[Emas]

Hello,

I am doing a titration of Vitamin C some time in the next couple of weeks. I have done some rough calculations to see roughly how much 'stuff' I am going to need. Just wondering if they are correct?

I will be doing a back titration with I₂.

To give an overview of the method:
1) Create a known amount of I₂.
2)Add Vitamin C to the I₂, which will oxidize SOME of it.
3)Titrate this left-over I₂ with Na₂S₂0₃.5H₂0

Some useful equations:

IO3^- + 5I- + 6H+   --> 3I₂ + 3H2O  (iodine creation)

Vitamin C + I2   -->   [Vitamin C]- + 2H+ + 2I-   (reduction of iodine)

2S₂O₃^2- + I₂  --> 2I- + S₄O₆^2-

Mr of Vit. C is 176.12g mol
I will use 0.02mol L KIO3 and 0.11mol L S2O3_2-

OK. Here goes.

The packet states that there is 1000mg of Vitamin C per tablet. This would be put into a 200mL glass of water usually(in real life). Which means there is:
1g Vitamin C per 200mL
0.125g Vit. C per 25mL

number of moles of Vitamin C in 25mL:

n(Vit. C) = m/Mr
=0.125/176.12
=7.097x10^-4mol

number of moles of IO3-:

n(IO3-)= cv
=0.02x0.025
=5x10^-4mol

IO3^-             :  I₂     (molar ratios)
         1                         :          3
5x10^-4mol     : 1.5x10^-3mol

nuber of moles of I2 left over:

=(original) - (used)         (remember Vit C: I2 = 1:1 ratio)
=(1.5x10^-3)-(7.097x10^-4)
=7.093x10^-4mol

S2O3                                    :    I2
  2                                        :    1
1.5806x10^-3mol    : 7.903x10^-4mol

volume of S203:      (the titre)
=n/c
=1.5806x10^-3/0.11
=0.01437L
=14.37mL(minimum)

this is the miminum titre required.

maximum titre is needed when vitamin C content is 0.

nuber of moles of I2 left over:

=(original) - (used)         (remember Vit C: I2 = 1:1 ratio)
=(1.5x10^-3)-(0)
=1.5x10^-3mol

S2O3                                    :    I2
  2                                        :    1
3.0x10^-3mol          : 1.5x10^-3mol

volume of S203:      (the titre)
=n/c
=3.0x10^-3/0.11
=0.02727L
=27.27mL(maximum)


These values work well for me. Becasue i need the minium to be above 11mL and the maximum below 30mL.

But is my working right??

I know its a bit of a mission, but any help/checking would be greatly appreciated.

P.S. I have concerns about how hard it will be for other students to do this experiment, as they are doing things, such as Vitamin C in vegetables, which have considerably less Vit. C content. That means they need to use weaker solutions, and that brings in the problem of accuracy, seeing as out scales only weigh to 2decimal places. your thoughts on this would be appreciated too.

thanx.

[xiankai]

yeah its all good i suppose.

wierd that they only ask u to find the max./min. , but im just ignorant

[Emas]

they dont ask for just max and minimum, but its my way of ensuring that all of the possible values i CAN get, are appropiate.