JCE Classroom Activity: #90
Instructor Information
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How Does Your Laundry Glow? Richard B. Weinberg Wake Forest University School of Medicine, Winston-Salem, NC 27157;
[email protected] In this Activity, students examine the effect of pH on the intensity and color of the emission of fluorescent dyes in liquid laundry detergent. They perform two titrations using vinegar to estimate the pH at which the fluorescence properties change. In the second titration, sodium bicarbonate is added to buffer the detergent solution. Optical brighteners present in most laundry detergents are dyes that glow with a bright blue fluorescence when excited by ultraviolet (UV) light present in sunlight. Dilute solutions of detergent also exhibit this fluorescence. Students may be familiar with indicators that change color at a certain pH, but they are less likely to know that pH can also affect the behavior of fluorescent chemicals. Acidifying the detergent solutions with vinegar causes a noticeable brightening of the fluorescence and a change in the color from blue-violet to blue-green. These changes in fluorescence emission are caused by the protonation of the optical brightener, which in turn alters its intramolecular electron distribution. A complete discussion of the chemistry involved, and its use in a fluorescent clock reaction, is presented in this issue and the references cited therein (1).
Integrating the Activity into Your Curriculum This Activity illustrates the principles of UV-excited fluorescence, pH indicators, and the effect of buffers. It can also be used as a starting point for discussions of the environmental impact of consumer chemicals present in wastewater.
About the Activity Miniature key-chain sized UV LED flashlights can be used to observe the detergent solution fluorescence. They are available from internet vendors (2). Other UV light sources can also work well. Instructors can confirm that a UV light source will be satisfactory if it yields a blue-violet glow when it illuminates a white shirt. Students titrate a detergent solution with vinegar while monitoring the pH to identify the pH at which the change in fluorescence emission occurs. Without any buffers in the solution, the pH rapidly falls from ~6.0 to 4.0 after the addition of several drops of vinegar, which makes it difficult to identify the transition midpoint (pH 4.6–4.8). When the titration is repeated in the presence of sodium bicarbonate (baking soda), the amount of vinegar needed to lower the pH greatly increases, resulting in a more informative titration curve. Sample titration curves are in this issue of JCE Online.W Arm & Hammer perfume- and dye-free detergent was used in testing. Narrow-range pH test strips are used to produce titrations suitable for home use. ColorpHast pH indicator strips, pH 4.0–7.0, are available from various sources (3). Alternatively, pH meters or probes could be used. About 25–30 strips are needed to conduct the two titrations. Distilled or de-ionized water produces the best results. Baby food jars or small plastic cups work well as containers.
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Answers to Questions 1. The detergent solution glows bright blue. The glow is due to fluorescence. Fluorescence occurs when a molecule absorbs UV light energy, becomes “excited”, and then almost immediately re-radiates the energy as visible light. 2. Adding vinegar lowered the pH of the solution and resulted in a brightening of the fluorescence and a color change to blue-green. The acid in the vinegar changes the structure of the fluorescent dye in the detergent, which alters its fluorescent properties. This is similar to how acid changes the color of pH indicators. 3. The plot of pH vs drops of vinegar changes more slowly in the presence of baking soda. Sodium bicarbonate acts as a buffer which “soaks up” hydrogen ions as soon as they are added, according to the reaction: H+ + HCO3− H2CO3 CO2(g) + H2O. Thus, much more vinegar must be added to the buffered solution to lower its pH. 4. The fluorescence color transition occurs between pH 4.8 (blue-violet) and pH 4.6 (blue-green). 5. You could add a drop of detergent to a sample of unknown pH. If the mixture gave a blue-violet fluorescence, its pH is greater than 4.8. If the mixture gave a blue-green fluorescence, its pH is less than 4.6. 6. Laundry detergent dyes exhibit detectable fluorescence even at extremely low concentrations. A UV light could be used to test river and lake water samples (4). A sample that exhibited a blue fluorescence would indicate that household wastewater contaminated the water source.
This Classroom Activity may be reproduced for use in the subscriber’s classroom.
