Demonstration Extensions Based on Color-Changing Goldenrod

Dec 5, 2018 - ... to red in basic solutions, to black in very acidic solutions, to off-white in sodium hypochlorite solutions, and to purple-black in ...
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Demonstration Extensions Based on Color-Changing Goldenrod Paper Donald K. Schorr and Dean J. Campbell* Mund-Lagowski Department of Chemistry and Biochemistry, Bradley University, Peoria, Illinois 61625, United States

J. Chem. Educ. Downloaded from pubs.acs.org by UNIV OF GOTHENBURG on 12/05/18. For personal use only.

S Supporting Information *

ABSTRACT: For many years, a type of goldenrod-colored paper has been used as an indicator to demonstrate acid−base behavior, turning yellowish in acidic conditions and reddish in basic conditions. This pH sensitive color change has been the basis of demonstrations involving making hidden messages and pictures using techniques such as printing (e.g., to make a “bloody” red handprint), spraying, wax resist, and electrolytic processes. Candidate dye compounds for the goldenrod paper that feature a pH sensitive yellow/red color change include Direct Yellow 4 and curcumin. Direct Yellow 4, curcumin, and the color-changing goldenrod paper also all exhibit additional changes to darker colors in very acidic solutions (pH < 0). This paper provides evidence that Direct Yellow 4 (or a closely related compound) and not curcumin is the dye present in the color-changing goldenrod paper. These color changes of the Direct Yellow 4 in the goldenrod paper, in conjunction with the fact that paper sheets can display twodimensional images or shaped into three-dimensional objects, lead to novel ways to demonstrate acid−base and other chemical concepts to audiences. For example, paper flowers made from the goldenrod paper will change from yellow to red in basic solutions, to black in very acidic solutions, to offwhite in sodium hypochlorite solutions, and to purple-black in iodine solutions. KEYWORDS: General Public, First Year Undergraduate/General, Demonstrations, Analogies/Transfer, Acids/Bases

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BACKGROUND Color-changing goldenrod paper has been a staple of chemistry demonstrations for decades.1,2 This type of paper has a yellow (goldenrod) color as purchased and maintains this color when exposed to mildly acidic or neutral solutions. When exposed to basic solutions, the paper turns red. This pH sensitive paper is available in letter-sized 21.6 cm × 27.9 cm (8.5 in. × 11 in.) sheets, which are much larger than typical indicator paper strips. As a result, the paper is readily used in large-scale chemistry demonstrations. One common demonstration using this paper is the “bloody handprint”, in which basic solution (e.g., dilute ammonia solution) is sprayed on a gloved hand and then the wet hand is placed in contact with a sheet of goldenrod paper to leave behind a red handprint.1,2 For those who want to avoid the “bloody” focus, adding legs and a beak to the handprint can make it into a “hand turkey”, which might be more appropriate for all ages. Another common demonstration using this paper is to use paraffin wax or a clear crayon to write a message on the paper that is effectively invisible to spectators. (One reviewer has noted that watercolor resist media can work instead of crayons or candle wax.) When the paper is sprayed with a basic solution, the paper turns red except where it is masked by the layer of wax. The net result is a goldenrod-colored message on a red background.2 The red color can then be turned back to goldenrod with acid solution. For example, the Bradley University chemistry demonstration group begins its outreach events by spelling out the word “WELCOME” using © XXXX American Chemical Society and Division of Chemical Education, Inc.

household ammonia solution spray (which typically contains a few percent ammonia) on goldenrod paper, and then erases the message using vinegar spray. What is not as well-known about the color-changing goldenrod paper is its transition from yellow to purple-black in very acidic conditions (below a pH of approximately 0). This goldenrod paper can also be bleached by exposure to sodium hypochlorite bleach solution. The additional colorchanging properties of this paper open up a wealth of possibilities for additional demonstrations. Most goldenrod-colored paper sold today does not have color-changing capability; however, the color-changing goldenrod paper is still available from suppliers such as Educational Innovations, Bethel, CT.1,2 The specific dye used in this colorchanging goldenrod paper was not divulged by the company, but a few Internet sources claim it is Direct Yellow 4. This dye has a CAS number of 3051-11-4 and has a number of other names, including Brilliant Yellow, Paper Yellow CB, Color Index 24890, and 2,2′-(1,2-ethenediyl)bis[5-[(4hydroxyphenyl)azo]-benzenesulfonicacid disodium salt.3,4 The structure of Direct Yellow 4 in neutral conditions is shown in Figure 1A.5,6 In basic conditions, a hydrogen ion is removed from each phenol group (shown in red).5,6 In very Received: May 8, 2018 Revised: November 4, 2018

