Chemistry Iodometric Determination of Vitamin C

Transcription

1 Chemistry 3200 Triiodide, I 3, is a mild oxidizing agent that is widely used in oxidation/reduction titrations. Triiodide is prepared by combining potassium iodide, KI, and potassium iodate, KI 3, in acidic solution according to the following stoichiometry: I I + 6 H + 3 I H 2 (1) In preparing triiodide, excess KI is used, so the concentration of I 3 - is determined by the amount of KI 3 added to the solution. Triiodide reacts with ascorbic acid (vitamin C, a mild reducing agent) to form dehydroascorbate and three iodide ions according to the reaction: H H - H 2 + I I - + 2H + H H H H (2) Notice that one mole of iodine is consumed for each mole of ascorbic acid. In this experiment, you will determine the amount of ascorbic acid in a vitamin pill using the triiodide reaction in a back titration. After extracting the ascorbic acid from vitamin pills with acid, you will convert it to dehydroascorbate using a known excess of triiodide. The amount of triiodide remaining after reaction 2 will be determined by titration of the triiodide with a standardized thiosulfate solution. Note that you do not titrate the analyte directly, but rather titrate an added reagent after excess has been added. This is known as a back titration. The back titration reaction is I S I + S (3) Note that 2 moles of thiosulfate are consumed for each mole of triiodide present. The endpoint is determined using a starch indicator. Mixtures of starch and triiodide have a deep violet color, but when the triiodide is consumed the solution becomes colorless. ver time the starch-triiodide complex can stabilize, and it becomes difficult to reduce all of the triiodide. Therefore it is preferable to add the starch just before the endpoint. Fortunately the triiodide solution itself has a yellow-to-brown color, depending on concentration. When the solution turns pale yellow, you know that most of the triiodide has been consumed, and you are near the endpoint. Then you can add the starch indicator. You know how much I 3 - is added to the vitamin sample, and with the titration results you can determine how much is left after the oxidation of ascorbate. The difference between these is the amount of triiodide consumed in the oxidation of ascorbate, which is related to the amount of vitamin C present in the sample by the stoichiometry of reaction 2.

2 Procedure Preparation of Reagents 1. Starch indicator will be provided 2. Solid potassium iodide will be available M H 2 S 4 will be available 4. ~0.04 M Sodium thiosulfate solution will be provided. You should be able to complete the experiment with 250 ml of this solution. 5. Preparation of 0.01M KI 3 Solution: a. Accurately weigh approximately g of solid reagent and record the mass to 4 decimal places. b. Deliver the KI 3 to a 250 ml volumetric flask and add 100 ml of DI water. Swirl to dissolve, then dilute to volume. c. Compute the molarity of the solution. (FW = ) This solution is used for both steps; do not do more than 4 titrations without consulting with your TA. Standardization of the Thiosulfate Solution 1. Collect 250 ml of the thiosulfate solution. You must use the same solution for the entire experiment. 2. Pipet ml of the KI 3 solution into each of 3 Erlenmeyer flasks. Add 1 g of KI and 20 ml of 0.3 M H 2 S 4 to each flask. 3. Calculate the volume of titrant (thiosulfate) required for titrations by using the calculated KI 3 concentration and assuming that the thiosulfate concentration is 0.04 M. This gives the approximate endpoint. 4. Titrate the triiodide with the thiosulfate solution until the brown solution becomes pale yellow. Then add 2 ml of the starch indicator solution and titrate until the violet color of the starch iodine complex just disappears. This is the endpoint. 5. Repeat this procedure for a total of three precise titrations. D NT do more than four titrations without consulting with your TA.

3 Analysis of the Vitamin C 1. Weigh a sufficient number of vitamin tablets so that approximately 500 mg of ascorbic acid is obtained (normally one tablet your TA may tell you how many tablets to use). 2. Grind the tablets with a mortar and pestle. Transfer the Vitamin C powder to a weigh boat. Record the mass of the powder and the weigh boat. Then transfer the powder to a 250 ml volumetric flask. Re-weigh the weigh boat (empty weigh boat with some powder residue) again. By subtracting the masses before and after transferring the powder, it gives you the accurate mass of Vitamin C powder that is actually transferred and analyzed. 3. Add 100 ml of 0.3 M H 2 S 4. Swirl the flask for about 10 minutes, and then let it stand for several minutes. Swirl again, and then dilute to the mark with 0.3 M H 2 S 4. Because of the fillers and binders used in vitamin tablets, your solution may be cloudy this is normal. 4. Deliver ml of the vitamin C solution to an Erlenmeyer flask. 5. Add 1 g of KI and ml of standardized KI 3 to the flask. Titrate the remaining triiodide with the standardized thiosulfate solution as above, taking care to add the starch solution just before the end point. 6. Repeat this titration twice for a total of three precise determinations. 7. Calculate the average mass of vitamin C in each tablet and the uncertainty in the determination, as indicated on the report sheet.

4 Student Name: Chemistry 3200 Date: Lab Instructor: Section: Preparation of KI3 Mass of KI3: Molarity of KI3 solution: Standardization of Na2S23 Volume of thiosulfate solution used for standardization: Average volume of titrant: Molarity of thiosulfate solution: Analysis of Vitamin C Mass of the Vitamin C tablet(s): Mass of Vitamin C powder in volumetric flask: Volume titrant used in each determination: Average volume of titrant used: Moles of ascorbic acid titrated: Grams of ascorbic acid titrated: Mass percent of ascorbic acid in pill: Mass of ascorbic acid in pill:

5 Student Name: Calculation for the standardization of the thiosulfate: Calculation for the mass of ascorbic acid in pill:

6 Student Name: Calculation for error analysis (Include a list of the errors and their sources):