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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: cz on December 22, 2009, 05:04:56 PM

Title: Determining Vitamin C content by Redox Titration with Iodate
Post by: cz on December 22, 2009, 05:04:56 PM

Hello, everyone:
  I got a worksheet for determining the amount of ascorbic acid in a Vitamin C tablet.
  I have the average volume of KIO3- used to titrate the acids,and I have the concentration of KIO3-. I'm asked to calculate the number of moles of iodate reacting and the number of moles of iodine liberated.
  Then I'm asked to calculate the number of moles of ascorbic acid oxidized.

I found the half reactions for the equations

Here are the equations:
C6H8O6   +   I2   +   2H2O   -> C6H10O8   +   2I -   +   2H+
I2 + I- → I3-
IO3- + 5I- + 6H+ ---> 3I2 + 3H2O

balanced half reactions:

Iodate and iodide
12H+ + 10 e- + 2IO3- ---> I2 + 6H2O
10I- ---> 5I2 + 10 e-

Ascorbic acid and the liberated iodine
C6H8O6 --> C6H6O6 + 2H+ + 2e-
2e- + I3- --> 3I-

I do not understand which mole to mole ratio is appropriate to use.
      
Your help would be really really appreciated!~
Please help me!

Title: Re: Determining Vitamin C content by Redox Titration with Iodate
Post by: Borek on December 22, 2009, 05:24:07 PM

Quote from: cz on December 22, 2009, 05:04:56 PM

I do not understand which mole to mole ratio is appropriate to use.

Iodate to ascorbic acid. It may be a little bit difficult to find, as there are intermediates, but that's what you should look for.

Title: Re: Determining Vitamin C content by Redox Titration with Iodate
Post by: cz on December 22, 2009, 05:30:03 PM

I think I'm supposed to use 1 mole of C6H8O6 to 1 mole of I2, from the balanced half reaction equations, but I'm not sure.

Title: Re: Determining Vitamin C content by Redox Titration with Iodate
Post by: Borek on December 22, 2009, 05:48:14 PM

I₂ is not iodate.

Write FULL balanced reactions, not half reactions - molar ratio would be obvious then.

Title: Re: Determining Vitamin C content by Redox Titration with Iodate
Post by: cz on December 22, 2009, 09:40:51 PM

Then the full equation would be:

C6H8O6 + I3- + H2O --> C6H6O6 + 3I- + 2H+

and

IO3- + 5I- + 6H+ ---> 3I2 + 3H2O

Title: Re: Determining Vitamin C content by Redox Titration with Iodate
Post by: cz on December 22, 2009, 09:49:38 PM

But iodine and triiodide will remain in the solution after all the Vitamin C is oxidized, so that the starch would turn colour?  That's why I thought I had to use Iodine (I2) as the ratio.  Sorry I'm saying dumb things..I'm so lost

Title: Re: Determining Vitamin C content by Redox Titration with Iodate
Post by: Borek on December 23, 2009, 03:51:18 AM

You are titrating with iodate. When you add iodate to the solution, it reacts to produce I₂. Iodine either oxidizes ascorbic acid or reacts with starch showing blue color. However, reaction with starch is easily reversible, so as long as there is ascorbic acid in the solution, it will reduce iodine - thus solution is colorless. Once ascrobic acid is completely oxidized, small excess of iodine gives relatively stable color - there is no reducing agent that could remove iodine from teh solution.

Now, take a look at your reactions. If you start with one molecule of iodate - how many molecules of iodine will be produced, and how many molecules of ascorbic acid will be oxidized?

BTW, check this reaction:

Quote from: cz on December 22, 2009, 09:40:51 PM

C6H8O6 + I3- + H2O --> C6H6O6 + 3I- + 2H+

It is close, but not entirely correct.

Title: Re: Determining Vitamin C content by Redox Titration with Iodate
Post by: cz on December 23, 2009, 11:35:15 AM

1 molecule of iodate will produce 3 molecules of iodine?
1 molecule of ascorbic acid will be oxidized when react with 1 molecule of iodine?

Title: Re: Determining Vitamin C content by Redox Titration with Iodate
Post by: Borek on December 23, 2009, 12:30:50 PM

So, how many molecules of ascorbic acid per molecule of iodate? What is molar ratio?

Title: Re: Determining Vitamin C content by Redox Titration with Iodate
Post by: cz on December 23, 2009, 02:33:49 PM

1 mole of iodate to 3 moles of ascorbic acid

Title: Re: Determining Vitamin C content by Redox Titration with Iodate
Post by: Borek on December 23, 2009, 03:59:41 PM

So you are ready to solve the question.

Title: Re: Determining Vitamin C content by Redox Titration with Iodate
Post by: cz on December 23, 2009, 04:24:36 PM

Thank you so much,Borek!!!!!!!!!!!!!!!!!!!!!!!!!