edited by
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JAMES 0 . SCHRECK Univeffiity of Northern Colorado Greeley, CO 80639
Crepe Paper Colorimetry David L. pringlel Department of Chemistry and Biochemistry, University of Northern Colorado, Greeley, CO 80639 Kathy Chaloupka Aspen High School, Aspen, CO Maryann Varanka-Martin Estes Park High School, Estes Park, CO Most of us have attended a homecoming football game or parade in the rain and noted that the dyes in crepe paper used on the goal posts or floats are water soluble. During an NSF-sponsored workshop, one teacher, while studying a spectrophotometry module, noted that some elementary and middle schools used crepe paper to introduce the basic concepts of the physics of color. This data, coupled with the solubility of many of the crepe paper dyes, resulted in the ited by availability of a;ariety of colors, the need to weigh very small amounts of compounds or do successive dilutions, the use of chromophores and solvents that create disposal problems, or the need for complex chemical reactions to produce the desired color. These limitations divert the students' attention away from the intended task. The use of crepe paper avoids all of these common pitfalls. Because crepe paper comes in a wide variety of colors, it provides a ready source of samples for the study of colorimetry. Furthermore, standard streamer crepe paper can be cut into easily handled lengths from one to 20 centimeters, depending on the colors, to provide solutions with ahsorbances from near zero to about one when dissolved in 100-mL water. The resulting solutions can safely he poured down the drain. An additional benefit to the use of crepe paper is the cost. One 100-foot roll of crepe paper will last many years. Experiment Three basic experiments have been tried using crepe paper: (1) exploration of spectral characteristics of various colors, including black and white; (2) determination of calibration curves along with the verification of Beer's Law; and (3) introduction of simultaneous colorimetry. Some or all of these experiments have been used with students in chemistry courses ranging from high school chemistry to college-level analytical chemistry 'Author to whom correspondence should be addressed.
722
Journal of Chemical Education
Spectral Characteristics Students are asked to cut strips of crepe paper (purple, blue, green, yellow, orange, and green have been used) about five centimeters long and dissolve the dye used to color them in 100-mL water. The strips of blue crepe paper need to be cut to two-centimeter widths due to the intensity of the color. I t is important not to stretch the crepe paper, but simply measure it as it comes from the roll. The dissolution of the dye is rapid; solutions are ready to use in r~huurlive m i n u r e s . ~ ~ h undissolved e paper can I,; rcmoved making r(.;~dings,or the superfrom t l ~ cs o l u t ~ o tbcfirrr ~ natant can be decanted into thecuvette.A ruler, scisiors, 250-mL beakers, and 100-mL graduated cylinders are all that are needed to prepare samples. If a recording spectrophotometer is available, each student or student group is asked to take the transmission and absor~tionsoectra of each solution. If a single-beam ~ ~ e c t r o p h o t b m e tsuch e ~ , as a Suectronic 20 is used. each student or student m o u can ~ determine the spectral plot of a single color, and &e resulting spectrum can be shared by the entire class. Upon completion, a comparison of the wavelength regions of transmission a n d absorption for each color can be made. Adventurous students might be asked to speculate about and determine the spectral characteristics of black and white crepe paper. Beer's Law and the Determination of an Unknown Using a brief discussion of Beer's Law (1)and the data collected above, students are asked to prepare a series of five solutions of a single color, such that absorbance of the most concentrated solution does not exceed 1.00 when the . then take the absorbance data is taken a t ha,Students data, plot a calibration curve, and determine the slope, intercept, and correlation coefficient from the graph. Crepe paper of unknown lengths can be easily and quickly prepared by the instructor. Simultaneous Colorimetry After completion of the analysis of a single-component system, the idea of the simultaneous determination of a
Summary of Spectrophotometric Results for Crepe Paper
Color Red Orange Yellow Green Blue Purple Notes
Amax.nm
Length, cm
Corr. Coefficient
Slope, cml-
Intercept, A. U.
