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JCE Classroom Activity #110: Artistic Anthocyanins and Acid Base Chemistry Jenna Lech*,† and Vladimir Dounin‡ † ‡
Ontario Institute for Studies in Education, Toronto, Ontario M2J3Z1 Canada Department of Physical and Environmental Sciences, University of Toronto, Toronto, Ontario M1C 1A4 Canada
bS Supporting Information ABSTRACT: Art and science are sometimes viewed as opposing subjects, but are united in many ways. With an increased awareness of the benefits of interdisciplinary studies in education, it is desirable to show students how different subjects impact one another. Visual arts are greatly connected to chemistry in several ways. Pigments are usually synthetically produced to conjure all the colors of the rainbow for creating many great masterpieces. However, these hues were originally derived from naturally occurring minerals and plants. Students can still paint a “green” or environmentally friendly picture, using colors obtained without synthesis. The fun procedure outlined here illustrates how red cabbage juice, along with other types of produce, can be used to prepare an inexpensive canvas that can be transformed into works of art while using acids and bases to modify the chemical structure of the anthocyanin pigment within the produce. Using this hands-on classroom activity, students will understand the benefits of using natural pigments, investigate how colors can be manipulated, make color gradients, and explore how using different media can affect an individual’s artwork. Importantly, students will also develop understanding of the interconnection between science and art. KEYWORDS: Elementary/Middle School Science, General Public, High School/Introductory Chemistry, Interdisciplinary/ Multidisciplinary, Hands-On Learning/Manipulatives, Inquiry-Based/Discovery Learning, Acids/Bases, Applications of Chemistry, pH, Green Chemistry FEATURE: JCE Classroom Activity
’ BACKGROUND An indicator is a chemical compound or mixture that has color characteristics that are specific to the pH of its immediate environment, where the pH is the logarithm expression of the dissolved hydronium ion concentration. Solutions with a pH greater than 7 are considered basic, while those with a pH lower than 7 are considered acidic. Anthocyanins are naturally occurring substances that have indicator properties. The pH conditions determine the protonation states of their ketone and hydroxyl substituents, which in turn leads to color changes. The use of anthocyanins from red cabbage juice has been previously reported for classroom activities in which students can visualize color changes that occur when the anthocyanins are exposed to different pH conditions.1,2 More sophisticated possibilities include staining paper to have a desired color by using an anthocyanin solution with a particular pH value,3 dipping anthocyanin-stained paper into solutions with different pH values,4 and writing messages in invisible acid base ink that are revealed when an anthocyanin solution is applied.5 In this activity, rather than staining paper with an anthocyanin solution or dipping the Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
paper into solutions to change color, students are immersed in creating their own artworks on an easily prepared, dry anthocyanin-stained canvas using brushes with common colorless acids and bases as unconventional “paints”. This exercise incorporates visual arts as an interdisciplinary link that allows connections to be made to practical applications of science while portraying the learning material in a fun and engaging manner in the form of art.
’ INTEGRATING THE ACTIVITY INTO THE CURRICULUM This exercise can be easily integrated into a classroom at nearly any level. Concepts including acids, bases, and pH are investigated while doing this activity, as well as the extraction of a compound (anthocyanin in red cabbage juice). This activity can provide a segue into a unit on light and spectra. This procedure Published: October 12, 2011 1684
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Figure 1. Anthocyanin color at three different pH values.
can also be used to integrate ideas of gradients, spectrum of colors, and color change.
