Demonstration pubs.acs.org/jchemeduc
Demonstration of the Spectrophotometric Complementary Color Wheel Using LEDs and Indicator Dyes W. Russ Algar,* Caitlyn A. G. De Jong, E. Jane Maxwell, and Chad G. Atkins Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada S Supporting Information *
ABSTRACT: A quick, simple, and engaging demonstration of the spectrophotometric complementary color wheel is presented. Colored indicator dye solutions are illuminated under ambient room light and with different colors of light-emitting diodes (LEDs) in a dark room. The solutions are observed to be colored, black and opaque, or transparent and colorless in accordance with the color wheel, their optical absorption spectra, and the illumination conditions.
KEYWORDS: First-Year Undergraduate/General, Second-Year Undergraduate, Demonstrations, Analytical Chemistry, Hands-On Learning/Manipulatives, Dyes/Pigments, pH, UV−Vis Spectroscopy
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INTRODUCTION Color is one of the most striking features of our world. In nature, organisms from plants to animals use color for functions such as camouflage, signaling, and physiological regulation.1 Color also has aesthetic value to humans and exerts psychological effects.2 Given the many vibrant roles for color in our world, it is no surprise that many favorite chemistry demonstrations feature a color change (for example refs 3−6), and that the chemical and physical basis of dyed/pigmented color (cf. structural) is taught in undergraduate courses such as general,7,8 analytical,9,10 and inorganic chemistry.11,12 Articles in this Journal have explored the relationship between color, light absorption, and chemistry,13−15 and full courses dedicated to the chemistry of color have been taught.16,17 Here, we describe a simple yet effective demonstration for illustrating the relationship between the observed color of a dye solution and its optical absorption spectrum. Many undergraduate chemistry textbooks introduce color using a complementary color wheel.7,9−12 A typical six-sector color wheel is shown in Figure 1, and more detailed color wheels are available.15 Solutions that strongly absorb a particular color of light will generally appear to be the complementary color to the human eye. For example, a solution that strongly absorbs green light will be red/pink in appearance. Although the color of the solution is generally obvious to students, it is more difficult to demonstrate that the solution absorbs the complementary color of light. Demonstrations and activities that relate wavelength and color,18,19 differentiate color from absorption versus fluorescence,20 and address color synthesis21,22 have been reported, but these do not directly address the color wheel concept with respect to light absorption. The most rigorous demonstration of the color(s) of light absorbed by a solution is measurement of its © XXXX American Chemical Society and Division of Chemical Education, Inc.
UV−visible absorption spectrum; however, such a demonstration may seem abstract to students (especially those without a spectroscopy background), is not particularly engaging, and may not be feasible in a classroom or lecture hall if a portable spectrophotometer is not available. Light-emitting diodes (LEDs) emit light that typically appears to be a single color to the naked eye. In reality, the emission spectra of LEDs have full widths at half-maximum (fwhm) in the range of 20−35 nm. LEDs are convenient to use in the classroom or lecture hall because they are small in size, have low power requirements, and are widely available at minimal cost and in a wide variety of colors/wavelengths. We have found that the pseudo-monochromatic light from LEDs enables an effective, nonabstract demonstration of the relationship between the color(s) of light absorbed by a solution and the color it appears. In our demonstration, a colored solution in a clear glass dish is placed on top of a document printed in black text on white paper. In ambient room light, it is possible to read the printed text through the solution, and the color of the solution is obvious against the white paper background. The appearance of the solution is quite different when the room is darkened and the glass dish is illuminated with a color LED. If the color of light emitted by the LED is absorbed by the solution, it appears black and opaque, and the printed text is invisible. If the color of light emitted by the LED is not absorbed by the solution, then it appears as though the dish is filled with a clear liquid and the printed text is visible. This demonstration can be done with multiple dye solutions and colors of LED to illustrate the color wheel concept, and the observations can be subsequently related to the emission