A

DOI: 10.1021/acs.jchemed.8b00341 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Demonstration

color chemistry of both yellow dyes and the color-changing goldenrod paper are briefly described in the Experimental Section and the Supporting Information. These experiments can be easily replicated in undergraduate laboratory courses. Demonstrations based on the additional color-changing properties of the goldenrod paper are described in the Discussion section.

acidic conditions, a hydrogen ion is added to each azo group (shown in blue).7

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EXPERIMENTAL SECTION

Observations of Color-Changing Goldenrod Paper and Related Dyes

The properties of the dye used in the color-changing goldenrod paper (Educational Innovations, Bethel, CT) were compared with those of Direct Yellow 4 (TCI America, Portland, OR) and curcumin (Alfa Aesar, Tewksbury, MA). The color-changing goldenrod paper was cut into small pieces with scissors, and these pieces were added to small amounts of deionized water and stirred with a PTFE-coated stir bar for several hours. The yellow aqueous solution was separated from the paper pulp by a variety of approaches, including centrifugation of the slurry, filtration through filter paper, and filtration through a pipet containing powdered alumina (which seemed to produce the best separation). Ultraviolet/ visible light spectra were collected with a Vernier UV−vis spectrophotometer.11 The maximum absorbance wavelengths are given in Table 1. The light absorption data for the paper extract resembles the Direct Yellow 4 more than the curcumin. The spectra used to acquire these values are shown in the Supporting Information. Hydrochloric acid (6 M) and sodium hydroxide (0.1 M) were added to solutions of color-changing goldenrod paper extract, Direct Yellow 4, and curcumin in order to find their color transition points. Note that the curcumin had to be dissolved in ethanol, as it has poor water solubility. The approximate pH values of these color transitions based on these measurements are also given in Table 1. Infrared spectra were taken of the extracted and dried dye, dry curcumin, and dry Direct Yellow 4. The spectra were measured on a Thermo Nicolet Nexus 470 Fourier transform infrared spectrophotometer with an attenuated total reflectance accessory. The spectra are shown in the Supporting Information. The IR spectra of Direct Yellow 4 and the dye extracted from the paper more closely resemble each other than the IR spectrum of curcumin. Finally, it should be noted that sodium hypochlorite solutions can bleach the dyes, presumably by oxidation. Since sodium hypochlorite is also basic, the dyes can also turn red for a time before they bleach to colorless. The response of the color-changing goldenrod paper to the bleach is notably slower than the responses to acids or bases. Given that others have made color-changing paper by treating paper with turmeric solution,2 it seemed reasonable to

Figure 1. (A) Structure of Direct Yellow 4 dye in neutral conditions. The locations of the hydrogen ions removed in basic conditions are shown in red. The locations where the hydrogen ions are added in very acidic conditions are shown in blue. (B) The keto and enol tautomers of curcumin. The locations of the hydrogen ion removed in basic conditions are shown in red. The locations where the hydrogen ions are added in very acidic conditions are shown in blue.

Some sources have pointed out that turmeric, with its yellow-colored compound curcumin, can be used to make a yellow-colored paper that changes to red in basic solution.2,8 The structure of curcumin in neutral conditions is shown in Figure 1B.9 This compound exhibits keto−enol tautomerism. In basic conditions, one or more hydrogen ions are lost from the phenol groups (shown in red).9 In very acidic conditions, both of the oxygen atoms near the middle of the molecule have hydrogen atoms (shown in blue), and the dye darkens to a red.9 Curcumin can also change color in the presence of aqueous boron species.10 A number of chemists have been confused as to which dye is used in the commercially available color-changing goldenrod paper: Is it Direct Yellow 4, curcumin, or some other compound? Understanding the identity of the dye used in the paper provides insights into the chemistry of the paper and potentially expands the possible experiments and demonstrations that can be performed using that paper. Evidence that the dye in question is Direct Yellow 4 and more details about the

