EXPERIMENT 13. Vitamin C

Transcription

1 EXPERIMENT 13 Vitamin C INTRDUCTIN Vitamin C is an essential component of the diet for humans and some animals because we lack the enzymes to make this substance from its natural precursor, glucose. A deficiency of vitamin C leads to a disease known as scurvy, characterized by hemorrhages throughout the body, which are especially noticeable on the gums and around the mouth and other areas of the skin with abundant vascularization (blood vessels). It is essential in the synthesis of the protein collagen, which connects cells in the body, especially in the blood vessels. The disease scurvy was described as early as 1500 BC. In the winter of 1535, a group of 110 French explorers with Jacques Cartier were stranded along the St. Lawrence River and became afflicted with scurvy. Several of the men died and many were totally incapacitated by the disease before the Native Americans came to their rescue and recommended a concoction from a local evergreen tree that cured the survivors. The common name for the tree is arborvitae, or tree of life. A surgeon named Lind performed a scientific investigation of the causes of scurvy among British sailors in He found that sailors given citrus fruits regularly did not develop scurvy, while sailors receiving various other dietary supplements did. He concluded there was some "antiscorbutic" factor in the citrus fruits that prevented the disease. The chemical structure for the active antiscurvy factor was not determined until the early 1930's and was named ascorbic acid because it was an acidic compound with ascorbutic (antiscurvy) properties. Ascorbic acid is a chiral compound, meaning it has "handedness". The natural vitamin is L-ascorbic acid, meaning it has a "levo" or left handed structure, as opposed to D-ascorbic acid, which has a "dextro" or right handed structure. The recommended daily (or dietary) allowance (RDA) for vitamin C is 60 mg for adults, and higher for pregnant and lactating women. This is an amount that is sufficient to prevent scurvy, although there is a lot of controversy over how much vitamin C is needed for a healthy individual. Some people claim that large doses of vitamin C will help to ward off colds, although there is not complete agreement on this, primarily because individuals usually respond differently to various drugs. 53

2 When vitamins are used in megadoses, they should be considered drugs and not everyone will respond the same way. In this laboratory you will measure the amount of vitamin C in various beverages. Some beverages will be supplied, but each student is encouraged to bring in a sample of his or her choosing to analyze for its vitamin C content. You will use a titration method for the determination, similar to the titrations you did for antacid tablets. In this case you will use iodine solution as the titrant (in the buret), which reacts with ascorbic acid to produce iodide ion and dehydroascorbic acid. The ascorbic acid is readily oxidized by oxygen in the air under neutral or alkaline conditions, so you will be adding acetic acid to keep the medium acidic. Starch will be added to the sample, so when the ascorbic acid has completely reacted with iodine, any excess iodine added will form a deep blue color with the starch that is present. This color formation between starch and iodine was demonstrated in the CH 2 H CH 2 H previous laboratory with CHH carbohydrates. H CHH + I2 + 2 I- H MATERIALS NEEDED Three 125 ml Ascorbic Acid Dehydroascorbic Acid Erlenmeyer flasks, 10 ml pipet, 50 ml buret, ascorbic acid standard solution (1.00 mg/ml), 1% starch solution, 6 M acetic acid solution, iodine solution, buret, fruit juices or other samples to test for vitamin C. PRCEDURE Add 25 ml deionized water and 2 ml of 6 M acetic acid to each of three 125 ml Erlenmeyer flasks. Using a pipet, add exactly 10.0 ml of 1.0 mg/ml ascorbic acid standard solution to each flask and 2 ml of 1% starch solution to each. Set up a 50 ml buret and fill it with the iodine solution (see Titration Apparatus, Appendix I). Turn the stopcock of the buret to fill the tip before you start the titration, making sure you have no air bubbles in the tip. Record the initial level of the iodine solution in the buret. Now slowly add the iodine solution (several drops at a time) to the ascorbic acid standard 54

3 solution in the Erlenmeyer flask. You will see a deep blue color appear that slowly disappears as you swirl the solution. Continue adding the iodine solution a little at a time and swirl the flask after each addition until the blue color persists. As you get closer to the endpoint, it will take longer for the blue color to disappear. That means you should try to add smaller and smaller amounts of the iodine solution between swirling. When you have added enough, so the blue color remains for more than 30 seconds, record the final level of iodine solution in the buret. Subtract the initial level from the final level to determine the volume of iodine solution added. Repeat the procedure with each of the other two flasks containing the ascorbic acid standard solutions, making sure you have enough iodine solution in the buret before starting each titration. Be sure to record the initial and final level of iodine solution in the buret before each titration. Use the average amount of iodine solution needed for the titrations in your calculations later. How much error did you have in your measurements? Add 25 ml deionized water and 2 ml of 6 M acetic acid to a clean 125 ml Erlenmeyer flask. Using a pipet, add exactly 10.0 ml of your juice sample to the flask and about 2 or 3 ml of 1% starch solution. Titrate the sample in the same way that you did the ascorbic acid standard, making sure there is enough iodine solution in the buret for each titration. The final volume for the previous titration will be the initial volume for this titration unless you opened the stopcock of the buret between titrations. Be careful not to allow the level of the iodine solution to drop below the graduation marks on the buret. When you reach the endpoint for the sample, record the final volume in the buret and determine the volume of iodine solution added as you did above for the ascorbic acid standard. To calculate the amount of vitamin C in your sample you will compare the amount of iodine used to titrate the sample with the amount used to titrate the standard. Since the standard solution contains 1 mg ascorbic acid per ml, the calculation is as follows: Vol I 2 soln used for sample (ml) Vit C conc (mg/ml) = )))))))))))))))))))))))))))))))))) Vol I 2 soln used for standard (ml) To determine the total amount of vitamin C in your sample, multiply the concentration (mg/ml) by the total volume of sample, e.g., 12 oz = 375 ml, so multiply this number by

4 ml for a 12 oz sample. Repeat this procedure for other samples. You should analyze at least 2 different juice or beverage samples. You may also test vitamin C supplement tablets from home by dissolving the tablets in 90 ml of water and 10 ml of 6 M acetic acid. Crush the tablet into a powder before dissolving it. After it has dissolved, add enough of this solution to give about 20 mg of vitamin C in the titration flask, along with 25 ml deionized water, 2 ml of 6 M acetic acid and 2 ml of 1% starch solution. Check with the instructor to make sure your calculations are correct for the tablet you are using. YUR DATA SHEET SHULD LK LIKE THIS Ascorbic Acid Standard (1.0 mg/ml): Flask 1 Flask 2 Flask 3 Final level of iodine solution in buret (ml): Initial level of iodine solution in buret (ml): Total volume of iodine used (ml): Average: ml Juice Samples: Final level of iodine solution in buret (ml): Initial level of iodine solution in buret (ml): Total volume of iodine used (ml): Average: ml Questions to answer in your notebook: 1. What percentage of the RDA for vitamin C does each test sample provide in a normal serving? How large is a normal serving? 2. In the previous experiment you used an iodine solution to test for the presence of starch. What is the purpose of adding starch to the ascorbic acid solution prior to titrating it with the iodine solution in this experiment? 56

5 3.* What is the purpose of titrating the ascorbic acid standard solution prior to titrating the juice samples? Could you obtain the amount of vitamin C in the juice sample without first titrating the ascorbic acid standard solution? 57