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My vitamin C titration with iodate turned red. Help!

  1. Jan 9, 2017 #1
    So for my IA, I dissolved a 1000mg vitamin c tablet in 355mL of water, took 20mL of it and diluted it to 100mL making it about 0.0032mol/L. I then added 5mL of 0.5mol/L potassium iodide, 5mL of 1.0mol/L hydrochloric acid and some 6 drops of 1 gram of starch dissolved in 100mL of water.

    I then titrated that entire sample with potassium iodate to determine the concentration of vitamin c. The endpoint was suppose to be when the solution turned blue (iodine+starch) but it ended up slowly turning yellow and then becoming darker until it eventually turned red. I tried increasing the concentration of potassium iodate but it just lead to the solution becoming red faster. I think I should have enough I had quit my IA and am excpected to redo it sometime later. Does anyone have any idea what has happened to my experiment? Any help would be appreciated. Thanks!
  2. jcsd
  3. Jan 9, 2017 #2


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    What is "IA?"
  4. Jan 10, 2017 #3


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    The color you describe is that of a free iodine dissolved in water. Sounds like it doesn't react with neither ascorbic acid nor the starch, as if they were not present in the solution.
  5. Jan 10, 2017 #4
    Thanks! It turned out that I didn't heat up the water before dissolving the starch in it. This led most of the starch not dissolving and settling on the bottom where it was unable to react with the iodine being formed at the top. Once I heated the starch solution, it worked fine!
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