Table 1. Maximum Absorbance Wavelengths for the Basic, Acidic, and Highly Acidic Forms of the Goldenrod Paper, Direct Yellow 4, and Curcumin Dyes Sample Paper extract in water Direct Yellow 4 in water Curcumin in ethanol

Very Acidic Form, λmax (nm)

Approximate pH Transition

No Acid or Base Added, λmax (nm)

Approximate pH Transition

Basic Form, λmax (nm)

540 570

−0.1 −0.3

397 403

8.2 8.3

491 487

369

0.5

427

7.9

467

B

DOI: 10.1021/acs.jchemed.8b00341 J. Chem. Educ. XXXX, XXX, XXX−XXX

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Figure 2. (Back row) Paper flowers made by dipping color-changing goldenrod paper in various solutions: (left to right) 3 M hydrochloric acid solution, undipped, sodium carbonate solution, household bleach solution. (Front left) Direct Yellow 4 in aqueous solutions added to (left to right) hydrochloric acid, deionized water, sodium hydroxide solution, and household bleach. (Front left) Curcumin in ethanol added to (left to right) hydrochloric acid, deionized water, sodium hydroxide solution, and household bleach.

treat white paper with an aqueous solution of Direct Yellow 4 dye in an attempt to make homemade color-changing goldenrod paper. When white copier paper and white paper towel were dipped in a Direct Yellow 4 dye solution, the copier paper took on a more orange hue than the white paper towel (see the Supporting Information). This might be due to an interaction between calcium carbonate in the copier paper and the dye. Drops of the dye solution placed on powdered calcium carbonate also turned somewhat orange in color while drops of the dye solution placed on titanium(IV) oxide powder were more yellow in color (also in the Supporting Information). Powder X-ray diffraction, performed on a Rigaku Smartlab diffractometer using Cu Kα radiation, was used to study paper samples; the diffraction patterns are shown in the Supporting Information. Diffraction measurements of white copier paper showed signals associated with calcium carbonate; measurements of the paper towel did not show such signals. Additionally, at least four different brands of copier paper bubbled when exposed to hydrochloric acid solution (as would be expected for calcium carbonate reacting with the acid to produce carbon dioxide). The copier paper samples turned the dye orange, whereas the paper towel (which did not exhibit bubbling) failed to turn the dye orange. Interestingly, powder X-ray diffraction of the color-changing goldenrod paper did not show signals associated with calcium carbonate, whereas a sample of the modern NON-color-changing goldenrod paper did show signals associated with calcium carbonate. Perhaps Direct Yellow 4 dye and calcium carbonate are less compatible with each other in the same “goldenrod” paper samples because the basic calcium carbonate causes the dye to shift in color to a more orange hue. The Direct Yellow 4 dye placed on paper turned red when exposed to basic solutions, whether the paper contained calcium carbonate or not. A drop of 6 M HCl placed on a dye-treated paper sample containing calcium carbonate both produces bubbles and turns the dye purple. Green paper soaked in Direct Yellow 4 or curcumin solutions turned a different shade of green when dry; this green paper turned reddish when exposed to basic solutions. So, it is

possible to make a version of color-changing goldenrod paper with Direct Yellow 4 solutions, but for easily used sheets of uniform consistency it might be easier to purchase the commercially available paper.

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SAFETY All solution containers should be clearly labeled. Precautions, including proper personal protective equipment such as goggles, should be used when working with acids, bases, bleach, and iodine solutions. Avoid spilling the solutions on clothing. Avoid skin contact and wear gloves while working with the solutions. Always wash your hands after completing the demonstrations. With the exceptions of mixing vinegar and ammonia, and Vitamin C and iodine, the solutions should generally not be mixed with one another. This is ESPECIALLY true for acids and sodium hypochlorite bleach, which can produce chlorine gas. Mixing bleach and ammonia can produce hazardous chloramines. Solutions that can release noxious vapors, such as hydrochloric acid, aqueous ammonia, and sodium hypochlorite bleach, should not be used in confined, unventilated spaces. Hydrogen chloride vapors from acidsoaked goldenrod paper have been observed to alter the color of nearby fresh goldenrod paper. It should also be noted that the term “brilliant yellow” can refer to other color-producing substances than Direct Yellow 4, for example, a mixture containing cadmium and sometimes lead.17 This reinforces the need for caution when dealing with nonstandardized nomenclature.

