Return to Lab Menu. Chemiluminescence and Anti-oxidants

Transcription

1 Return to Lab Menu Chemiluminescence and Anti-oxidants Objectives: -to explore a chemiluminescent reaction -to observe the effects of temperature on the rate of a reaction -to observe the effects of anti-oxidants on the rate of a chemiluminescent reaction Equipment/Materials Procedure I: 2-3 green or red/orange lightsticks ice water bath warm water bath (temperature < 70 C) Procedure II: 4 green or red/orange lightsticks 1 Vitamin C tablet (250 mg or 500 mg) (no sweetener) 1 Vitamin C tablet, chewable (250 mg or 500 mg) w/ sweetner* ½ teaspoon (2.0 g) sucrose (table sugar) Duct tape sharp knife masking tape and/or permanent marker You will also need an area that can be darkened (such as a closet or interior bathroom) where the lightsticks can be observed. *Sweetener needs to be a sugar (such as dextrose). Artificial sweeteners such as Aspartame will not work. Both of the Vitamin C tablets need to be the same dosage (i.e. both 250 mg or both 500 mg.) Introduction Chemiluminescence If you have ever seen the glittery display of fireflies on a summer evening or the eerie nighttime glow produced by the wake of a boat or crashing waves in the ocean, you have observed a chemical reaction that is releasing energy. Many chemical reactions produce energy, typically in the form of heat. However a small class of reactions produces energy in the form of light; these reactions are called chemiluminescent reactions. Man-made versions include the glow sticks or lightsticks that are sold for camping, novelty, and roadside emergencies. The biological version of this process (e.g. fireflies, glow worms) is called bioluminescence. Chemiluminescent reactions typically involve the oxidation of a compound. The oxidized form is produced in a chemically excited state of higher energy. As it returns to a more stable form, it emits its extra energy as light. (See Figure 1.) The color of the light depends on the energy gap between the excited state and the lower energy ground state. Chemiluminescence can also result when energy from an excited molecule is transferred to another molecule. This AACE Copyright 2000 by Doris Kimbrough, all rights reserved Page 1 of 5

2 second molecule then emits the light energy as it returns to its ground state. (See Figure 2.) Most commercially available lightsticks utilize the latter process. Commercial lightsticks typically utilize dilute hydrogen peroxide to oxidize a phthallic ester in the presence of a dye. The high energy oxidation product transfers its energy to the dye molecule which then emits light, as is illustrated in Figure 2. Different dyes produce different colors. The lightstick is constructed of an outer thick plastic tube around a smaller inner tube made of glass which contains the hydrogen peroxide. The inner tube is cracked and the two chemicals are mixed and the reaction occurs. L(reduced) oxidation high energy L(oxidized) light L(oxidized) low energy Figure 1. Schematic representation of the chemiluminescent reaction of molecule, L. The oxidized form of L is produced in a high energy excited state. As it returns to the more stable ground state, it gets rid of the extra energy as light. L(reduced) high energy M oxidized L(oxidized) high energy + M energy transfer energy transfer L(oxidized) low energy + light L(oxidized) low energy + M high energy M low energy Figure 2. In this example the energy is transferred from the oxidized form of L to a different molecule, M. M then returns to a more stable, lower energy form and emits light in the process. The light released in this case is often a different color than the example in which the oxidized form of L emits light directly. Typically, lightsticks are wrapped in an airtight, light blocking foil wrapper. If the wrapper is damaged, light emission is reduced over time. The chemiluminescence of a commercially available lightstick can be slowed down by cooling and accelerated by heating. Furthermore the reaction can be inhibited by the addition of selected anti-oxidants. Antioxidants Antioxidants are a class of chemical compounds that are gaining increased media coverage for their importance in healthy cellular processes. They help to rid the body s cells of dangerous free radicals which, if left unchecked, can promote oxidative processes that harm the cell. Free radicals are a natural by-product of normal metabolism, and your body has several mechanisms for dealing with them. However, antioxidants such as vitamins A, C, and E can also AACE Copyright 2000 by Doris Kimbrough, all rights reserved Page 2 of 5

