BALLENTINE'S ASSAY OF ASCORBIC ACID (A Titrimetric Redox Analysis of Commercial Vitamin C)

Transcription

1 Chemistry 135 For this experiment: BALLENTINE'S ASSAY F ASCRBIC ACID (A Titrimetric Redox Analysis of Commercial Vitamin C) 1. Complete the Prelab and obtain a stamp before you begin the experiment. Clark College 2. Write your lab notebook prelab and get it initialed/signed before you begin the experiment. You should include the pertinent chemical/redox equations. 3. Weigh out 10 ascorbic acid tablets to obtain the mass of one tablet. 4. You may grind more than one tablet for the experiment, based on your required amount needed (from the prelab). 5. Titrate 4 samples of the Vitamin C solution with the prepared KI 3 solution. You may do N MRE than 5 titrations total. Perform 4 titrations, and then do your calculations before determining if you need to do a 5 th. 6. Your titrations must all be done on the same day. 7. Determine the %Vitamin C in the tablet for the four titrations, and determine the Average % Vitamin C, the standard deviation and the RSD. The RSD must be less than or equal to Enter your % Vitamin C data into the computer in the front of the lab. Introduction Turn in only the Data Report Sheet, your Notebook Sheets and the Stamped Prelab!! Ascorbic acid (Vitamin C) is readily oxidized to dehydroxyascorbic acid: Ascorbic Acid MW = g/mol 2 e Dehydroxyascorbic Acid MW = g/mol (Eq. 1) This reaction is reversible but dehydroxyascorbic acid can be further irreversibly oxidized to oxalic acid and 1-threonic acid. In the following analysis only partial oxidation according to Equation 1 is utilized. Even atmospheric oxygen causes slow oxidation of ascorbic acid so the sample must not be dried and solutions of ascorbic acid must be protected from oxidation by adding oxalic acid or metaphosphoric acid (P 3). In this analysis the ascorbic acid is titrated with standard potassium iodate (KI 3) in the presence of excess potassium iodide. The iodate ion oxidizes iodide ion to I 2 and is itself reduced to I 2 according to the reaction: I I I (Eq. 2) The ascorbic acid is quantitatively oxidized by the liberated iodine: + I 2 2 I (Eq. 3) Vitamin C/Ascorbic Acid Titration Revised Spring 2007 NF Page 1 of 6

2 The net reaction is: + I - 3 I (Eq. 4) When all of the ascorbic acid is consumed, the iodine reacts with a starch indicator to form a deep blue starch-iodine complex. Note that in the overall reaction, iodate is reduced to iodide, which is a gain of six electrons, so the equivalent weight of iodate is 1/6 of the formula weight. Each ascorbic acid molecule loses two electrons so the equivalent weight of ascorbic acid is 1/2 of the formula weight. Analysis of Vitamin C Tablets Pharmaceutical grade Vitamin C is generally compounded into tablets for ease of administering, although U.S.P. grade ascorbic acid crystals can be obtained. In compounding, the ascorbic acid is usually mixed with various resins or gels to form into granules for ease in blending. Fillers, such as dextrose, lactose and sucrose, are added to make the tablet a convenient size. Binders, such as acacia gum, are added to reduce the tendency of the tablet to crumble into powder, and finally, lubricants, such as magnesium or zinc stearate and talcum powder, are added to aid the blending of the components into an homogeneous powder before pressing into a pellet. The assay of pharmaceutical Vitamin C tablets will depend upon the amounts of the various components added in the compounding. For example, a nominal "100-mg" tablet may actually weigh about 400 mg (corresponding to a 25% assay) and a "250-mg" tablet may weigh about 500 mg (corresponding to a 50% assay). Your laboratory instructor may inform you of the approximate assay to expect or you can calculate it based on the mass of your tablets and the mass of Vitamin C listed on the bottle. Because of the gels used in granulating the ascorbic acid, and the gums as binders, Vitamin C tablets do not dissolve readily, and the tablets must be powdered before analysis. nce powdered, the ascorbic acid dissolves very readily; the solution may remain cloudy if insoluble fillers and lubricants have been used, but the ascorbic acid will all be in solution. The binders and fillers will typically settle to the bottom of the solution, and can be avoided. Vitamin C/Ascorbic Acid Titration Revised Spring 2007 NF Page 2 of 6

