University of Alberta General Chemistry Laboratories

Analysis of Vitamin C

Lab Summary

To compare the content of vitamin C (ascorbic acid) in commercial tablets and in Tang(c) with the content claimed by the manufacturer.

To determine the amount of other acids in Tang(c).

Outline of Experiment

1. The ascorbate part of the vitamin C molecule can react with iodine:

By adding KI, HCl, and a few drops of starch solution to the sample, and then titrating ( Helpful hints on reading burets are given on a site of Prof. O. Seely) with potassium iodate KIO₃:

1. Each iodate reacts with I^- and H⁺ to form three I₂.

   IO₃^- + 5I^- + 6H⁺ -> 3I₂ + 3H₂O

   Once formed, each I₂ is reduced immediately by a vitamin C molecule back to I^-.

   After all of the vitamin C in the sample has reacted, the blue-black starch-triiodide ion complex forms. The appearance of the blue-black color signals the end point of the titration. At the endpoint:

   moles of vitamin C initially in the solution titrated =
   

   3 x moles of iodate added to reach the endpoint

2. Vitamin C reacts with NaOH by

Titration with NaOH gives the amount of acidic protons in a vitamin C tablet and in Tang^(c). You will find that at the endpoint of our titration, one acidic proton has been removed for each vitamin C present.

3. Tang^(c) has both vitamin C and citric acid. By titrating samples of Tang^(c) with KIO₃ , the amount of vitamin C is determined directly:

1. By titrating with NaOH the total amount of acidic protons is determined. total moles of acidic protons = M(NaOH) x vol of NaOH to reach endpoint (L)

2. If we assume that vitamin C and citric are the only acids in Tang^(c), the amount of citric acid is calculated from the difference between the total amount of acidic protons and the moles of acidic protons from vitamin C. In the titration of vitamin C with NaOH, you were told to assume (and would confirmed from the titration of the vitamin C tablet) that each vitamin C has lost 1 acidic proton by the phenolphthalein endpoint. Hence
   • moles of acidic protons due to citric acid =
     total moles of acidic protons - moles of acidic protons from vitamin C

   • moles of acidic protons due to citric acid =
     total moles of acidic protons - moles of vitamin C

3. Finally you were told to assume that each citric acid would react with 3 OH^- by the endpoint of this titration, so moles of citric acid = (1/3) x moles of acidic protons due to citric acid

   The molar mass of citric acid 192.12 g/mol is used to convert moles to grams.

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Prelab Hints

#2 See the Glossary in Petrucci.

#3 Look at Eq(2) and (3) in the Lab manual. So how many molecule of vitamin C can each molecule of IO₃^- react with under the conditions of this experiment ( ie. assume I^- and H₃O⁺ were also added). Then how would you convert from moles to mass?

# 4: Vitamin C dissolves in water because what kind of bonding (see p. F-1)? Would you expect the same kind of bonding between vitamin C and methanol?

# 5: The Lewis dot structure for I₃^- is on Petrucci 9th ed, p. 928. What you need to do is show which would be the VSEPR structure. Your reasoning should be similar to that used on p. 400 - 401 for SF₄.

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Report Hints

Part I.
A.Recall the prelab. So how many molecule of vitamin C can each molecule of IO₃^- react with under the conditions of this experiment ( ie. assume I^- and H₃O⁺ were also added). Note that Part II.A. is the same calculation.
B. 1. You have moles in A. So how do you convert moles to mass (in e.g. grams).
B. 2. ie. much less, about the same or much more?

Part III. Recall that under the conditions of our experiment, only one acidic proton in each vitamin C will react with NaOH.

Part IVD. Recall how many mg of vitamin C you determined was in each gram of Tang.

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Last revised Oct 30, 2006.

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