Chemistry for Everyone edited by
JCE DigiDemos: Tested Demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
A Salicylate Sympathetic Ink from Consumer Chemicals submitted by:
Stephen W. Wright Pfizer Global Research and Development, Groton, CT 06340;
[email protected] checked by:
Robert Hammond Department of Chemistry, East Carolina University, Greenville, NC 27858-4353
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 sympathetic ink that produces a violet color upon development. The impetus for this demonstration was our desire to have a simple, relatively safe reagent system that could be used to make a brightly colored, highly visible “magic sign” 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. Composition of Sympathetic Inks Many reagent systems are known that can be used to prepare sympathetic inks. Probably the most popular reagent combination is the use of a ferric salt solution as a developer, combined with solutions of thiocyanate and ferrocyanide to produce red and blue colors, respectively (1). These reagent combinations require access to laboratory reagents, and many of these reagents, particularly the transition-metal salts required, are toxic to varying degrees. Sympathetic ink formulas have also been prepared from consumer chemicals (2); however, we desired a formula that was entirely colorless so that the writing would not be visible on a white background. We were willing to sacrifice the ability to produce a palette of bright colors for the ability to use consumer chemicals to produce a single bold color. To this end, we recognized that the ferric ion represented the only metal cation that would likely be available on the consumer market and safe enough to use for such a system and that ferric salts are known to produce colored complexes with many compounds, both inorganic and organic (3). The reaction of ferric ion with salicylic acid in aqueous solution affords a deep red–purple color. This reaction forms the basis of qualitative analysis tests for both aspirin and for salicylic acid (4). The product has been isolated as garnet-red crystals and is thought to be a solvated chelate containing two molecules of salicylic acid coordinated to each ferric ion (5). After some experimentation, we devised a means to prepare a salicylic acid sympathetic ink solution using aspirin 542
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O O OH OH O OH O
CH3
CH3CO2C6H4CO2H aspirin
HOC6H4CO2H salicylic acid
Figure 1. Structures and formulas of aspirin and salicylic acids.
tablets as the source of salicylic acid, and a means to prepare an appropriate ferric ion developing reagent from commonlyavailable iron dietary supplement tablets. Using this procedure, dry invisible writing on white paper is made to “magically” appear as deep purple writing when lightly sprayed with the developing reagent. This has particular audience appeal at the beginning or end of a science outreach program. Experimental Background The sympathetic ink solution is prepared by dissolving aspirin tablets in a water solution of sodium carbonate, which is available to the consumer as washing soda.1 Aspirin is the common name for acetylsalicylic acid, which can be hydrolyzed to salicylic acid and acetic acid (CH3CO2H) (6) (Figure 1). Upon standing in the sodium carbonate solution, the aspirin in the tablets is converted to sodium salicylate and sodium acetate, according to the following equations: CO32−(aq) + H2O(l)
OH−(aq) + HCO3−(aq)
CH3CO2C6H4CO2H(aq) + OH−(aq) CH3CO2C6H4CO2−(aq) + H2O(l) − − CH3CO2C6H4CO2 (aq) + OH (aq) − − HOC6H4CO2 (aq) + CH3CO2 (aq)
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Chemistry for Everyone
Complete hydrolysis is rather slow and only occurs after three or four days, as determined by monitoring the reaction by 13C NMR. However, a satisfactory ink solution is obtained after the mixture has been allowed to stand overnight. Heating the mixture rather than waiting overnight or longer is not recommended because the controlled heating of such a small volume of liquid is impractical and possibly unsafe. In addition, the actual extent of the hydrolysis may be highly variable. To prepare the developing solution, a solution of a ferric salt must be prepared. As iron dietary supplement tablets are the only readily available consumer product containing a soluble iron salt, the developing solution is prepared from these tablets. The tablets supply iron in the form of ferrous ion, typically ferrous sulfate.2 The ferrous sulfate must therefore be oxidized to ferric sulfate for this demonstration. This can be accomplished by oxidizing ferrous sulfate to ferric sulfate in acid solution, using sodium hypochlorite (household laundry bleach)3 as the oxidizing agent, according to the following equation: + − + 2Fe2 (aq) + OCl (aq) + 2H (aq) + − 2Fe3 (aq) + Cl (aq) + H2O(l)
White vinegar proved to be a suitable acid for this reaction.4 The reaction of sodium hypochlorite with acids, including vinegar, can generate poisonous chlorine gas if no reducing agent is present. To avoid this possibility, ferrous sulfate is used in excess during the preparation of the developing solution. Thus the developing solution is actually a mixture of ferrous sulfate and ferric sulfate; however, this has no adverse effect on the color forming reaction. As a variety of iron supplements are available, it is important to note that the iron supplement tablets must supply iron as ferrous sulfate. Other types of iron supplements are not satisfactory for this experiment. When the developing solution is applied to the paper, the ferric ion in the developing solution reacts with the salicylate ion on the paper to develop the dark red ferric salicylate complex ion:
Experimental Section
Chemicals White vinegar (5% acidity), washing soda, and Clorox liquid chlorine laundry bleach (6% sodium hypochlorite) were purchased from a local supermarket and were used as purchased. Generic aspirin tablets (325 mg) and ferrous sulfate iron supplement tablets8 were purchased from a local pharmacy and were used as purchased. Liquid volumes were measured using Nalgene polypropylene student-grade graduated cylinders or kitchen measuring implements At the conclusion of the experiment, the aspirin solution and the developing reagent solution may be saved for future use. If they are not retained, they may be disposed of in the sanitary drains. The paper sign should be disposed of in the trash. Careful cleanup and audience control are essential for maximum safety, particularly with elementary school age audiences.