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DOI: 10.1021/acs.jchemed.5b00665 J. Chem. Educ. XXXX, XXX, XXX−XXX
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Demonstration
DEMONSTRATION We present this demonstration to students after introducing the complementary color wheel in lecture, using a graphic such as Figure 1. The first experiment in the demonstration is a red or pink solution, such as phenolphthalein in aqueous base, because (i) most students are very familiar with this indicator dye, and (ii) we suggest that students use the mnemonic “Christmas/ Holiday colors” to learn that red and green are complementary. Under ambient room light, students are prompted to identify the color of the solution and predict the color of light absorbed from the color wheel. Many students suggest that the observed pink color can be approximated as red, although some students suggest violet. Methyl red has a less ambiguous red color in acidic solution and can be substituted or used in addition to phenolphthalein. Students are then asked to predict what will be observed when the phenolphthalein or methyl red solution is illuminated with green and red LEDs in a dark room. To test the predictions, the solution is placed on top of a printed document and illuminated from above with each color of LED. Figure 2A shows photographs of what is observed during this demonstration with phenolphthalein and methyl red solutions. The class is prompted to explain the observations in Figure 2A, and, based on this first experiment, make predictions for solutions of methyl orange and bromophenol blue. Photographs of what is observed for these two solutions are shown in Figure 2B. For each solution, the colored solution appears black and opaque when illuminated with the LED color that is complementary to the color observed under room light, and appears transparent when illuminated with the LED that corresponds to the observed color of the solution in ambient room light. Notably, the solutions of methyl orange and bromophenol blue, which have complementary colors under room light, absorb and transmit the opposite colors of the LED, highlighting that the eye and light bulb symbols really can be rotated around the color wheel in Figure 1. Moreover, the generality of the color wheel is demonstrated by the absorption and transmission of like colors of LED by the phenolphthalein and methyl red solutions, despite being different shades of red. In as little as 10 min, these four solutions can thus provide a convincing visual demonstration of the spectrophotometric complementary color wheel. To complete the demonstration, students are shown the emission spectra of the LEDs superimposed upon the absorption spectrum of each dye solution (see Figure 2), and asked to relate this data to their explanation of their observations. The key result is that, for the opaque cases, the emission spectrum of the LED falls under the absorption curve for the dye, whereas for the transparent cases, the emission spectrum of the LED falls outside the absorption spectrum.
Figure 1. Color wheel showing the approximate relationship between the color or wavelength of light absorbed and the color observed. The light bulb and eye are rotated synchronously around the wheel. As drawn, the figure shows that a solution that absorbs violet light will appear yellow to the eye.
spectra of the LEDs and absorption spectra of the solutions, making it a visually engaging and effective teaching tool.
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REAGENTS AND APPARATUS Solutions with orange, red, pink, and blue colors are obtained by dissolving methyl orange, methyl red, phenolphthalein, and bromophenol blue, respectively, in 0.1 M KOH (aq) or 0.1 M HCl (aq) to obtain the conjugate base or conjugate acid form as appropriate. A small diameter (50 mm), low-height (15 mm) crystallizing dish is used to hold the solutions. The small diameter ensures that the LED can illuminate the full area, and the low height facilitates top-down illumination and observation. Blue, green, orange, and red LEDs are used for illumination. Bright LEDs can be ordered from electronics wholesalers at a typical cost of $1 each or less. Two lengths of insulated wire are soldered to a 2-pin header connector, and the opposite ends of the wires are attached to the terminals of a battery. Different colors of LED can be inserted or removed from the header connector quickly and easily to conduct the demonstration. Additional details can be found in the Supporting Information.
