In the Laboratory
Sweet Chemistry Benedict Aurian-Blajeni* Science Department, BOOST, Naval Education and Training Center, Newport, RI 02841 Jonathan Sam and Michael Sisak Science Department, Naval Academy Preparatory School, Newport, RI 02841
One of the main goals of high school and freshman laboratory sessions is to create connections between everyday life and the science taught in the classroom. Upon completion of standard spectrophotometry laboratory sessions (1, 2), some students do not, on their own, make the fundamental connection between the obvious features of spectra and the colors of reagents. Only data collection and graph production occur; the student then moves on to the rest of the laboratory work. Moreover, one of the main challenges to the instructor is how not to turn the laboratory into a torture chamber while preserving scientific rigor as much as possible. Our strategy attempts to stimulate the students’ senses, yet provides adequate opportunity for quantitative verification of scientific principles. Students use food colorings and the dyes from M&M candies in order to: • • • • •
Verify the absorption of light by substances and its dependence on wavelength, Illustrate the change of light absorption with the concentration of dissolved substance (Beer–Lambert law), Explicitly connect solution colors with spectral features, Gain experience with superposition of graph curves, Explore the concept of nonprimary colors.
Some classical experiments that involve spectrophotometry deal with colored complexes of metals such as chromium, which create toxic waste (1, 3). Others involve substances with which the students are not familiar, such as indicators or the FeSCN2+ ion (4, 5). The laboratory work presented here uses perfectly safe chemicals and connects everyday-life objects and substances with laboratory measurements. This way, a familiar framework for basic chemical concepts is created (6 ). Experimental Procedure
Materials and Equipment This laboratory session requires the following materials and equipment: Spectrophotometer with cuvettes; we used a Spectronic 20 spectrophotometer from Bausch and Lomb. Food dyes: red (R), yellow (Y), and blue (B). We used the colors from the Decacake set that comprise Yellow #5, Blue #1, and Red #40 and #3. The foodstuff dye stock solutions are prepared in advance by the instructor, in concentrations of about 0.1% v/v. Water Plain, brown, M&M candy 10-mL graduated cylinders Test tubes A 50-mL beaker for extracting the food coloring from the M&M candy
Student Procedure Preparation of Solutions The students prepare the following mixed solutions from the original stock: R:B, 1:1; R:B, 2:1; R:B, 9:1. They compare the 5 tubes (2 pure colors and three mixtures) against a white paper background and note the change in hue. They will observe that the 9:1 solution is almost indistinguishable from the red solution. This demonstrates that even colors that seem “pure” to the eye may in reality be mixtures. The students also prepare a brown solution by mixing the three stock solutions in the proportions indicated in Table 1. Finally, they prepare an M&M extract by putting one M&M candy (being careful not to select a broken one) in 10 mL of water and swishing it for about 20 seconds. The supernatant is then poured into a cuvette. Spectrophotometric Measurements The students measure the absorbance of the primary solutions (1, 2, and 3), solution 4, the three-color mixture (7), and the M&M extract (8) from 340 to 720 nm, in steps of 20 nm. (N OTE: The absorbance must be set to 0 for the pure solvent (water, in our case) before each measurement, because the transmittance of the solvent itself changes with wavelength.) The absorbance at 620 nm is measured for solutions 5 and 6. The solutions used and the measurements to be performed by students are summarized in Table 1. The instructor may emphasize the fact that the color of a solution in natural light is given by the light which is not absorbed by the substance. Data Analysis and Interpretation The laboratory report should include qualitative observations regarding the change in color for solutions 3–6, graphs of the absorption spectra of the three primary dyes explained in terms of the electromagnetic theory, a graph of the spectrum of solution 4 with an explanation of the two observed peaks, Table 1. Solutions and Spectrophotometric Measurements Solution No.
Measurement
Yellow
Blue
1a
+
–
–
Full spectrum
2
a
–
+
–
Full spectrum
3a
–
–
+
Full spectrum
4
1 part
–
1 part
Full spectrum
5
2 parts
–
1 part
620 nm
6
9 parts
–
1 part
620 nm
7
2 parts
1 part
1 part
Full spectrum
–
–
–
Full spectrum
8 (M&M) *Corresponding author.
Dye Red
a Prepared
by the instructor.
JChemEd.chem.wisc.edu • Vol. 76 No. 1 January 1999 • Journal of Chemical Education
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In the Laboratory 2.0
M&M
1.4
1.8
1.2
Absorbance
1.0
Solution #7 0.8
0.6
Absorbance
1.6 1.4 1.2 1.0 0.8 0.6
0.4
0.4
0.2
0.2
0.0 300
350
400
450
500
550
600
650
700
750
0.0 0.00
0.02
0.04
0.06
0.08
0.10
Concentration (%)
Wavelength / nm Figure 1. Comparison between the absorption spectra of the M&M extract and the artificial brown (solution 7). The correspondence between the three peaks is easily observed.
Figure 2. Beer–Lambert law illustrated for the absorbance at 620 nm, for solutions 6, 7, 5, 4, and 3 (from left to right). The origin is the blank.
and a graph comparing the spectra of the M&M extract and the three-color mixture (see Fig. 1). The Beer–Lambert law is verified using the measurements for solutions 3–7. The student plots the absorption at 620 nm on the ordinate and the concentration of the blue dye in the solution on the abscissa. The result should be a straight line with 0 intercept, as shown in Figure 2. Time permitting, the instructor could use solutions prepared by successive dilution from a stock solution. The dilutions can be carried out by the instructor or by the students. The instructor might also wish to follow up by separating the dyes by paper chromatography (7), or by providing a graph of a “mystery” solution and asking the students to guess what should be the corresponding color (e.g., green).
Literature Cited
Conclusion We have used this laboratory session with more than a dozen classes, and numerous anecdotes from both students and lab instructors indicate that it does improve students’ appreciation and understanding of the related lecture material.
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1. Orna, M. V.; Schreck, J. O.; Heikkinen, H.; Ayers, C.; Breyer, A.; Himes, C. L. SourceBook; ChemSource: New Rochelle, NY, 1994; INST 4. 2. Department of Chemistry, University of Illinois at UrbanaChampaign. General Chemistry Experiments, Chemistry 102; Stipes Publishing: Champaign, IL, 1985; p 95. 3. Hunt, H. R., Jr. Introductory Chemistry Laboratory Manual; MacMillan: New York, 1974; p 193. 4. Beran, J. A.; Brady, J. E. Laboratory Manual for General Chemistry; Wiley: New York, 1982; p 261. 5. Leonard, C. B., Jr. In Modern Experiments for Introductory College Chemistry; Neidig, H. A.; Kieffer, W. F., Eds.; Division of Chemical Education of the ACS, 1967; p 79. 6. Irgolic, K.; Peck, L.; O’Connor, R.; Glenn, P. Fundamentals of Chemistry in the Laboratory, 2nd ed.; Harper and Row: New York, 1977; p vii. 7. Orna, M. V.; Schreck, J. O.; Heikkinen, H.; Ayers, C.; Breyer, A.; Himes, C. L. SourceBook; ChemSource: New Rochelle, NY, 1994; FOOD 10.
Journal of Chemical Education • Vol. 76 No. 1 January 1999 • JChemEd.chem.wisc.edu