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DISCUSSION

Related Demonstrations

Safety must be seriously considered if one is to use bleach solutions and highly concentrated acid solutions. Certainly no one would want to spray these solutions onto the colorchanging paper as the risk of inhalation of hazardous aerosols would be too great. The handprint demonstration described earlier can be hypothetically modified to use the acid or bleach C

DOI: 10.1021/acs.jchemed.8b00341 J. Chem. Educ. XXXX, XXX, XXX−XXX

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Figure 3. (Top) A paper lantern can be made from color-changing goldenrod paper. The lantern has been splashed with sodium carbonate solution to produce red spots. (Bottom) A gloved hand is swabbed with an aqueous iodine solution, which is then printed onto color-changing goldenrod paper. Starchlike molecules in the paper turn iodine solution in the handprint to a blue-black color.

flowers, see Supporting Information. Care must be taken to prevent contact between incompatible solutions, e.g., of hypochlorite ions and acids. Figure 3 shows a paper lantern made from the colorchanging goldenrod paper. The papers in these sorts of lanterns have been sprayed with ammonia solution or, as shown in the figure, splashed with a little sodium carbonate solution. This variation on the demonstration was developed and used around the Lunar New Year (also known as Chinese New Year). The demonstration was verbally presented in the context that red is considered to be a lucky color in Chinese culture,12 and then the yellow paper lantern was sprayed with ammonia solution to temporarily turn it red. As mentioned above, green paper soaked in aqueous Direct Yellow 4 turned reddish when exposed to basic solutions. This could be the basis of other seasonal demonstrations, as illustrated in the Supporting Information. Some household items can be used to write on the goldenrod paper. For example, the insect bite treatment After Bite (Tender Corporation, Littleton, NH) can contain ammonia and can be used to draw red lines on the goldenrod paper. A bleach pen containing chlorine-based bleach (e.g., produced by The Chlorox Company, Oakland, CA) can be used to draw lines on paper that are initially red and then turn white. See Supporting Information for photographs. One final color change variation is not based on the dye in the goldenrod paper, but rather on the paper itself. As with many paper samples, iodine (e.g., in povidone-iodine solution) will interact with starch-like compounds in the paper to turn the brownyellow iodine solution to blue-black in color.13 One way to present this is shown in Figure 3. The iodine solution was

solutions to produce prints of different colors. Heavy fabric gloves or knit gloves over disposable waterproof gloves are definitely recommended over the use of bare hands. Thick water-absorbent fabric in the shape of a hand that is mounted to a plastic block has been used for printing. This assembly can help decrease direct contact with the solutions, but the chemically wetted fabric stamps should be washed with water after the printing to minimize contamination of other surfaces. An alternative demonstration that can showcase all of the colors available from this demonstration is to make paper flowers from the color-changing goldenrod paper, Figure 2. The 21.6 cm × 27.9 cm sheets were cut into quarters, and then these smaller sheets were folded at about 1 cm intervals into parallel grooves. These corrugated sheets were pinched in the middle, affixed to supporting sticks or wires, and flared out at their edges. There are many additional ways to construct paper flowers, which can add much artistic diversity to this demonstration. To demonstrate the color changes, these paper flowers were briefly dipped into the acid, base, and bleach solutions and set out to dry upright in containers to be used as vases. If the paper flowers from different solutions were placed too close to each other while wet, cross-contamination could occur. For example, hydrogen chloride vapors from an acid-wetted paper flower appeared to be able to produce color changes in other neighboring paper flowers. To keep the red color of the base-treated flower, a sodium carbonate solution was used. Ammonia solutions changed the paper to red, but the red color eventually reverted to yellow as the ammonia vaporized from the paper into the air. If one dips a paper flower into one solution, then not as deeply into a second solution, it is possible to produce patterns such as stripes in the D

DOI: 10.1021/acs.jchemed.8b00341 J. Chem. Educ. XXXX, XXX, XXX−XXX

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spread onto the palm and fingers of a hand, so an audience could be shown the solution’s brown-yellow color. When the hand was used to print the wet iodine solution onto the goldenrod paper, the resulting stain was blue-black. This handprint demonstration also worked on some forms of white paper (e.g., copier paper) but not dollar bills, and was similar to the handprint demonstrations such as the hand turkey described above. This demonstration has been used to segue into related demonstrations. The hand that was still stained with iodine solution after printing has been sprayed with spray starch to turn the stain blue-black. This hand stain was then turned colorless by contact with vitamin C tablets, as the ascorbic acid in the tablets reduced the iodine to iodide ions.14 These concepts can be taken still further to descriptions of counterfeit detection pens, clock reactions using iodine, or oscillating reactions using iodine.15,16

