Experiment 5: Iodimetric Titration of Vitamin C
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EJ Habina Quantitative Anal. Lab MW 2:00 Experiment 5: Iodimetric Determination of Vitamin C Purpose: The purpose of this experiment is to determine the manufacturer’s claim for the specifications of a source of Vitamin C are correct or not. We will determine the validity of the claims through determining the content of ascorbic acid (vitamin C) in an unknown sample and compare that to the known content in a supplemental source. Procedure: 1. Standardization of Thiosulfate Solution: a. As a group, a starch indicator as well as 0.07 M sodium thiosulfate (approx. 8.7 g in 500 mL boiled water w/ 0.05 g sodium carbonate) solution. Another solution of 0.01 M KIO3 was made using 1 g of solid reagent and dissolving in 500 mL flask. The thiosulfate was then standardized by pipetting 50 mL of KIO3 into an Erlenmeyer with 2 g solid KI and 10 mL of 0.5 M sulfuric acid and titrated until solution lost all of its color. 2 mL of starch indicator was then added to each and the titration is repeated. 2. Analysis of Vitamin C: a. A sample of ascorbic acid required for a 30 mL titration is weighed and then dissolved into 50 mL of 0.3 M H2SO4. 2 g of KI and 50 mL of standard KIO3 is added and the sample was then titrated as before until endpoint. This procedure was then repeated again for a vitamin C tablet. Reactions: πΌπ3− + 8 πΌ + 6 π» + → 3 πΌ3− + 3 π»2 π πΌ3− + 2 π2 π3−2 → 3 πΌ − + π4 π6−2 πΆ6 π»8 π6 + πΌ3− + π»2 π → πΆ6 π»8 π7 + 3 πΌ − + 2 π» + Data: Mass of KIO3 = 1.0116 g Mass of Thiosulfate = 8.7049 g Mass of Na2CO3 = 0.0548 g Standardization of Thiosulfate Solution: Trial # 1 2 3 KI (g) 2.0290 2.0022 2.0101 Thiosulfate Added (mL) 36.50 40.31 39.75 Average Molarity of Thiosulfate Solution 0.0777 0.0703 0.0714 0.07313 Analysis of Vitamin C: Ascorbic Acid w/ Sulf. Acid Trial # Ascorbic Acid (g) 1 KI (g) 0.1871 Thiosulfate Added (mL) Wt. % of Ascorbic Acid 2.0052 12.40 90.85% KI (g) 2.0900 2.0220 2.0042 Thiosulfate Added (mL) 14.8 14.8 14.9 Wt. % of Vitamin C 82.32% 82.32% 82.32% KIO3 & KI w/ Thiosulfate Trial # 1 2 3 Vitamin C (g) 0.1878 0.1875 0.1874 Calculations: Molarity of KIO3: πππ π πΎπΌπ3 × Ex: 1.0116 π πΎπΌπ3 × 1 πππ πΎπΌπ3 1 × = π πΎπΌπ3 214.00 π 0.05 πΏ 1 πππ πΎπΌπ3 1 × = 0.009454 π πΎπΌπ3 214.00 π 0.05 πΏ Molarity of Sodium Thiosulfate: π πΎπΌπ3 × 0.05 πΏ × Ex: 0.009454 π × 0.05 πΏ × 3 πππ πΌ3 1 πππ πβππ. 1π × × = π πβπππ π’ππππ‘π 1 πππ πΌπ3 1 πππ πΌ3 πβππ. π΄ππππ 3 πππ πΌ3 1 πππ πβππ. 1π × × = 0.0777 π πβπππ π’ππππ‘π 1 πππ πΌπ3 1 πππ πΌ3 0.0365 πΏ Amount of Ascorbic Acid Required: 1 πππ πΌ3− 1 πππ π΄π ππππππ π΄πππ 176.12592 π π πβπππ π’ππππ‘π × 0.03 πΏ × × × − 2 πππ πβππ. 1 ππππ πΌ3 1 πππ π΄π ππππππ π΄πππ = π π΄π ππππππ π΄πππ Ex: 1 πππ πΌ3− 1 πππ π΄π ππππππ π΄πππ 176.12592 π × × − 2 πππ πβππ. 1 ππππ πΌ3 1 πππ π΄π ππππππ π΄πππ = 0.18757 π π΄π ππππππ π΄πππ 0.07313 π × 0.03 πΏ × Weight % of Ascorbic Acid (also for Vitamin C): πΏ πβππ. π΄ππππ π πβππ. 1 πππ π2 π3 1 πππ πΌ3 × × × = πππππ πΌ3 1 1 1 πππ πβππ. 1 πππ π2 π3 π πΎπΌπ3 × 0.05 πΏ × 3 πππ πΌ3 = ππ₯πππ π πππππ ππ πΌ3 1 πππ πΎπΌπ3 ππ₯πππ π πππππ ππ πΌ3 − πππππ πΌ3 = πππππ πΌ3 πππππππππ πππππ πΌ3 πππππππππ × 1 πππ π΄π ππππππ π΄πππ 176.1256 π × = π π΄π ππππππ π΄πππ 1 πππ πΌ3 1 πππ π΄π ππππππ π΄πππ π ππ π΄π ππππππ π΄πππ × 100 = ππ‘. % πππ π ππ π πππππ Ex (using Trial 1 data): 0.0124 πΏ 0.07313 π 1 πππ π2 π3 1 πππ πΌ3 × × × = 0.000453 πππππ πΌ3 1 1 1 πππ πβππ. 1 πππ π2 π3 0.009454 π πΎπΌπ3 × 0.05 πΏ × 3 πππ πΌ3 = 0.001418 ππ₯πππ π πππππ ππ πΌ3 1 πππ πΎπΌπ3 0.001418 ππ₯πππ π πππππ ππ πΌ3 − 0.000453 πππππ πΌ3 = 0.000965 πππππ πΌ3 πππππππππ 1 πππ π΄π ππππππ π΄πππ 176.1256 π × 1 πππ πΌ3 1 πππ π΄π ππππππ π΄πππ = 0.16997 π π΄π ππππππ π΄πππ 0.000965 πππ πΌ3 × 0.16997 π ππ π΄π ππππππ π΄πππ × 100 = 90.85 % 0.1871 π Claimed Manufacturer Wt. %: 0.25 π‘πππππ‘ × πππ π ππ π‘πππππ‘ × 0.25 π‘πππππ‘ × 0.52 π × 1 × 100 = ππ‘. % 0.303 π 1 × 100 = 42.9 % 0.303 π Conclusion: The purpose for this experimental procedure was to determine the content of both commercial Vitamin C tablet as well as reagent grade ascorbic acid. After completing the procedure, the results for the amount of reagent grade ascorbic acid was calculated to be 90.85 %, which is a desired value due to the assumed purity of a reagent grade chemical. As for the accuracy of the commercial vitamin C tablet results, the value of 82.3 % from the data in this procedure was much higher than the calculate manufacturer claimed weight percent of 42.9 %. As for the particles that did not dissolve, they are what are known as fillers. These act as a supplemental substance in tablets so that the tablet can reach a desired size, weight, etc.
