CH 112 Special Assignment #5 The Case of the Suspicious Painting

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1 CH 112 Special Assignment #5 The Case of the Suspicious Painting PRE-LAB ASSIGNMENT: Make sure that you read this handout and bring the essentials to lab with you. You may find it helpful to read parts of Chapter 17 in Adventures in Chemistry, specifically pp Here are the pre-lab questions for this week. 1. Last week you examined the relationship between absorbance of visible light and the color of a chromophore. What is this relationship? 2. Predict the wavelengths of light absorbed by a red dye, a yellow dye, a green dye, and a blue dye. 3. Which dye would absorb light of the highest energy? Explain briefly. Alfredo Fettuccini (artist) INTRODUCTION: CRaC s assignment this week comes from the Colby Art Museum, which has been negotiating for the acquisition of a rare masterpiece by the great Italian finger painter, Alfredo Fettuccini. This painting is in the collection of noted art dealer Ebenezer Wheezingeezer, who lost a bundle in the stock market and is selling off several valuable paintings. The director of the art museum wants to ensure that Wheezingeezer isn t trying to pull a fast one and unload a worthless substitute on them. That s where we come in. You will analyze several Ebenezer Wheezingeezer (art dealer) ????

2 Figure 1. Many painting pigments are solids derived from minerals, such as the copper-containing mineral azurite (left). Prussian Blue (right) was the first modern synthetic pigment, discovered by accident in pigment samples with relevance to this investigation to help determine whether the art museum should spend several million dollars on the painting in question. Pigments are the colored substances in paint. Pigments usually start out in powdered form, such as finely ground mineral particles, and are added to a colorless liquid before being used for painting. Humans have used many different types of pigments throughout history, but many have been replaced over time. For example, blue pigments that contain copper, such as azurite, were used before 1800, while blue pigments that contain iron, such as Prussian Blue, are relatively new inventions (Figure 1). Note that Fettuccini, like all artists of his time, mixed his own paints and was legendary for a copper-containing blue named after him. Certainly the finding of an ironcontaining blue paint in a work attributed to an artist of his time would be enough to show that the work is a fake. Pigments appear colored because of how they interact with visible light, absorbing certain wavelengths of visible light and reflecting other wavelengths. If a pigment is red, it reflects red light and absorbs all other colors (Figure 2). Our eyes see the reflected light, so our brains tell us that the pigment as red. Because of differences in the chemical structures of compounds that may appear to be similar colors, the absorbance spectrum of a pigment can often be used like a fingerprint for identification purposes. The pigments that you will analyze in this assignment are the following: Pigment 1: Blue pigment from the questioned work Pigment 2: Blue pigment from a modern painting Figure 2. Materials appear colored because they reflect visible light of certain wavelengths. This apple reflects red light, and so it looks red to us. Pigment 3: Blue pigment found on the floor of Wheezingeer s gallery (from a touch-up job?) Pigment 4: Genuine sample of Fettuccini blue from one of his preserved brushes 2

3 You will measure the absorbance of light by these pigments using the same spectrophotometers that you used last week, using the absorbance spectrum, particularly the λmax, to characterize each pigment. You will also do some chemical tests on the pigments, including tests for iron and copper. Remember that iron is used in some modern blue pigments (like Prussian Blue), but Fettuccini is known to have used only copper-containing blue pigments. The results of your experiments will help you to determine what pigments are present in the questioned painting and whether they are consistent with those available to Fettuccini. LEARNING GOALS: Be able to: Characterize different pigment samples based on their absorbance spectra. Characterize different pigment samples based on their chemical properties. Synthesize the results of different tests to reach a conclusion based on the data. PROCEDURE: Part 1. Relationship Between Pigment Color and Color of Light Absorbed 1. Calibrate the spectrophotometer with deionized water. Choose Calibrate Spectrometer from the Experiment menu. The calibration dialog box will display the message: Waiting 60 seconds for lamp to warm up. Following the instructions in the dialog box to complete the calibration, use a cuvette filled about ¾ full with deionized water. Check to make sure the non-frosted, clear sides are in the light path. The cuvette should be inserted all the way into the sample chamber. You should feel that the cuvette is gently, but firmly, held in place so that you cannot twist the cuvette. Click Finish Calibration and then click OK. This step blanks the spectrophotometer with water. 2. Using a pipette, fill a cuvette 2/3 full with deoinized water, then add a few drops of red food coloring. Cover the cuvette with Parafilm and mix well by inverting several times. Place the cuvette in the sample chamber of the spectrophotometer. Click on and then. 3. Sketch the absorbance spectrum in your notebook, carefully noting how many peaks are present. Read the absorbance using the Examine tool, by clicking on and then moving the cursor along the spectrum. The wavelength and absorbance will be displayed in the new dialog box in the data window. Record the wavelength at which the absorbance value is the greatest. This is the wavelength of maximal absorbance, or λmax. To what color of light does this correspond? Note that if there are two clear peaks, then you should record two values for λmax at the top of each peak. 4. Repeat steps 2 and 3 for yellow and green food coloring. 3