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DISCUSSION This demonstration can be modified in several ways to accommodate instructional needs, preferences, and resources. Foremost, the indicator dyes in Figure 2A are suggestions. There are a multitude of other dyes with similar colors and absorption spectra that can be substituted. For example, bromocresol green can be substituted for bromophenol blue, and its absorption spectrum better overlaps with the orange LED. In addition, the demonstration is scalable, and has successfully used larger vessels (e.g., 100 mm crystallizing dishes) and smaller vessels (e.g., clear plastic 96-well microtiter plates). The advantage of scale-down, in addition to less
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HAZARDS Institutional chemical safety practices for corrosive solutions, solid powders, and waste disposal should be observed. Recommended personal protective equipment includes chemical resistant gloves, safety goggles, and a lab coat. Material safety data sheets should be reviewed prior to undertaking the demonstration. The LEDs pose no special optical or electrical hazards, and standard electrical safety practices should be observed. B
DOI: 10.1021/acs.jchemed.5b00665 J. Chem. Educ. XXXX, XXX, XXX−XXX
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Figure 2. Photographs of solutions of (A) methyl red (in 0.1 M HCl) and phenolphthalein (in 0.1 M KOH) under room light and green and red LED illumination; (B) methyl orange and bromophenol blue (both in 0.1 M KOH) under room light and blue and orange LED illumination. The normalized absorption spectrum for each dye (black line) and the normalized emission spectrum for each LED (filled curves) are also shown.
Figure 3. Examples of utilizing changes in pH to change the light absorption properties of a dye solution: (A) neutralization of 0.1 M KOH (aq) with phenolphthalein through the addition of 1 M HCl (aq) from a Pasteur pipet; (B) conjugate base (b) and conjugate acid (a) forms of bromophenol blue under room light and illuminated with blue and orange LEDs.
reagent consumption and waste, is that the illumination of multiple solutions can be viewed simultaneously. The drawback is that a camera must be available to clearly project the image at a scale large enough for the class to see. In general, the demonstration works well with document/demonstration cameras, which often turn mostly opaque solutions (by eye) into completely opaque solutions (on camera). An expanded version of this demonstration can illuminate the various dye solutions with more than two colors of LED, and relate the observations to the width of the dye absorption band and the wavelength-dependent molar absorption coefficient. An example of such a demonstration is provided in the Supporting Information. The demonstration can also
capitalize on the acid−base indicator properties of the dyes. As shown in Figure 3A, when an aliquot of strong acid is added to basic solution with phenolphthalein under green LED illumination, the neutralization and acidification can be observed as black/opaque and transparent contrast. The inversion of colors of LED light absorbed and transmitted with a conjugate acid−conjugate base transition can also be demonstrated, as shown in Figure 3B for bromophenol blue. Other indicator dyes can also be used for these experiments. The demonstration can also function as a springboard for discussion of related concepts. One simple example is the difference between clear and colorless solutions, which students sometimes have difficulty grasping. The solution of the C
DOI: 10.1021/acs.jchemed.5b00665 J. Chem. Educ. XXXX, XXX, XXX−XXX
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(3) Stoddard, R. L.; McIndoe, J. S. The Color-Changing Sports Drink: An Ingestible Demonstration. J. Chem. Educ. 2013, 90, 1032− 1034. (4) Peyser, J. R.; Luoma, J. R. Flame Colors Demonstration. J. Chem. Educ. 1988, 65, 452−453. (5) Pearson, R. S. Manganese Color-Reactions. J. Chem. Educ. 1988, 65, 451−452. (6) Briggs, T. S.; Rauscher, W. C. Oscillating Iodine Clock. J. Chem. Educ. 1973, 50, 496−496. (7) Petrucci, R. H.; Harwood, W. S.; Herring, F. G.; Madura, J. D. General Chemistry: Principles and Modern Applications, 9th ed.; Pearson Education: Upper Saddle River, 2007; p 1021. (8) Oxtoby, D. W.; Gillis, H. P.; Nachtrieb, N. H. Principles of Modern Chemistry, 4th ed.; Saunders College Publishing: New York, 1999; p 616. (9) Harris, D. C.; Lucy, C. A. Quantitative Chemical Analysis, 9th ed.; W.H. Freeman and Company: New York, 2016; p 437. (10) Christian, G. D.; Dasgupta, P. K.; Schug, K. A. Analytical Chemistry, 7th ed.; John Wiley & Sons: Hoboken, 2014; p 478. (11) Housecroft, C. E.; Sharpe, A. G. Inorganic Chemistry, 1st ed.; Pearson Education Limited: Harlow, 2001; p 437. (12) Brisdon, A. K. Inorganic Spectroscopic Methods; Oxford University Press: Oxford, 1998; p 57. (13) Orna, M. V. Chemistry and Artists Colors 0.1. Light and Color. J. Chem. Educ. 1980, 57, 256−258. (14) Orna, M. V. Chemical Origins of Color. J. Chem. Educ. 1978, 55, 478−484. (15) Brill, T. B. Why Objects Appear as They Do. J. Chem. Educ. 1980, 57, 259−263. (16) Orna, M. V. Molecular-Basis of Form and Color - Chemistry Course for Art Majors. J. Chem. Educ. 1976, 53, 638−639. (17) Gelabert, M. C. Color Science, a Course for Nonscience Majors. J. Chem. Educ. 2006, 83, 1155−1157. (18) Paselk, R. A. Demonstrating Color-Wavelength Relations. J. Chem. Educ. 1982, 59, 383−383. (19) Malerich, C. J. Using the Colorimeter to Illustrate the Wave Nature of Light and the Relationship between Color and Light Absorbed. J. Chem. Educ. 1992, 69, 163−163. (20) Sturtevant, J. L. What Color Are Fluorescent Solutions. J. Chem. Educ. 1989, 66, 511. (21) Suding, H. L.; Buccigross, J. M. A Simple Lab Activity to Teach Subtractive Color and Beers-Law. J. Chem. Educ. 1994, 71, 798−799. (22) Williams, D. L.; Flaherty, T. J.; Jupe, C. L.; Coleman, S. A.; Marquez, K. A.; Stanton, J. J. Beyond Lambda(Max): Transforming Visible Spectra into 24-Bit Color Values. J. Chem. Educ. 2007, 84, 1873−1877.
conjugate acid of phenolphthalein transmits all wavelengths of visible light and is thus both clear and colorless, whereas the solution of its conjugate base is clear and colored because it only transmits some wavelengths of visible light. Another example is color theory, including differences in how color is perceived under bright and dark conditions, additive and subtractive color mixing, and the trichromic basis of human vision, electronic displays, and digital cameras. Regarding subtractive color, the difference in color between solutions of methyl red (acidic) and phenolphthalein (basic) under room light (see Figure 2A) is a good starting point. The absorption band of phenolphthalein is relatively narrow, absorbing almost exclusively green light, and thus acts as a magenta filter and appears that color. The absorption band of methyl red is comparatively broad, absorbing mostly green light but also some blue light, and therefore acts as a combination of magenta and yellow filters to appear red. The yellow (acidic) and blue (basic) solutions of bromophenol blue (see Figure 3B) also illustrate subtractive color, as the conjugate acid acts as yellow filter, absorbing blue light, and the conjugate base acts as a combination of cyan and magenta filters, absorbing red light and green light. Overall, this demonstration is an effective and engaging method of teaching the spectrophotometric complementary color wheel, and the relationship between color and wavelength in the context of spectrophotometry. It is inexpensive, well suited to classrooms or lecture halls of all sizes, relatively nonhazardous, and easy to set up and perform. The demonstration is also versatile and can be modified or expanded by instructors to suit a variety of curricula and pedagogical environments.
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ASSOCIATED CONTENT
S Supporting Information *
Additional information and movies of representative demonstrations. The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.5b00665. Additional experimental details (PDF) Demo in Figure 3A (MPG) Demo with phenolphthalein and bromocresol green (MPG)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the Department of Chemistry and the Carl Wieman Science Education Initiative (CWSEI) at UBC for their support.
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REFERENCES
(1) Stuart-Fox, D.; Moussalli, A. Camouflage, Communication and Thermoregulation: Lessons from Colour Changing Oraganisms. Philos. Trans. R. Soc., B 2009, 364, 463−470. (2) Elliot, A. J.; Maier, M. A. Color Psychology: Effects on Perceiving Color on Pyschological Functioning in Humans. Annu. Rev. Psychol. 2014, 65, 95−120. D
DOI: 10.1021/acs.jchemed.5b00665 J. Chem. Educ. XXXX, XXX, XXX−XXX