Preparation of the Aspirin Ink Solution A mixture of 1 teaspoon (about 5 grams) of washing soda (sodium carbonate 10-hydrate) and six 325-milligram aspirin tablets (about 2 grams of aspirin) in 2 tablespoons (30 mL) of water is prepared in a small glass jar.9 The mixture is allowed to stand overnight with occasional stirring or shaking to facilitate solution.
Preparation of the Iron Developing Solution
+ Fe3 (aq) + 2HOC6H4CO2−(aq) Fe(HOC6H4CO2)2+(aq)
The message is printed on a sheet of paper using the aspirin solution as ink. A small watercolor brush or cotton swab can be used to apply the solution to the paper. White construction paper and white blotter paper work well, as does laboratory filter paper. Writing or printer papers tend to be less satisfactory as their surfaces are coated with a water resistant finish. The paper is allowed to dry thoroughly. When ready for use, the paper is sprayed lightly with a mist of the iron developing solution.5 A layer of paper toweling or newspaper behind the sign can be used to keep the developing solution off of the surrounding surfaces. The aspirin solution may turn brown upon prolonged storage; however, no difference in its use as a sympathetic ink has been noted. A sympathetic ink that gives a black color can be prepared from powdered instant tea,6 as has been described by
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numerous sources (7). The tea solution is a very dark brown, however, and clearly shows when used on white paper. It is less noticeable when used on colored papers, and a Halloween jack o’lantern can be prepared using orange construction paper. The sympathetic ink and the developing solution may of course be prepared using laboratory reagents if they are available. In this case salicylic acid and a ferric salt may be used directly to produce the color. A 5% (w兾v) solution of salicylic acid in ethyl alcohol7 or a 5% (w兾v) solution of sodium salicylate in water can be used for the sympathetic ink. A 5% (w兾v) of ferric chloride or ferric ammonium sulfate in water may be used for the developing solution.
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Only iron supplement tablets that supply iron in the form of ferrous sulfate are suitable for this experiment. Six iron supplement tablets (equivalent to about 2 grams of ferrous sulfate 7-hydrate) are dissolved in 1兾2 cup (125 mL) of white vinegar. After the tablets have dissolved,10 a slightly cloudy green solution is obtained. To this mixture is added 1兾2 teaspoon (2.5 mL) of household bleach while stirring with a disposable spoon. The mixture turns a dark orange immediately and is ready for use. The solution is transferred to a spray bottle for use. Note: If the color of the writing produced by this developing solution is not bright enough, another 1兾4 teaspoon (1.2 mL) of household bleach may be added to it to further increase the concentration of ferric ion.
