OBJECTIVES: Visitors learn what an antioxidant is and how it behaves. They also learn how to test for the presence of vitamin C..

Transcription

1 Vitamin C Visitors use iodine to compare the reactivity of two starch solutions one with vitamin C added, one without vitamin C. OBJECTIVES: Visitors learn what an antioxidant is and how it behaves. They also learn how to test for the presence of vitamin C.. SCIENCE TOPICS PROCESS SKILLS VOCABULARY Antioxidants Chemical Reactions Oxidation Observing Measuring Investigating Comparing Molecule Anti-oxidant UNIT 8 EVERYDAY CHEMISTRY U8.1

2 Vitamin C Procedure: 1. Always wear safety goggles. 2. Rinse the two flasks and the graduated cylinder in the sink. 3. Use the graduated cylinder to measure 10 ml of starch solution and add it to Flask Measure 10 ml of starch solution again, and add it to flask Add 10 drops of vitamin C solution to Flask 2 only. 6. Add one drop of iodine solution to Flask 1. Swirl the flask to mix the solution. What happens? 7. Count each drop as you continue adding drops of iodine to Flask 1. Swirl the flask after each drop until the solution turns dark blue. How many drops did you need to add to make the solution turn blue? 8. Repeat step 5, adding iodine to Flask 2. How many drops did you need to turn the solution in Flask 2 dark blue? 9. Empty and rinse the flasks and graduated cylinders in the sink. U8.2 UNIT 8 EVERYDAY CHEMISTRY

3 What is vitamin C? How does it work? A Closer Look Vitamin C, or ascorbic acid (C 6 H 8 O 6 ), is an antioxidant. Antioxidants are very reactive molecules that are broken down by oxygen. Antioxidants, like vitamin C, act as a first target of reactive molecules like oxygen or iodine. This protects other molecules critical to our health. In this experiment, the iodine (I 2 ) drops in Flask 1 react quickly with the starch to form a blue complex. In Flask 2, the vitamin C prevents the iodine from reacting with the starch. Once all of the vitamin C has reacted, any additional iodine reacts with the starch to form the blue complex. Vitamin C does this in our bodies, too. It protects us by reacting with toxic compounds before they can harm us Vitamin C is found in many fruits and vegetables. It is oxidized by exposure to air, heat, and light. Oxidation destroys the function of vitamin C. Thus, fresh, uncooked fruits and vegetables are the best sources of vitamin C. MATERIALS UNIT 8 EVERYDAY CHEMISTRY U8.3

4 (with amounts to have on hand) Two 100-ml Erlenmeyer flasks One 100 ml dropper bottle One 10-ml graduated cylinder One 250-ml squeeze bottles (see-through) One 30-ml dropper bottle One 500-ml brown (opaque) plastic bottle Vitamin C crystals (keep 50 g on hand) Soluble starch (soluble potato powder) (keep 100 g on hand) Povidone iodine (keep 250 ml on hand) One 1000-ml storage bottle Setup/Takedown Procedures ORIGINAL SETUP WEEKLY SETUP Color code and label the two 100-ml Erlenmeyer flasks Flask 2 (white) and Flask 1 (white). Label the 10-ml line on the 10-ml graduated cylinder Label the 100-ml dropper bottle vitamin C (red) Color code and label one 250-ml squeeze bottle Starch Solution (blue). Label the 30-ml dropper bottle Iodine. Prepare 0.3% starch solution (see Materials Prep). Prepare 0.2% stock vitamin C solution (see Materials Prep) Check to see that the experiment works. (When 10 drops of vitamin C solution is added to 10 ml of starch solution, ~5 drops of iodine should be used to make the solution blue. If drops of iodine are required to turn the starch/vitamin C solution blue, the solution has too much vitamin C; dilute it with water. If less than 5 cause it to turn blue, dilute the iodine solution slightly.) U8.4 UNIT 8 EVERYDAY CHEMISTRY

5 Fill the squeeze bottles and dropper bottle with the appropriate solutions. Store the vitamin C solutions IN THE REFRIGERATOR DAILY SETUP Set out the visitor instructions in a Plexiglas holder. Get the squeeze bottle of cold 0.2% vitamin C solution from the refrigerator. On a tray lined with a white mat, set out the following: Two labeled flasks One labeled graduated cylinder One small, labeled iodine dropper bottle One labeled squeeze bottle. One small labeled vitamin c dropper bottle Check to see that the experiment works. (When 10 drops of vitamin C solution is added to 10 ml of starch solution, ~5 drops of iodine should be used to make the solution blue. If drops of iodine are required to turn the starch/vitamin C solution blue, the solution has too much vitamin C; dilute it with water. If less than 5 cause it to turn blue, dilute the iodine solution slightly.) Check to see whether there is plenty of stock 0.2% vitamin C solution and 0.3% starch solution under the counter. Prepare more of each solution as needed (see Materials Prep). DAILY TAKEDOWN Store the bottles tightly capped in an upright position. Clean the glassware and return all equipment and instructions to the tub under the counter. Return the vitamin C solution TO THE REFRIGERATOR. WEEKLY TAKEDOWN Dispose of any leftover starch and vitamin C solutions, including those in the refrigerator. Empty and clean the 250-ml squeeze bottle. Clean all of the glassware and bottles. UNIT 8 EVERYDAY CHEMISTRY U8.5

6 Clean and return the mat to storage. Clean the tray and leave it at the station. RUNNING SUGGESTIONS Check the squeeze bottles periodically and refill them as needed (the starch solution will be used up rather quickly). If the cold vitamin C solution requires drops of iodine to turn blue, the solution has too much vitamin C, dilute it with cold water. If the cold vitamin C solution turns too quickly, you may need to prepare fresh vitamin C solution (see Materials Prep). EXTENSIONS You can check the vitamin C content of various fruit juices. Try the experiment with old vitamin C solution or vitamin C solution exposed to heat (on the hot plate for 15 minutes), light, or air. It should take fewer drops of iodine to create the blue starch complex Safety & Disposal MATERIALS PREP To prepare 0.2% stock vitamin C solution: Weigh 2 g vitamin C crystals (from the organic cabinet). Add 1000 ml H 2 O to the vitamin C crystals. Place the solution in a labeled/dated brown (opaque) bottle. Store the solution in the REFRIGERATOR. To prepare 0.3% starch solution: Boil 900 ml dh 2 O (deionized water). Weigh 3.0 g soluble starch. Mix the starch with 10 ml of dh 2 O to form a paste. Add the paste to the 900 ml of boiling water. Heat and stir the solution until it becomes clear. U8.6 UNIT 8 EVERYDAY CHEMISTRY

7 Cool the solution. Add dh 2 O to bring the total volume to 1000 ml. Place the solution in a labeled/dated, 1000-ml storage bottle. UNIT 8 EVERYDAY CHEMISTRY U8.7