’ ABOUT THE ACTIVITY Students will use special paper (preparation instructions follow below) to explore how indicators work, primarily by using red cabbage juice, which is an inexpensive source of an anthocyanin indicator. Clear, translucent liquids differing in pH can be applied to the special paper to change its color. There are many possible sources of acids and bases, including lemon juice, vinegar, and soda pop for lower pH solutions, and tap water and baking soda for higher pH solutions. (See the online Supporting Information, SI Appendix 1.) Students can explore the acidity and basicity of various household products for the purposes of producing multicolor art while following the underlying principles of green chemistry to minimize the impact on the environment. If you decide to allow the use of products at the extremes of the pH scale, such as drain cleaner, appropriate safety precautions should be enforced to protect the students and yourself. Never mix household chemicals that contain ammonia and bleach, as the reaction produces hazardous gases. Students should be educated and reminded about safety precautions and the dangers of mixing such chemicals before being introduced to this activity. Wear protective gloves, goggles, and a lab jacket to avoid skin contact with these substances. ’ PREPARATION OF PAPER To dye the paper with pH indicator,6 take a minimum of onequarter of a head of red cabbage, chop, and puree it with 2 cups of boiling water in a blender. Both tap water and distilled water can be used when blending the red cabbage. Different water hardness
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and pH levels will give varying results for the initial color of the anthocyanin solution, which will affect the stain color of the paper (Figure 1). In general, highly acidic and basic water should not be used to stain the paper for the sake of safety. The boiling water improves the extraction of the anthocyanin compounds from the red cabbage. Let the mixture stand for at least 10 min. Filter the mixture, keeping the liquid extract and discarding the solid cabbage portion. Pour the liquid extract into a pan such as a cookie sheet, broiler pan, or baking dish that is large enough to hold the type of paper you plan to soak. Ordinary printer (bond) paper or watercolor paper can be used. Watercolor paper can be found with art supplies in hobby, craft, or discount stores. It is typically sold as a pad of multiple sheets, commonly sized at 9 12 in.: to reduce costs, a single sheet could be cut in quarters for small-scale student artwork. Watercolor paper is preferable for the student artwork, because it produces a more even color when acids and bases are “painted” onto it. Place a sheet of paper into the pan. Let the sheet of paper sit in the liquid; printer paper should soak for approximately 20 min, watercolor paper for approximately 2 h. The pan and liquid can be agitated occasionally. Depending on the paper used, over time the liquid may change from purple to blue. This is due to the pH change caused by bleach leaching from the paper. Using archival quality (neutral pH) watercolor paper may eliminate this color change, although this is a costly choice. Alternatively, one can periodically add vinegar to lower the pH and adjust the color back to purple. Hang the sheets of paper to dry over an area that can catch the dripping indicator. One possibility is to extend a piece of string over a sink or bathtub and hang the papers using binder clips. Printer paper is fragile after soaking and care should be used while hanging it. Having a second person to assist can be helpful. Once dry, the paper can be stored in a cool, dark, and dry location to keep the anthocyanin pigments as stable as possible. If available, vacuum packing the papers after they are dry can increase their shelf life considerably. The papers can be cut into smaller pieces to allow students to practice before working on the larger pieces of paper. Excess cabbage juice liquid extract can be used as a liquid indicator for other related demonstrations.1 5
’ MODIFICATIONS AND ENRICHMENT Postsecondary Chemistry
Use the Henderson Hasselbach equation to estimate how much citric acid and sodium bicarbonate are needed to produce the range of colors for the cabbage juice indicator. (See the online Supporting Information, SI Appendix 4.) Test the estimations and discuss any unexpected results. Additional Indicators
In this activity, anthocyanins from red cabbage juice are used to dye the paper and subsequently have a particular type of color spectrum for painting. Allow students to investigate what will happen to the color spectrum if more than one type of indicator were used to stain the same paper. (The colors should blend according to color theory. Try with common lab indicators such as phenolphthalein and bromothymol blue. Colors will change in proportion to the concentration of the indicators. The idea of this exercise is to build up to the concept of universal pH paper, which is a blend of various indicator compounds to produce smooth color changes at specific pH intervals.) 1685
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’ ASSOCIATED CONTENT
bS
Supporting Information Student activity worksheet; answers to student questions; related color swatches; example artwork made using the red cabbage indicator. This material is available via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ REFERENCES (1) Fortman, J. J.; Stubbs, K. M. J. Chem. Educ. 1992, 69, 66. (2) JCE Staff. J. Chem. Educ. 2003, 80, 40A 40B. (3) Forster, M. J. Chem. Educ. 1978, 55, 107. (4) Suzuki, C. J. Chem. Educ. 1991, 68, 588. (5) McCall, B.; Diener, L.; Gimm, J. A. J. Chem. Educ. 2009, 86, 464A. (6) How To Make Red Cabbage pH Indicator. http://chemistry. about.com/od/acidsbase1/a/red-cabbage-ph-indicator.htm (accessed Sep 2011).
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