3 PRCEDURE: Note: The ascorbic acid must be titrated within a few hours after the solution has been prepared. 1. Calculate the amount of powdered vitamin C needed for a 35 ml titration with 0.01 N KI 3 (you have already done this in the prelab). See the paragraphs above and the example calculations at the end of the experiment. If you need to use more than one tablet to get enough ascorbic acid for the titration, you may do so! 2. Now that you know how many tablets you need, grind your Vitamin C tablet(s) with a clean mortar and pestle. You may use weighing paper for this experiment to weigh out your sample to ±0.1 mg. Transfer the sample to a clean 100-ml volumetric flask, and half-fill the flask with distilled water. 3. With a graduated cylinder, add 10.0 ml of 2.0% oxalic acid to the volumetric flask. Shake the solution occasionally (every minute) for about 30 minutes until all the powder dissolves except for the residual binder and lubricants. This will ensure that all of the ascorbic acid will dissolve from the sample. 4. Fill the flask to the mark with distilled water, rinsing down the neck and stopper, to form a solution of ascorbic acid in 0.20% oxalic acid. Mix thoroughly. Allow the insoluble material to settle for about 30 minutes. Some binder may remain floating on the surface. 5. While the ascorbic acid solution is settling, prepare a standard ~0.01 N potassium iodate solution by pipeting ml of a standard N KI 3 solution into a clean 250-ml volumetric flask. Do not pipet directly from the reagent bottle! Add distilled water, swirl the solution thoroughly, and dilute to the mark with additional distilled water to make the solution. Calculate the exact normality of your standard potassium iodate solution to ppth precision. 6. Pipet ml aliquots of the clear, upper portion of the prepared ascorbic acid solution into each of four 250-ml Erlenmeyer flasks. Add 25 ml of distilled water, 1 ml of 10% potassium iodide solution, 2 ml of 2 N sulfuric acid, and 2 ml of 1% starch indicator solution. The volumes of water, KI, 2S 4 and starch can be measured with a graduated cylinder, a marked plastic (beral) pipet, or with a dropper at 20 drops/ml. All reagents must be in each flask or the titration will not work! 7. Titrate the ascorbic acid solutions with your prepared, standard 0.01 N potassium iodate solution (from step 5). The end point is the appearance of the first permanent blue color due to the formation of the starch-iodine complex. 8. Calculate the assay (or percent) of the Vitamin C tablet and report the average assay with a relative error of 10 parts per thousand or less. In this experiment we are not using calibrated volumetric flasks and the oxidation of Vit.C will make our RSD less precise than in previous experiments. If the assay is 50% or greater, round to ±0.1% and if it is less than 50%, round to ±0.01%. NTES Sufficient mixing is the name of the game here. It will take about 30 minutes of near constant shaking and swirling to get the ascorbic acid dissolved away from binders and fillers. In addition, after the sample is diluted to the mark, you must up-end (flip it upside down and right-side up) the volumetric flask at least 25 times, at the absolute minimum, to ensure a homogeneous solution of ascorbic acid for analysis. Vitamin C/Ascorbic Acid Titration Revised Spring 2007 NF Page 3 of 6

4 Sample Calculations 1. Calculate the sample weight that would be required for a sample of approximately 25% assay. (Remember...35 ml titration, impure sample, 10 ml aliquot diluted to 100 ml) Equiv Wt. Ascorbic Acid = g mol x mol = g 2 equiv e - equiv 0.01 equiv KI 3 L Equiv KI 3 = Equiv Acid x L = X g tablet x 25 g Asc. Acid 100 g tablet x equiv g Asc. Acid x 10 ml sample 100 ml tablet soln Sample Weight = 1.2 g tablet! Note: The sig figs are incorrect, based on the values given. This is just an estimate! 2. Prepare a solution of KI 3 from a N primary standard solution, using a 10-mL volumetric pipet and a calibrated 250 ml volumetric flask. N 1 V 1 = N 2 V 2 ( N)(10.00 ml) = N 2 (250.0 ml) N 2 = N 3. A gram sample of ascorbic acid was dissolved in a ml calibrated volumetric flask. A 10.0 ml aliquot was taken with a calibrated pipet and required ml of a N KI 3 solution. Calculate the % assay of the sample. Start with the titration volume and normality of KI 3 to get equivalents of KI 3, which gets converted to equivalents of Ascorbic acid, and finally grams of ascorbic acid eq KI 3 L buret volume! x L x " " 1 eq Asc. A 1 eq KI 3 x g Asc. A equiv The diluted conc. Normality means everything is 1:1 of KI 3 (in the 250 ml vol flask) = g Asc. Acid The amount of ascorbic acid calculated here is the amount in the 10.0 ml aliquot (sample) that was directly titrated. We need to do a reverse dilution calculation to relate the amount of ascorbic acid in the 10.0 ml portion back to the amount in the original ml solution g Asc. Acid in aliquot x ml original solution 10.0 ml aliquot = g Asc. Acid in original solution Finally, this amount of ascorbic acid came from the amount of tablet weighed out at the beginning. To find the % assay of the sample, we simply find a mass percent: g asc. acid g tablet x 100% = 22.33% Vitamin C/Ascorbic Acid Titration Revised Spring 2007 NF Page 4 of 6

5 Chem 135 Vitamin C Analysis DATA REPRT SEET NAME Enter your data into spread sheet at the front of the lab. Don t forget to include the sample calcs! Normality of KI 3 (N) Mass of Vitamin Sample Titration: Volume of KI 3 % Vitamin C Average % Vitamin C Standard Deviation Class Average (Instructor will fill this in) ppth difference from Class Average (Instructor will fill this in) RSD Accuracy Score NTE: Circle Q-test/5ppth test rejected data. Attach Q-test and 5 ppth Test calculations on a separate sheet (lab notebook is fine). Calculation for Normality of KI 3: Sample Calculation of Ascorbic Acid Assay: Vitamin C/Ascorbic Acid Titration Revised Spring 2007 NF Page 5 of 6

6 Chem 135 Vitamin C Analysis PRELAB 1. Determine the sample weight needed for a 35-mL titration with 0.01 N KI 3. For this calculation, you may approximate the % assay at 75% Sample Weight = 2. What is the purpose of: a. The oxalic acid? b. The sulfuric acid? 3. A gram sample of ascorbic acid tablet was dissolved in a ml calibrated volumetric flask. A ml aliquot was taken with a calibrated pipet and required ml of a N KI 3 solution. Calculate the % assay of the sample, and report your answer to ppth precision. % Assay = Vitamin C/Ascorbic Acid Titration Revised Spring 2007 NF Page 6 of 6