ANALYSIS OF VITAMIN C

Transcription

1 Purpose To learn how to analyze food for vitamin C content and to examine various sources for vitamin C content. Caution Handle the glassware with caution to prevent breakage. When using a burner in the laboratory, be absolutely sure that loose hair and clothing do not come close to the flame. It is easy to forget this simple rule when concentrating on measurements. The solutions used in this experiment are not particularly caustic to the skin if washed off immediately, but the KIO 3 and HCl can both damage eye tissue. Wear your goggles at all times. Be sure the gas is turned off completely when you are through with it since in a closed laboratory a small gas leak can create an explosive mixture. Even though the substances being analyzed are foodstuffs, do not taste or eat any of the materials as they may become contaminated with undesired chemicals. Introduction Vitamins are complex organic molecules required in small amounts by the body in order to maintain health and well being. Generally, the daily requirements of the various vitamins are very small quantities, but whenever these small quantities are not available, the body cannot function properly. For example, if a person s diet does not include a sufficient amount of vitamin C (the Food and Drug Administration recommends around 50 milligrams per day per person) a disease called scurvy results, which causes sores to develop in and around the mouth and gums. If enough vitamin A is not available, bones will not grow properly (among other problems) and this will lead to a malady called rickets. Vitamin C is also known as ascorbic acid, and has the structure shown below: In this experiment you will learn to analyze a sample of material for vitamin C content. The analysis makes use of the reducing qualities of the vitamin C. Both iodide (I ) ions and iodate (IO 3 ) ions are added to a solution containing the vitamin C. The iodide and iodate ions react with each other to form elemental iodine (I 2 ) (Equation 1). The ascorbic acid (vitamin C, C 6 H 8 O 6 ) quickly reacts to reduce I 2 to I, destroying the vitamin C in the process (Equation 2). When all of the vitamin C has reacted, the next tiny amount of I 2 formed reacts with a small amount of starch, which has been added as an indicator, to Revision SP11 Page 1 of 7

2 produce a deep blue color (Equation 3). The iodate (as KIO 3 solution) is added with a buret, and the blue color of the resulting starch iodine product signals the endpoint of the titration. IO I + 6 H + 3 I H 2 O (1) I 2 + C 6 H 8 O 6 C 6 H 8 O 6 * + 2 H I (2) I 2 + starch (I 2. starch) (deep blue color) (3) * oxidized product of vitamin C Equipment: 250 ml conical flask 400 ml beaker 100 ml beaker 50 ml buret with stand glass stirring rod analytical balance balance mortar and pestle 500 mg vitamin C tablets 5.00 x 10 2 M KIO 3 solution 0.60 M KI solution 1.0 M HCl solution starch indicator solution (freshly prepared) natural vitamin C source(s) cheesecloth Procedure Numbers in parentheses refer to entry numbers on the data sheet. PART A: ANALYSIS OF COMMERICAL VITAMIN C TABLETS Weigh a small piece of paper on an analytical balance (1). Then place a 500 mg vitamin C tablet on the paper and weigh the two together (2). What does the tablet actually weigh (3)? Dissolve the tablet in about 150 ml of distilled water using a 250 ml conical flask. Crush the tablet carefully with a stirring rod to help it dissolve faster. Note that some insoluble material remains from the tablet. How does this relate to the weight of the tablet noted above? Add to the flask 5 ml of 1.0 M HCL (this speeds up the reaction between the I and the IO 3 ), 10 ml of 0.60 M KI solution, and 2 drops of freshly prepared starch indicator solution. Clean, rinse, and fill a buret with 5.0 x 10 2 M KIO 3 solution and take an initial buret reading (4). Carefully add KIO 3 solution from the buret until the solution takes on a permanent bluish purple color. Record the final buret reading (5). What is the volume of the KIO 3 solution required for the titration (6)? Calculate the titre value of the tablet (the titre value for a substance is the volume of titrant divided by Revision SP11 Page 2 of 7