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Background
References and Additional Related Activities (URLs accessed Feb 2007) 1. Weinberg, Richard B. An Iodine Fluorescence Quenching Clock Reaction. J. Chem. Educ. 2007, 84, 797–800. 2. A reliable source is http://www.stevespanglerscience.com/product/1433, WBLK-100 mini black light. 3. VWR LabShop, http://vwrlabshop.com, catalog # EM-9582-1; Fisher Scientific, http://fishersci.com, catalog # M95823. Based on inquiries, shipping through VWR was significantly less expensive. 4. Fluorometric detection of optical brighteners as an indicator of human sources of water pollution. http:// www.ext.vt.edu/news/periodicals/cses/2005-11/part1.html JCE Classroom Activities are edited by Erica K. Jacobsen and Julie Cunningham
www.JCE.DivCHED.org •
Vol. 84 No. 5 May 2007 •
Journal of Chemical Education
800A
JCE Classroom Activity: #90
Student Activity
How Does Your Laundry Glow? Did you know that most laundry detergents, even some labeled as “dye free”, may contain dyes known as optical brighteners? These dyes glow with a bright blue fluorescence when excited by ultraviolet (UV) light present in sunlight. The function of these dyes is to neutralize yellowness in laundered fabrics, thereby giving them a whiter appearance. In this Activity, you will examine how pH affects the properties of a fluorescent dye present in laundry detergent.
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photo by Richard B. Weinberg
You will need: distilled or de-ionized water; 3–4 small, clear, colorBe Safe! Eyes should be less containers; 2–3 plastic pipettes; measuring spoons or a 10-mL shielded at all times from graduated cylinder; liquid laundry detergent that is labeled “perfumedirect exposure to UV light and dye-free”; white vinegar; baking soda; narrow-range pH test strips sources. Never look directly (ColorpHast strips, 4.0–7.0); plastic coffee stirrers; miniature ultraat a UV light source. Liquid violet (UV) LED flashlight; graph paper. Perform the following steps laundry detergent is irritatin a darkened room. ing to open cuts and the __1. Add 2 tablespoons (30 mL) of water to a small, clear, colorless eyes. Wash your hands container. Shine a UV LED at the liquid through the side of thoroughly if you get deterthe container. What do you observe? Using a plastic pipette, gent on them. add 5 drops of “perfume- and dye-free” liquid laundry detergent to the water, and stir well with a plastic stirrer. Shine the UV LED through the side of the container. What do you observe? Shine the UV LED on white clothing. What do you observe? __2. Dip the end of a narrow range (4.0–7.0) pH test strip into the solution from step 1. Immediately compare the color with the scale on the package and record the pH. __3. While shining the UV LED at the container from step 1, add 1 teaspoon (5 mL) of white vinegar and stir. What do you observe? Measure the pH of the solution with a test strip and record. __4. In a new container, add two tablespoons (30 mL) of water and 5 drops of liquid laundry detergent. Stir well. Using a clean plastic pipette, add 1 drop of white vinegar to the container and stir. Measure the pH of the solution with a test strip and record. Shine the UV LED on the solution. What do you observe? Add a second drop of vinegar, stir, check and record the pH, and again examine the solution with the UV LED. Continue to add additional drops of vinegar, checking and recording the pH and the appearance of the solution under UV light until no further change occurs or you reach the end of the pH range of the test strips. __5. Make a graph of pH vs the number of drops of vinegar added to the solution. Mark on the graph where you noticed a change in the solution’s appearance under UV light. __6. In a new container, add two tablespoons (30 mL) of water and 5 drops of liquid laundry detergent. Add a small pinch of baking soda and stir well. Repeat the procedure from step four, but this time add 5 to 10 drops of vinegar at a time between each time you check the pH and the appearance of the solution under UV light. Record the number of drops added, the pH, and the appearance of the solution each time. __7. Again, make a graph of pH vs the number of drops of vinegar added to the solution. Mark on the graph where you first noticed a change in fluorescence properties, and where you no longer noticed any further change.
Questions 1. What did you observe when you shined the UV light on the detergent solution? What is this phenomenon called? What causes it? 2. What happened when you added vinegar to the detergent solution? Why? 3. Compare the two graphs of pH vs drops of vinegar added. What effect did the addition of baking soda have? Why? 4. Estimate the pH range over which the detergent fluorescence changes. 5. How could you use laundry detergent to roughly estimate the pH of a solution? 6. How could environmental scientists use UV light to determine whether household wastewater has contaminated a river or lake?
Information from the World Wide Web (accessed Feb 2007) Introduction to fluorescence. http://probes.invitrogen.com/resources/education/tutorials/1Introduction/player.html Optical brightener. http://en.wikipedia.org/wiki/Optical_brightener Acid–base indicators. http://www.chemguide.co.uk/physical/acidbaseeqia/indicators.html Buffers. http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/buffer12.swf This Classroom Activity may be reproduced for use in the subscriber’s classroom.
800B
Journal of Chemical Education •
Vol. 84 No. 5 May 2007 •
www.JCE.DivCHED.org
