Determination Of Vitamin C

Determination of Vitamin C

Haylee Stiffler SCH 121-01 1/29/2018

Abstract: The amount of Vitamin C, also known as ascorbic acid, was found in both a known sample and an unknown drink sample using titration, which was based on the oxidization of Vitamin C by Iodine. A starch indicator was used to determine when the iodine had reacted completely with the Vitamin C.

Introduction: The main objective in this experiment was to determine how much Vitamin C was in an unknown drink sample as well as a known sample through titration. Vitamin C is a coenzyme in the human body that is used to deliver hydride ions. The recommended daily allowance of Vitamin C is 60 mg. Vitamin C deficiency results in scurvy, which weakens the muscles. Having an excess of Vitamin C causes kidney stones. Titration is a quantitative technique used to determine concentration. In order to perform a titration, a buret must be filled with a titrant (iodine solution) and placed above a titrane (known vitamin C sample or drink sample), which includes an indicator solution. The indicator solution used in this experiment was starch, which caused the Vitamin C solution to turn blue when the excess iodine reacted with the starch in the Vitamin C solution. The titration in this experiment was based on the oxidation of vitamin C by elemental Iodine, shown below. IO3-(aq) + I2(aq)  C6H6O6(aq) + 2H+(aq) + 2I-(aq)

Due to iodine being difficult to weigh on its own, it was created in a solution by mixing potassium iodate and potassium iodide in an acidic solution, as seen below. This experiment required accurate measurements in order to make sure the calculations would be correct. In order to have the measurements be as accurate as accurate as possible, a volumetric pipette was used throughout. IO3-(aq) + 5I-(aq) + 6H+(aq)  3I2(aq) + 3H2O(l)

Experimental: 0.01 M Potassium Iodate (50 mL) 10% potassium iodide (20 mL), and 0.03M sulfuric acid (25 mL) were all transferred into a 100-mL volumetric flask using a volumetric pipette. The Volumetric flask was then filled to the mark with distilled water, before being covered with Parafilm and inverted, allowing the contents to mix. After observations of Vitamin World brand Vitamin C were made, it was transferred to a tared piece of weighing paper, which was then placed on the scale. After the mass was recorded as 0.26 g, the Vitamin C was transferred into a clean 50-mL volumetric flask, using a funnel to prevent spillage. The 50-mL volumetric flask was the filled to the mark with distilled water, covered with Parafilm, and inverted, allowing the Vitamin C to mix with the distilled water. Vitamin C solution (10 mL) was then measured with a volumetric pipette, and was transferred into a clean 250-mL Erlenmeyer flask. 0.3 M H2SO4 (5 mL) and 1% starch solution (2 mL) were then added to the 250-mL Erlenmeyer flask containing the Vitamin C solution. The flask was then swirled to allow the solution to mix. The buret was then conditioned with the iodine solution prepared previously. Iodine was then poured into the buret and the initial volume was measured and recorded. The iodine solution was then used to titrate the solution. The iodine

solution was added dropwise until the solution turned from clean to a reddish brown, signaling the end point. The final buret reading was then recorded. Observations were recorded about the Welch’s White Grape juice, including the amount of Vitamin C found on the label. The drink sample (50 mL) was measured with a volumetric pipette and placed into a clean 250-mLErlenmeyer flask. 0.3 M H2SO4 (10 mL) and 1% starch solution (2 mL) were then added to the 250-mL Erlenmeyer flask containing the drink sample solution. Iodine was then poured into the buret, and the initial volume was recorded. The iodine solution was then used to titrate the drink sample solution, by adding the iodine solution dropwise until the drink sample solution turned from transparent yellow in color to dark blue, showing that the end point had been reached. The final buret reading was the recorded.