3 the weight of the sample) (7). For example, if your tablet weighed g and required ml of KIO 3 solution for titration, the tire value would be calculated as shown below: ml/ g = ml/g The tire value can be used as a measure of purity or for comparing the amount of vitamin C in different sources. Repeat the analysis with a second tablet, and compare your titre values (8 14). Weigh a third tablet and dissolve it in distilled water as before. Boil the solution for 15 minutes; cool it to room temperature; add the KI, HCl, and starch as before. Titrate to the blue endpoint (15 21). Is vitamin C affected appreciably by cooking (22)? PART B: ANALYSIS OF NATURAL SOURCES OF VITAMIN C Instructions are given below for analyzing several natural products for vitamin content. Your instructor will assign particular products to be analyzed (23 and 29). (Different students may analyze different products so that more foods may be examined by the class.) Be careful with these titrations. Only small amounts of titrant will be needed in each case. 1. Citrus juices: Most citrus juices contain pulp from the fruit, which makes the endpoint hard to see. Remove the pulp before using the juice by straining the juice through cheese cloth. Then measure out 100 ml of the juice (24), add 50 ml of distilled water, 5 ml of 1.0 M HCl, 10 ml of KI solution, and 4 drops of starch. Titrate carefully as before (25 and 26). Remember, the color of the solution will be a composite of the juice color and the color of the indicator. The endpoint will be indicated by a color change, but not necessarily to purple. Juices one can use (and compare) include fresh and frozen orange juice, reconstituted lemon juice, vitamin C enriched fruit drinks such as Hi C or Tang, grapefruit juice, etc. 2. Fruits and Vegetables: Weigh out about 100 g of fruit or vegetable (30). Crush with a mortar and pestle to break down the cellular structure, then wash juice and pulp into a 400 ml beaker. Fill to the 150 ml mark and boil for 15 minutes. Cool, strain through cheese cloth, and analyze in the fashion as for the fruit juices (31 and 32). One can determine a value for the vitamin C content of the foodstuffs as follows. Assuming the tablets contained exactly 500 mg of vitamin C (The Food and Drug Administration has something to say about this!), one can determine the amount of vitamin C in the sample of food titrated using a sample ratio Revision SP11 Page 3 of 7

4 method. The volume of titrant needed for the food sample should be in the same ratio to the vitamin C in the food as the volume needed for the tablet is to the vitamin C in the tablet. EXAMPLE CALCULATION: 500 mg vitamin C =? mg vitamin C vol. titrant need for tablet vol. titrant needed for food Suppose your 500 mg tablet took ml of titrant, and the sample of food required 3.45 ml of titrant. The vitamin C in the food sample would be 500 mg =? mg (food) ml 3.45 ml? mg (food) = (500 mg) x (3.45 ml) = 76.8 mg (in food) ml Determine the vitamin C content of each of the foods analyzed (28 and 34). Waste Disposal and Cleanup: Dispose of solutions according to the instructions of your lab instructor. Clean all glassware thoroughly and return to their original locations. References for Images Accessed January 12, content/uploads/2009/07/vitamin c1.png, Accessed January 12, 2011 Revision SP11 Page 4 of 7

5 DATA SHEET Name: Partner: PART A Tablet 1 Tablet 2 Tablet 3(boiled) Weight of paper (g) Paper + tablet (g) Weight of tablet (g) Initial buret reading (ml) Final buret reading (ml) Volume of KIO 3 (ml) Titre value PART B Natural Product 1 Natural Product 2 (citrus juice) (Fruit or vegetable) Product Volume (ml) or wt. (g) Initial buret reading (ml) Final buret reading (ml) Volume of KIO 3 (ml) Vitamin C content Revision SP11 Page 5 of 7

6 POST LAB QUESTIONS Name: Partner: 1. When weighing the vitamin C tablet in this experiment, students frequently find that it weighs more than the stated value. For example, a 500 mg tablet might weigh more than 500 mg. Explain. 2. Often when dissolving vitamin C tablets some of the tablet does not dissolve. Does this mean that some vitamin C cannot be dissolved or is there another explanation? Revision SP11 Page 6 of 7

7 PRE LAB ASSIGNMENT Name: 1. What precaution should be taken if any of the solutions in this experiment are spilled on your skin? 2. Why should you not taste any substance used in this experiment? 3. What substance actually reacts with the vitamin C in this reaction and what is the vitamin C converted into? 4. In this reaction, a titer value is determined. What is the titer value? Revision SP11 Page 7 of 7