503 477 427 629 628 540
2.0,4.0,6.0,8.0,10.0 2.0,4.0,6.0,10.0 2.0,6.0,10.0,14.0,18.0 2.0,5.0,8.0,11.0,14.0 1.0,3.0,5.0,7.0,9.0 1.0,3.0,5.0,7.0,9.0
0.9991(0.9986) 0.9999 (0.99931 0.9999 (0.9995) 0.9997 (0.9984) 0.9998 (0.9991) 0.9995 (0.9993)
0.0936 (0.0968) 0.0892 (0.0824) 0.0496 (0.0532) 0.0651 (0.0688) 0.1086 (0.0906) 0.1083 (0.1110)
-0.001 (0.004) 0.010 (0.0261 0.002 (0.001) -0.004 (0.015) 0.015 (0.040) -0.004 (0.012)
1. Lengths were cut from standard streamer paper. Blue paper strips were cut into 2.0-cm widths
before dissolving in water. 2. Correlation coenicients, slopes, and intercepts data reported without parentheses were obtained
using a Varian Super Scan Ill. The data reported in parentheses were collected using a Bausch and Lomb Spectronic 20.
mixture of two different colors of crepe paper is undertaken (2).Mixtures can be chosen for maximum or minimum spectral interferences using the spectra of individual colors of crepe paper obtained in part one. Mixtures of purple and orange or red and green work well. Students are asked to prepare and analyze solutions of the individual colors and to determine t h e absorptivities (in terms of mWcm of crepe paper1 a t the two wavelengths chosen for the analysis. They are then asked to prepare a mixture with known lengths of crepe paper and to analyze this mixture. After calculating the results and comparing them with the lengths they cut out, they are asked to comment on the accuracy of the method. Students finish the experiment by analyzing a mixture prepared by the instructor. Introduction of these experiments into undergraduate analvtical chemistrv has eenerated a meat deal of enthusiasm and discussion. Students enjoy working with the variety of colors and comparing their spectra. Most students are amazed that the wavelengths a t which various colors absorb are diametrically opposed to their predictions. Confusion between wavelength absorbed by the solution and its color has stimulated discussions between students and instructor that were lacking with prior introductory spectrophotometric experiments. By the end of the experiment, students have a genuine appreciation for the difference hetween absorption and transmission spectra and their relationship to color. Understanding of the visually enhanced colored system leads to discussions of the less obvious topic of ultraviolet spectrophotometry. Results obtained from preparation of calibration curves for the various colors of crepe paper are shown i n the table. Graphs of length of the crepe paper versus absorbance show excellent linearity for each color. Results for unknowns show good accuracy Average percent error obtained by one class was: red, 2.5%; orange, 4.4%;yellow, 2.5%; green, 4.9%;blue, 3.0%; and purple, 3.0%. Most of the error is probably due to the lack of care i n cutting the crepe paper. It is evident from the correlation coefficients,
however, that the crepe paper is very uniform, and with a little care, good results can he obtained. The simultaneous determination showed similar results. For mixtures of orange and purple, in which the orange crepe paper ranged in lengths between 4.5 and 6.8 cm and the purple paper ranged between 3.9 and 4.4 em, the average error was 4.1% (range 1.0 to 11%)for the orange paper and 4.5%(range 1.0 to 16'961 for the purple paper. Mixtures of yellow and orange paper gave a n average error of 4.2% for the yellow paper and an average error of 7.1% for the orange paper. Preliminary results for red and green mixtures gave errors of approximately 2%for both colors. Summary U s i n g crepe p a p e r for t h e introduction of spectrophotometric concepts has been shown to be useful. Dyes used in the manufacture of the crepe paper dissolve rapidly in water (provided the run-proof papers are not used1 to produce solutions of colors unmatched i n variety and simplicity by experiments routinely used to introduce spectrophotometry. The rich variety of colors not only spark student interest, but provide spectra in the visible spectrum that allow students to grasp concepts of absorption and transmission unmatched by using solutions of single colors. Ease of solution preparation allows for the study of many spectrophotometric concepts in a relatively short time because no extensive chemistry or time is needed for color development. The laboratory activity is inexpensive, safe for students of any age, and disposal of materials is simple. Acknowledgment This work was initiated under a teacher enhancement grant provided by the National Science Foundation, DPE8470011. Literature Cited 1. Skoog. 0. A : Lea% J. J.P~lneiplerdI"rtrumenloiAnnlysir; Savnders College Publishing: New York. 1992:pp 123-126. 2. S k o q . D.A . pp 127-129.
Volume 72 Number 8 August 1995
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