4 Part 2. Characterization of Absorbance of Sample Pigments 1. Using a pipette, fill a cuvette 2/3 full with Pigment 1. Place the cuvette in the sample chamber of the spectrophotometer. Click on and then Sketch the absorbance spectrum in your notebook. 2. Record the λmax. To what color of light does this correspond? 3. Repeat until you have the absorbance data for all four pigment samples. Part 3. Chemical Characterization of Sample Pigments 1. Use a dropper and put 1 drop of each pigment sample on a ph test strip. Recall that ph paper turns red in the presence of acids, turns blue in the presence of bases, and stays orange in neutral solutions. Record the approximate ph of each pigment in your notebook. 2. Add 10 drops of each pigment to a different well of the spot plate. 3. Add 1 drop of 0.2 M potassium ferrocyanide [K 4 Fe(CN) 5 ] to each well containing pigment. This reagent forms different colored products with iron and copper. If a pigment has iron in it, it will turn dark blue. If a pigment has copper in it, it will turn brick red. Record your results for each pigment in your notebook. Since all the pigments are blue to begin with, it may be difficult to know for certain if a pigment contains iron. If you think you see some of the pigments turn darker, however, you can verify your findings with a second test for iron. 4. Add 5 drops of each pigment to a separate clean well of the spot plate. 5. Using a mortar and pestle, grind up an aspirin tablet and add a few grains to each pigment in the spot plate. Aspirin tablets contain acetylsalicylic acid, which forms a purple product with iron. Watch for a color change to purple, which confirms the presence of iron. Record your results for each pigment in your notebook. References: Harmon, Katy (2008) The Case of the Stolen Masterpiece. Colby College CH 151 website. Harmon, K. J., Miller, L. M., and Millard, J. T. (2009) Crime Scene Investigation in the Art World: The Case of the Missing Masterpiece. J. Chem. Educ. 86,

5 CraC Special Assignment #5 REPORT SHEET NAME PARTNER DATE DATA SUMMARY Pigment/Dye Red Food Coloring Yellow Food Coloring Green Food Coloring Pigment 1 Pigment 2 Pigment 3 Pigment 4 λmax Pigment Approximate ph Color of potassium ferrocyanide test Color of aspirin test Pigment 1 Pigment 2 Pigment 3 Pigment 4 POST-LAB QUESTIONS 1. Re-visit the general relationship between the color of light absorbed and pigment color that you presented in your pre-lab. Did all the food coloring samples follow this relationship as predicted? Explain. 2. What did your absorbance data tell you about the four blue pigments? 5

6 3. Which blue pigment(s) contained copper? How do you know? 4. Which blue pigment(s) contained iron? How do you know? 5. Metal ions in aqueous solutions generally form acidic species. Were your ph results consistent with this trend? Explain. 6. Why was it necessary to do more than one kind of test to solve this case? 7. Taken together, what do your data suggest about the painting in question? CONCLUSIONS Use the space below to report on your experiment to the art museum, making sure that you address the original goals of this work. 6