Data/Results: Table 1: Known Vitamin C sample observations and mass used Brand

Vitamin World

Observations

White, crystal appearance, glittery

Amount of Vitamin C per serving

5000 mg/5 g

Mass of vitamin C used

0.26 g

Table 2: Drink Sample Observations and volume used Drink Type

Welch’s White Grape juice

Observations

Transparent yellow appearance

Amount of Vitamin C per serving

60 mg/295mL

Volume used

50 mL

Table 3: Titration of Known Vitamin C Sample Volume of 0.3M

Initial Buret

Final Buret

Actual Volume

H2SO4

Reading

Reading

of Iodine

Trail 1

4.90 mL

6.40 mL

32.70 mL

26.30 mL

Trial

5.00 mL

12.4 mL

30.00 mL

17.70 mL

Table 4: Titration of Drink Sample with Iodine Volume of 0.3M

Initial Buret

Final Buret

Actual Volume

H2SO4 and drink

Reading

Reading

of Iodine

Trial 1

9.90 mL

21.40 mL

31.30 mL

9.90 mL

Trail 2

9.80 mL

20.50 mL

29.90 mL

9.40 mL

Known Vitamin C sample trial calculations: 1. Expected Vitamin C concentration in the 50-mL flask: 200g x (5000mg/5g) = (1300mg vitamin C/50mL) = 5.2mL 2. Expected amount of Vitamin C in the Erlenmeyer flask: (5.2mg/1mL) x 10mL = 52mg Vitamin C 3. Amount of Vitamin C found in the titrated sample: (0.01 mol IO3-/1L) x 0.05L = 0.0005 mol IO30.0005 mol IO3- x (3 mol I2/1 mol IO3-) = 0.0015 mol I2 (0.0015 mol I2/0.1 L) = 0.015 mol/L I2 Trial 1: 0.0263 L I2 x (0.015 mol I2/1L) = 0.000395 mol I2

0.000395 mol I2 x (176.1 g I2/1 mol I2) x (1000mg/1g) = 69.5 mg Vitamin C Trial 2: 17.7 mL x (1L/1000mL) x (0.015 mol I2/1L) x (176.1g/1mol I2) x (1000 mg/1g) = 46.7 mg Vitamin C

Drink sample trial calculations: 1. Expected Vitamin C concentration in drink sample: 50mL x (60mg/295mL) = 10.17mg Vitamin C 2. Expected amount of Vitamin C in Erlenmeyer flask: 10.17 mg Vitamin C 3. Amount of Vitamin C found in the titrated sample: Trial 1: 9.9 mL x (1L/1000mL) x (0.015 mol I2/1L) = 0.00149 mol I2 0.00149 mol I2 x (176.1g/1mol I2) x (1000mg/1g) = 26.62 mg Vitamin C Trial 2: 0.00940 L x (0.015 mol I2/1L) = 0.00141 mol I2 0.00141 mol I2 x (176.1g/1mol I2) x (1000mg/1g) = 24.3 mg Vitamin C

Comparison of results: 1. Average amount of Vitamin C found in the known sample trials: (69.5 mg + 46.8 mg)/2 = 58.15 mg Vitamin C 2. Calculate the % error of your results: 58.15 mg – 52 mg/ 58.15 mg x 100 = 10.58% 3. Calculate the average amount of vitamin C found in the drink sample trials: (26.2 mg + 24.8 mg)/2 = 25.5 mg Vitamin C 4. Calculate the % error of your results

25.5 mg – 10.17 mg/ 25.5 mg x 100 = 60.1%

Discussion: The main objective of this experiment was to find out how much Vitamin C was in a drink sample, which we did successfully. Both my partner and I used the same drink sample, and found the average Vitamin C content to be 25.5 mg, which is higher than 10.17 mg, our expected Vitamin C content. A possible source of error would be other parts of the drink reacting to the iodine, causing the color change to occur sooner than it would have without those other chemicals. In order to avoid this for future experiments, we could try to find out what else was reacting with the iodine, and attempt to remove what that was. We could also potentially do this experiment with a drink that wouldn’t contain anything else that might react, allowing our results to be more accurate. Another potential source of error could be allowing too much of the iodine solution to come into contact with the vitamin C mixtures, which caused our known sample to turn a reddish-brown color instead of the light blue it should been. This could be avoided in the future by making sure we’re swirling the mixture enough while the iodine solution is being added dropwise. While we did use a volumetric pipette for all exact measurements, it’s possible that there was human error that caused our measurements to be slightly off, which could be avoided by being as precise as possible. Our average Vitamin C content in our unknown drink sample ended up being 25.5 mg, which is just under half of the recommended daily allowance of 60 mg. One bottle of our sample was 295 mL. In order to consume 60 mg of Vitamin C, you would need to have 2.35 bottles, or 693.25 mL, of our sample.