration was a suggestion to adapt the “Vitamin C Clock Reaction” (1) into a simple, relatively safe reagent system that could be used to make “magic writing” for use in science outreach programs. We wished to employ only consumer-available chemicals that might be purchased at a supermarket or pharmacy and commonly found in the home. The use of consumer products also permits stimulating chemistry to be carried out in distance-learning situations. Many reagent systems are known that can be used to prepare sympathetic inks (2). However, we are not aware of a sympathetic ink that uses a “clock reaction” to cause the writing to appear after a predictable and controllable period of time. Having developed a clock reaction using ascorbic acid (vitamin C), hydrogen peroxide, iodide ion, and starch, we decided to use this reagent system to determine whether it could provide the desired “clock reaction writing” effect. Ascorbic acid reacts with iodine according to the following equation: The byproduct of the ascorbic acid oxidation is hydriodic acid (HI), which is readily oxidized to elemental iodine and water by many oxidizing agents, including hydrogen peroxide: Thus the reaction between hydrogen peroxide and ascorbic acid (vitamin C), mediated by an iodide–iodine electron carrier system, provides the basis for the clock reaction. When † Current address: University of Connecticut, Storrs-Mansfield, CT 06269.
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the ascorbic acid is exhausted, the iodine produced in the second equation reacts with starch to produce a blue–black color. The necessary reagents were vitamin C supplement tablets to supply the ascorbic acid, 10% povidone–iodine topical disinfectant to supply iodine, and 3% hydrogen peroxide. The necessary starch would be supplied by the paper itself, as starch is incorporated in nearly all commercial papers as a filler or sizing material. In order to produce the desired clock reaction effect, we decided that the most practical approach would be to apply the vitamin C and iodide ion to the paper as the sympathetic ink, and to use a spray of hydrogen peroxide as the developing agent. As different papers may or may not contain alkaline materials that could inhibit the desired clock reaction, dilute acetic acid (white vinegar) was added to the hydrogen peroxide solution to provide a constant, weakly acidic pH for the clock reaction to proceed.1 In order to identify a mixture of vitamin C and iodide that would give a satisfactory clock reaction on paper after being sprayed with hydrogen peroxide solution, we prepared vitamin C solutions of various concentrations (1 M, 0.1 M, 0.01 M, and 0.001 M) and combined them with various dilutions of the povidone–iodine solution (1:1, 1:2, 1:4, and 1:8) in a 2:1 volume ratio to provide test sympathetic ink solutions containing ascorbic acid and iodide ion. We then screened the different mixtures to determine which, if any, would afford the desired clock reaction sympathetic ink effect when sprayed with hydrogen peroxide. We immediately found that the 1 M and 0.1 M vitamin C solutions were too concentrated to produce a clock-reaction effect, as the writing was not developed upon treatment with the hydrogen peroxide spray. The combinations of 0.01 M and 0.001 M vitamin C with various dilutions of povidone–iodine gave satisfactory ink mixtures, with varying times elapsing before the writing appeared (Table 1). The various combinations of different concentrations of vitamin C with different dilutions of the povidone–iodine afford ink solutions that contain a range of final concentrations of vitamin C. As the povidone–iodine mixture that is added is made increasingly dilute, less vitamin C is consumed by the iodine, resulting in a higher final concentration of
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Chemistry for Everyone Table 1. Results of Var ying the Reactant Concentrations in the Clock Reaction Iodine-Water
Vitamin C Solution 0.01 M
0.001 M
1:1
Writing appeared almost immediately after spraying
Unsuitable; solution still contained free iodine
1:2
Writing appeared within 15 seconds after spraying
Writing appeared almost immediately after spraying
1:4
Writing appeared within 25 seconds after spraying
Writing appeared within 15 seconds after spraying
1:8
Writing appeared within 40 seconds after spraying
Writing appeared within 30 seconds after spraying
vitamin C in the ink solution. This results in increasing the time that elapses before the writing appears. It is therefore possible to use various combinations of vitamin C and povidone–iodine to make different parts of a message appear at different times after being sprayed with the hydrogen peroxide. For example, a sign may be prepared that reads “EXAM NEXT TUESDAY” almost immediately when sprayed. Following the expected chorus of groans from the class, the word “CANCELLED” will appear. As many of the experimental conditions will vary from user to user, such as the type of paper used, the quantity of ink applied, and the quantity of hydrogen peroxide applied, it will be necessary for each user to try the reaction to determine what mixtures work best in his or her hands. It should be noted that the writing does not appear suddenly, as is customary for a conventional clock reaction carried out in solution, but instead the writing appears more gradually. We believe that the explanation for this phenomenon is simply that the reaction is not being conducted in a homogeneous aqueous solution. When the hydrogen peroxide and vinegar solution is sprayed onto the paper, the peroxide and acetic acid must diffuse into the fiber matrix to react with the iodide ion and vitamin C that are distributed within the paper fiber matrix at the beginning of the demonstration. Subsequently, the reactants and reaction intermediates, particularly the product(s) resulting from the oxidation of iodide ion, must also diffuse through the fiber matrix. The net result is that the reaction mixture absorbed on the paper is not homogeneous, and therefore the endpoint is reached at different times in different microscopically small locations. We also explored the use of the vitamin C–iodine system to produce a conventional sympathetic ink. In this case, solutions of varying concentrations of vitamin C were applied to the paper and allowed to dry. The paper was subsequently sprayed with varying dilutions of iodine solution. The writing with the vitamin C solution appeared as white writing against a dark background. The best results were obtained by using a 0.1 M solution of vitamin C as the sympathetic ink and a 1:8 dilution of povidone–iodine as the developer. More dilute solutions of vitamin C (0.01 M, 0.001 M) or more concentrated povidone–iodine sprays (neat or 1:1) produced progressively lighter and less discernible writing on the paper.