3 assist your cells in protecting against free radical damage. Ascorbic acid, or Vitamin C, is an important water soluble antioxidant that you ll use in this experiment. If an antioxidant is present, oxidation reactions are inhibited. This means that they either don t occur at all, or occur more slowly than if the antioxidant weren t present. Other compounds enhance oxidation processes. Since a chemiluminescent reaction gives off light, you ll be able to observe the presence and absence of an antioxidant. Procedure I. Effect of temperature on the behavior of lightsticks. Unwrap the lightsticks and follow the manufacturer s instructions to initiate the reaction. This typically involves cracking the inner glass tube by bending the outer plastic container and shaking to mix the two solutions of chemicals. Place one lightstick in the warm water bath. To avoid melting the plastic casing, do not exceed 70 C. Place a second light stick in an ice water bath. The optional third lightstick can be used as a control for comparison. Allow the lightsticks to equilibrate to the temperature of the baths (~10 minutes). Record the effect of temperature on the intensity of light emitting from each of the lightsticks. II: Effect of anti-oxidant compounds on chemiluminescence. CAUTION: In this procedure you will be using a sharp knife to cut open the lightsticks. Use caution! CAUTION: The chemicals contained within the lightsticks are not toxic. However they will cause discomfort in the case of eye contact. As with all procedures you should WEAR SAFETY GOGGLES WHEN PERFORMING THIS EXPERIMENT. Crush the regular Vitamin C tablet in a bowl or on a plate. Crush the chewable Vitamin C as well, taking care to keep the two powdered tablets separate. Measure or weigh out the sucrose. Unwrap the lightsticks and follow the manufacturer s instructions to initiate the reaction. This typically involves cracking the inner glass tube by bending the outer plastic container and shaking to mix the two solutions of chemicals. Secure one of the lightsticks vertically by stabilizing it against a wall or the refrigerator. Very carefully (Don t cut yourself!) cut the top of the stick ¾ of the way off. (See Figure 3.) Leave a portion of the cap intact so it can be used to cover the tube for the purposes of mixing later in the experiment. Use caution when cutting the top of the lightstick off both to prevent knife injuries and to prevent the chemicals within the lightstick from splashing out. Securing the lightstick AACE Copyright 2000 by Doris Kimbrough, all rights reserved Page 3 of 5

4 against a hard vertical surface is very important, as is wearing safety goggles during the entire procedure. After cutting into the top of the lightstick, fold back the cap and pour the sucrose into the tube. Fold the cap back into place and seal with duct tape. Try to ensure that your seal is airtight. Gently upend and the stick for seconds to mix, label the tube and set aside. Repeat the above process for each of the two Vitamin C samples. The fourth lightstick will act as the control, but you should open the tube and seal it with duct tape as you did the others. (Why?) Record the relative (to each other) brightness of each of the four tubes. Note their relative appearance after one hour, after two hours and after several hours. Further experimentation for interested students. Read an article on antioxidants. Repeat this experiment with some of the foods, herbs, and/or vitamins that are considered to have antioxidant behavior. AACE Copyright 2000 by Doris Kimbrough, all rights reserved Page 4 of 5

5 Report Download Data Sheet 1. Discuss the effect of temperature on the intensity of light from the lightsticks. What conclusions can you draw about the rate of reaction with respect to temperature? Why does placing activated lightsticks in the freezer prolong their life? 2. Discuss the effect of the added compounds in Procedure II, on the course of the reaction. Make a table that lists each compound and the corresponding effect on the chemiluminescent reaction. When (if ever) was the reaction inhibited? When (if ever) was the reaction enhanced? Provide a plausible explanation for the differences that you observed. 3. Why did you cut open the lightstick that was acting as your control? 4. Suggest other compounds that might enhance or inhibit this reaction? Postulate as to how you could use this reaction as a way to monitor the presence of antioxidants in foods. If you tried any additional substances, report on your results. AACE Copyright 2000 by Doris Kimbrough, all rights reserved Page 5 of 5