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(4) Terrific Science. Take-Home Challenge: Goldenrod Isn’t Always! http://www.terrificscience.org/lessonpdfs/Goldenrod.pdf (accessed Dec 2018). (5) Hurst, G. H. The Benzidine and Allied Colours, Their Composition, Properties and Application. J. Soc. Dyers Colour. 1888, 4, 14−24. (6) Green, F. L. The Sigma-Aldrich Handbook of Stains, Dyes and Indicators, 1st ed.; Aldrich Chemical Company, Inc.: Milwaukee, WI, 1990; pp 164−165. (7) Sawicki, E. Physical Properties of the Aminoazobenzene Dyes. IV. The Position of Proton Addition. J. Org. Chem. 1957, 22, 365− 367. (8) Marshall, J. What’s that stuff? Holi colors. Chem. Eng. News February 26, 2018; pp 28−29. (9) Lee, W.-H.; Loo, C.-Y.; Bebawy, M.; Luk, F.; Mason, R. S.; Rohanizadeh, R. Curcumin and its Derivatives: Their Application in Neuropharmacology and Neuroscience in the 21st Century. Curr. Neuropharmacol. 2013, 11, 338−378. (10) Alyea, H. N.; Hornbeck, L. G. Boron and silicon. J. Chem. Educ. 1968, 45, A151. (11) Vernier Software & Technology, LLC. Vernier UV-VIS Spectrophotometer. http://www.vernier.com/products/sensors/ spectrometers/ultraviolet-range/vsp-uv/ (accessed Dec 2018). (12) Lee, C. Why is Red Considered a Lucky Color for the Chinese? https://medium.com/story-of-eggbun-education/why-is-redconsidered-a-lucky-color-for-the-chinese-2ebe2b044275 (accessed Dec 2018). (13) Kenda, M.; Williams, P. S. Cooking Wizardry for Kids; Barron’s Educational Series, Inc.: Hong Kong, 1990; pp 52−54. (14) University of Canterbury College of Science. Determination of Vitamin C Concentration by Titration. https://www.canterbury.ac.nz/ media/documents/science-outreach/vitaminc_iodine.pdf (accessed Dec 2018). (15) Carmeli, D. Method of Detecting Counterfeit Paper Currency. US5063163A, 1990. (16) Wright, S. Tick Tock, a Vitamin C Clock. J. Chem. Educ. 2002, 79, 40A. (17) Museum of Fine Arts Boston. Brilliant Yellow. http://cameo. mfa.org/wiki/Brilliant_yellow (accessed Dec 2018).

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00341.

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UV−vis spectra of Direct Yellow 4, curcumin, and colorchanging goldenrod paper extract in acidic, neutral, and basic conditions; IR spectra of solid Direct Yellow 4, curcumin, and color-changing goldenrod paper extract; photos of Direct Yellow 4 solution placed in contact with samples containing and not containing calcium carbonate; and additional color-changing experiments and X-ray diffraction patterns of paper samples (PDF, DOCX)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Dean J. Campbell: 0000-0002-2216-4642 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Bradley University and the MundLagowski Department of Chemistry and Biochemistry with additional support from the Illinois Heartland Section of the American Chemical Society, the Ruby Worner Foundation, and the Illinois Space Grant Consortium. Additionally, many thanks to Max Palmer, Eilish Alexander, Kayla Lippincott, and Emily Rosengarten for assisting in the experiments and the figures.

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REFERENCES

(1) Educational Innovations, Inc. Color-Changing Goldenrod Paper. https://www.teachersource.com/product/color-changing-goldenrodpaper/chemistry (accessed Dec 2018). (2) Perkins, R. Goldenrod Paper (blog post at Educational Innovations). http://blog.teachersource.com/2009/10/01/goldenrodpaper/ (accessed Dec 2018). (3) World Dye Variety. Direct Yellow 4. http://www. worlddyevariety.com/direct-dyes/direct-yellow-4.html (accessed Dec 2018). E

DOI: 10.1021/acs.jchemed.8b00341 J. Chem. Educ. XXXX, XXX, XXX−XXX