In the Laboratory
Using Metals To Change the Colors of Natural Dyes
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Jennifer E. Mihalick* Department of Chemistry, University of Wisconsin, Oshkosh, Oshkosh, WI 54901; *
[email protected] Kathleen M. Donnelly Department of Theatre, University of Wisconsin, Oshkosh, Oshkosh, WI 54901
Although tea bags are easily obtained, some advance planning is required to save summer marigold flowers. The flowers can be picked as they wilt, but they must be dry before storage to avoid mold. Yellow and orange flowers both work; UV-vis spectra of the dye solutions show the same broad absorbance peaks, with a higher intensity from orange flowers. The sources for metal ions are alum (potassium aluminum sulfate), tin(II) chloride, and clean (not seasoned for cooking) cast iron pans. Iron(II) sulfate or an iron nail soaked in vinegar can be used instead of an iron pan, but we found that the color of the fabric was uneven. We also tried copper(II) sulfate and manganese(II) chloride with marigold dyes, but they had less effect on the final color of the fabrics. Chrome salts are used in commercial dyeing but because of their high toxicity we do not use them. A small quantity of cream of tartar (tartaric acid) is traditionally used with alum to maintain color brilliance, especially in the wool samples.
While developing labs for a nonmajor general chemistry course, we were inspired by previous articles emphasizing chemistry in the visual arts (1, 2) and the chemistry of fabrics (3) and dyes (4–6). The study of polymers is a focus of materials science, and a dyeing lab offers particularly interesting opportunities for liberal arts students. In the lecture component of the course, the historic and economic significance of each polymer material is studied. Throughout history the most common use of polymers has been in fabrics, and they are almost always dyed. Plant dyes have been used since ancient times, and their economic value influenced European voyages of exploration and colonization. Prior to the experiment, the students learn about the chemical structures of natural and synthetic fibers and the intermolecular attractions generated by their functional groups. A lecture period is also used to discuss the nature of light and learn how colors arise from selective absorption of visible wavelengths. It was discovered in medieval times that some metal salts could assist with binding dyes to fabrics and could also change the fabric color. The binding agents are called mordants, meaning “biting”. Alum was prized by medieval dyers because it did not change the dye color. Tannins found in black tea and red wine are also effective mordants for many fibers. This particular experiment is based on published recipes for home dyers (7). Fabrics dyed with two common plant materials, tea leaves and marigold flowers, are found to turn different colors when small quantities of metal salts are added. Students observe that the structure of the fiber, the structure of the dye, and the identity of the metal all influence the final result. Dyeing projects make good lab experiments for a nonmajors course because the reagents are easily obtained; the necessary cooking-type techniques have already been mastered by most high-school and college students; and students learn something useful that they can do in the future in their own kitchens. Comments on student surveys showed this to be one of the most popular laboratories, primarily because they liked making things. One student wrote, “It was interesting to see how simple colors were actually complex.”
Each group of students prepares a different dye bath, either tea or marigold, with or without a metal. The plant samples are added to water in a beaker, then heated on hot plates. Metal salts are weighed and added to simmering dye baths. Iron is introduced in the traditional way, by heating a dye bath with vinegar in a cast iron pan. After dye has been extracted into the water from the leaves or petals, fabric swatches are added to the dye baths. The samples are prepared in one 2-hour lab period, then left to soak for at least one day. Observations of the results can be made in a shorter class period. There is time to do another simple activity while the dye baths are simmering. One option is to have the students observe the color changes that occur when acetic acid and ammonia are added to pH indicator dyes. The Journal has published “Classroom Activities” using anthocyanins in tree leaves (8) and blueberries (9). These provide an interesting contrast with the tea and marigold dyes, which do not shift with a pH change.
Materials
Results
One of the advantages of working with fabrics and natural dyes is that the structure and reactivity of organic chemicals can be explored with inexpensive, nontoxic samples. A variety of undyed synthetic and natural fibers (especially wool) is needed to observe an interesting range of hues and intensities. White fabrics can be purchased at fabric stores or old clothes can be scavenged from thrift shops. Square swatches, about 5 cm on a side, are used.
Metallic mordants are most effective with protein fibers (silk and wool). The students will observe that those fabrics gain much more intense colors than the cellulose, nylon, polyester, or rayon samples. The tannins in tea are also better bound to protein fibers than to the others. These effects are due to the structure of the protein fiber. Proteins have the greatest variety of functional groups to participate in intermolecular attractions.
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Procedure
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In the Laboratory
The transition-metal ions (iron and tin) create metal complexes that shift the dye colors. Tea normally produces a tan dye, but iron changes it to dark gray. Marigolds make a yellow dye; iron changes it to green. Tin shows the most selectivity for a fiber, changing only wool to orange. Hazards Although the reagents are less toxic than many items found in chemistry laboratories, students should not ingest any. Alum and tin chloride could be irritants. Alum is discussed in a Chemical Laboratory Information Profile (10). Eye protection is necessary, especially when acids are being used. If concentrated acetic acid is diluted to make a vinegar solution, it should be done with proper ventilation and protective clothing; a Chemical Laboratory Information Profile for glacial acetic acid describes some of its hazards (11). Students who have not previously used a hot plate should be cautioned that the surface may heat quickly but cool slowly, so it would be best not to touch the surface. W
Supplemental Material
Instructor’s notes, including instructions for preparing the chemical and fabric samples and typical results, and the
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student worksheet with the procedures and spaces to record observations are available in this issue of JCE Online. Acknowledgment We thank the University of Wisconsin, Oshkosh, Faculty Development Program for funding the development of laboratory experiments. Literature Cited 1. Henchman, M. J. Chem. Educ. 1994, 71, 670. 2. Schrenk, J. L.; Malde, P.; Bordley, J. L. J. Chem. Educ. 1993, 70, 389. 3. Butler, S.; Malott, S. J. Chem. Educ. 1981, 58, 295–300. 4. Bonneau, M. C. J. Chem. Educ. 1995, 72, 724–725. 5. Epp, D. N. J. Chem. Educ. 1995, 72, 726–727. 6. Sequin-Frey, M. J. Chem. Educ. 1981, 58, 301–305. 7. Grae, I. Nature’s Colors: Dyes from Plants; Macmillan Publishing Co.: New York, 1974. 8. Journal’s Editorial Staff. J. Chem. Educ. 1997, 74, 1176A– 1176B. 9. Journal’s Editorial Staff. J. Chem. Educ. 1999, 76, 1688A– 1688B. 10. Young, J. A. J. Chem. Educ. 2004, 81, 1563. 11. Young, J. A. J. Chem. Educ. 2001, 78, 721.
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