Purpose: The goal of this lab is to determine the concentration of vitamin C in juices and Real Lemon. A redox titration, involving an iodometric method, will be used to do the analysis. The samples will be classified by their Vitamin C content.
Introduction: Although most mammals can synthesize vitamin C, or ascorbic acid (C6H8O6), from sugars, man must ingest considerable quantities of this substance. The National Academy of Sciences recommends the consumption of 60 mg of ascorbic acid per day. Vitamin C deficiency, which typically causes abnormalities in bones and teeth, was first characterized in sailors in the eighteenth century. These abnormalities were eliminated by compelling sailors to eat limes, a source of vitamin C. Many vegetables also contain large quantities of vitamin C, but ascorbic acid is commonly destroyed by many cooking processes, and hence citrus fruits are regarded as the most reliable source of vitamin C.
Vitamin C can be determined in food by use of an oxidation-reduction reaction. The redox reaction is preferable to an acid-base titration because a number of other species in juice can act as acids, but relatively few interfere with the oxidation of ascorbic acid by iodine. The solubility of iodine is increased by complexation with iodide to form triiodide
I2 (aq) + I- ºI3-
Triiodide then oxidizes vitamin C to dehydroascorbic acid
C6H8O6+ I3-+ H2O → C6H6O6+ 3I-+ 2H+(2)
Vitamin C dehydroascorbic acid The endpoint is indicated by the reaction of iodine with starch suspension, which produces a blue-black product. As long as vitamin C is present, the triiodide is quickly converted to iodide ion, and no blue-black iodine-starch product is observed. However, when all the vitamin C has been oxidized, the excess triiodide (in equilibrium with iodine) reacts with starch to form the expected blue-black color.
1. Preparation of iodine solution.
Dissolve 5.00 g potassium iodide (KI) and 0.268 g potassium iodate (KIO3) in 200 mL of distilled water in a 400 mL beaker. Add 30 mL of 3 M sulfuric acid. Then pour the solution into a 500 mL graduated cylinder, and dilute to a final volume of 500 mL with distilled water. Mix thoroughly and transfer to a 600 mL beaker. Do not put this solution in a volumetric flask!!!!
2. Preparation of vitamin C standard solution.
Dissolve 0.250 g vitamin C in 100 mL water. Dilute to volume in a 250 mL volumetric flask.
3. Standardization of the iodine solution with the vitamin C standard solution.
Add 25.00 mL of vitamin C solution into a 125 mL Erlenmeyer flask. Add 10 drops of 1 % starch solution. Rinse your buret twice with 5 -10 mL of iodine solution, and then fill it. Record your initial buret volume. Titrate the solution until the endpoint is reached (the first sign of blue color that remains after at least 20 s of swirling). Record the final volume. Repeat this titration at least three times. Results should agree to 0.1 mL.
4. Titration of juice samples.
Add 25.0 mL of your beverage sample into a 125 mL Erlenmeyer flask. Repeat the iodimetric titration until you have three good measurements (again to 0.1 mL). Use the Q-test to check for bad data.
5. Titration of Real Lemon.
Add 10.0 mL of Real Lemon into a 125 mL Erlenmeyer flask. Repeat the iodimetric titration until you have three good measurements (again to 0.1 mL).