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Experimental Section Generic 500 mg ascorbic acid (vitamin C) tablets,2 povidone–iodine antiseptic,3 3% hydrogen peroxide,4 and white vinegar,5 were purchased from a local supermarket. Liquid volumes were measured using Nalgene polypropylene student-grade graduated cylinders. If laboratory graduated cylinders are not available, satisfactory measures (such as those used for dosing liquid medicines) may be purchased at a pharmacy. Additional materials required are cotton swabs, watercolor paper, a trigger spray bottle, disposable plastic spoons for stirring, and containers in which to prepare the ink mixture. Laboratory ascorbic acid may be used if it is available.
Test for Starch in Paper Take a small piece of the watercolor paper and place a drop of the iodine on it. If a purple–black color change occurs, then the paper is suitable for the clock reaction writing experiment. Preparation of Ascorbic Acid Solutions Two 500 mg vitamin C tablets were combined with 600 mL of water to yield an approximately 0.01 M solution. The tablets were finely pulverized in the water until no particles remained. Fifty milliliters of the 0.01 M solution was diluted with 450 mL of water to afford the 0.001 M solution. Preparation of Iodine Solutions Pharmacy 10% povidone–iodine solution was combined with water to afford the iodine solutions. The 1:1 dilution was prepared from 10 mL of 10% povidone–iodine and 10 mL of water, the 1:2 dilution was prepared from 10 mL of 10% povidone–iodine and 20 mL of water, the 1:4 dilution was prepared from 10 mL of 10% povidone–iodine and 40 mL of water, and the 1:8 dilution was prepared from 10 mL of 10% povidone–iodine and 80 mL of water. Preparation of Clock Reaction Ink Solutions Dispense 10 mL of the 0.01 M Vitamin C solution into each of four small beakers or other containers. To the first small beaker, add 5 mL of 1:1 iodine–water solution. Repeat the process with the other three small beakers using the 1:2, 1:4, and 1:8 solutions of iodine–water. Clock Reaction Writing Use the ink solutions to write a message on the paper. If desired, the message can be very lightly sketched in pencil first. A small watercolor brush or cotton swab can be used to apply the solutions to the paper, taking care to saturate the paper in the areas where the writing is to appear. A different brush or swab should be used for each ink solution. White construction paper and white blotter paper are recommended. The paper is allowed to dry thoroughly at room temperature. When dry, the message paper is sprayed evenly with a mixture prepared by combining equal volumes of 3% hydrogen peroxide and white vinegar. A layer of paper toweling or newspaper behind the sign can be used to keep the developing solution off of surrounding surfaces.
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Chemistry for Everyone
Conventional Sympathetic Ink Reaction
Notes
Two 500 mg vitamin C tablets were combined with 60 mL of water to yield an approximately 0.1 M solution. The tablets were finely pulverized in the water until no particles remained. Using this solution as ink, a message is written on the paper. If desired, the message can be very lightly sketched in pencil first. A small watercolor brush or cotton swab can be used to apply the solutions to the paper, taking care to saturate the paper in the areas where the writing is to appear. White construction paper and white blotter paper are recommended. The paper is allowed to dry thoroughly at room temperature. When dry, the message paper is sprayed evenly with a mixture prepared by combining one volume of 10% povidone–iodine with eight volumes of water. A layer of paper toweling or newspaper should be placed behind the sign can be used to keep the iodine solution off of surrounding surfaces.
1. Previous investigation of this clock reaction showed that the reaction rate became much slower as the pH of the reaction medium approached neutrality. 2. Tablets marked as “chewable” or “flavored” should not be used. They contain much higher quantities of excipients, including sweeteners and flavors, which may interfere with the clock reaction. 3. Povidone–iodine (10%) contains 0.04 M I2 by iodometric titration. 4. 3% Hydrogen peroxide corresponds to approximately 0.9 M H 2 O2 . 5. White vinegar is a dilute (5%) solution of acetic acid in water, containing 0.9 M acetic acid when determined by acidimetric titration.
Hazards
Literature Cited
Elemental iodine is poisonous. Povidone–iodine must not be ingested. In case of accidental ingestion, seek professional assistance or contact a Poison Control Center immediately. Acknowledgments The authors wish to thank Ed Vitz for inspiring this experiment and Lyme-Old Lyme High School for enthusiastic support of this work.
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1. Wright, Stephen W. J. Chem. Educ. 2002, 79, 41–43. 2. See, for example: Tested Demonstrations in Chemistry; Alyea, H. N., Dutton, F. B., Eds.; Journal of Chemical Education: Easton, PA, 1965; p 46. Ford, L. A.; Grundmeier, E. W. Chemical Magic, 2nd ed.; Dover: New York, 1993; p 9. Summerlin, L. R.; Ealy, J. L., Jr. Chemical Demonstrations: A Sourcebook for Teachers; American Chemical Society: Washington DC, 1985; p 145.
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