“Magic” Writing The message is printed on a sheet of paper using the aspirin solution as ink. If desired, the message can be very
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lightly sketched in pencil first. A small watercolor brush or cotton swab can be used to apply the solution to the paper, taking care to saturate the paper in the areas where writing is to appear. White construction paper, white blotter paper, and laboratory filter paper are recommended. The paper is allowed to dry thoroughly at room temperature. When ready for use, the message paper is sprayed lightly with a mist of the iron developing solution. A layer of paper toweling or newspaper behind the sign can be used to keep the developing solution off of the surrounding surfaces. Hazards Chlorine bleach is poisonous (8). Iron supplement tablets can be poisonous if an overdose is taken. Chlorine bleach must not be combined with any other chemicals in any way, other than exactly according to the means detailed in the procedures. Other combinations involving these chemicals could result in the formation of hazardous byproducts or gases. All chemicals and the reaction mixtures must be kept away from the audience at all times, particularly children. Safety glasses must be worn when handling these or any other chemicals. Adequate ventilation must be ensured when handling chlorine bleach solutions. Notes 1. Washing soda is a hydrated sodium carbonate. 2. Iron is typically supplied as a dietary supplement in the form of ferrous salts or sometimes as iron metal, which dissolves in the stomach to form ferrous ion. This is because ferrous ion is readily absorbed by the gastrointestinal tract, whereas ferric ion is poorly absorbed. 3. Household liquid chlorine laundry bleaches containing 6% sodium hypochlorite have largely displaced the 5.25% sodium hypochlorite bleaches previously marketed. The 6% sodium hypochlorite corresponds to approximately 0.95 M sodium hypochlorite by iodometric titration. 4. White vinegar, or clear vinegar, is a colorless dilute solution of acetic acid in water. A 5% concentration is typically marketed for kitchen use. This corresponds to 0.9 M acetic acid as determined by acidimetric titration. Other varieties of vinegar, such as wine vinegar and cider vinegar, are not colorless and are therefore not suitable. 5. Small trigger-handle spray bottles can be purchased in most pharmacies and supermarkets. Alternatively, an empty bottle of a cleaning spray, such as a window cleaner bottle, can be rinsed thoroughly with water and used to apply the developing solution. 6. A solution of 1 teaspoon of instant tea powder in 2 tablespoons (30 mL) of water will give a good black color when sprayed with the developing solution. A solution of tannic acid (1兾4 teaspoon) in 2 tablespoons (30 mL) of water is better and is nearly
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colorless when dry; however, tannic acid may not be available locally. 7. Rubbing alcohol (70% isopropyl alcohol) may also be used. 8. Both Feosol and generic brand tablets containing 200 mg of dried ferrous sulfate (equivalent to 325 mg of ferrous sulfate heptahydrate, FeSO4⭈7H2O) were used. Some iron supplement tablets contain ferrous fumarate, ferrous gluconate, or other iron compounds. These were found by the checker to be unsuitable in this procedure. 9. Other bases do not work well in place of washing soda. Baking soda (sodium bicarbonate) does not hydrolyze the aspirin effectively and an inferior color is obtained with the developing solution. 10. The green coating on the tablets dissolves rapidly. The ferrous sulfate in the center of the tablet dissolves much more slowly. Solution will be hastened by stirring and by crushing the tablets in the vinegar after the coating has dissolved.
Literature Cited 1. 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. 2. See, for example: Sae, A. Chemistry Magic from the Grocery Store; Kendall兾Hunt Publishing Company: Dubuque, IA, 1998. 3. Shakashiri, B. Z. Chemical Demonstrations; University of Wisconsin Press: Madison, WI, 1992; Vol. 1, p 338 and references cited therein. 4. The United States Pharmacopeia USP 25; National Publishing: Philadelphia, PA, 2002; pp 165, 1548. 5. Hantzsch, A.; Desch, C. H. Justus Liebigs Ann. Chem. 1902, 323, 1–31. 6. The Merck Index; Merck & Co.: Whitehouse Station, NJ, 2001; Vol. 13, p 145. 7. The reaction of ferric salts with tannic acid (a component of tea) has been known for many years. Sympathetic inks that use tea and a ferric salt have been described in numerous references. See, for example: Sae, A. Chemistry Magic from the Grocery Store; Kendall兾Hunt Publishing Company: Dubuque, IA, 1998; pp 98–99. Chen, P. S. Entertaining and Education Chemical Demonstrations; Chemical Elements Publishing: Camarillo, CA, 1974; p 20. Porter, H. M. The Beginner’s Chemcraft Manual; Porter Chemical Company: Hagerstown, MD, 1958; p 10. 8. Young, J. A. J. Chem. Educ. 2002, 79, 1196. http:// www.jce.divched.org/Journal/Issues/2002/Oct/abs1196.html (accessed Jan 2005).
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