Chem 10X - Experiment F

Analysis of Vitamin C


Lab Summary

Outline of Experiment

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

reaction of ascorbate 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

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

  3. 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
reaction with NaOH

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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Last revised Jan 24, 2014.