1. Exp 13: Iodimetric Titration of Vitamin Cby
Oladoyin Ogunbayo

2. A Sample Calculation
A thiosulfate solution was standardized by dissolving g of KIO3 in a 500ml volumetric flask and by titrating with 50ml of KIO3 , the addition of 2g of KI, 10ml of 0.5M H2SO4 and 2mL of starch indicator and titrating the solution and a volume of 43.22L was used. During this experiment, g of vitamin C were dissolved in 60mL of 0.3M H2SO4 with the addition of 2g of KI, 50.00ml of standard KIO3 and 2mL of starch indicator and was titration with the same thiosulfate solution. Calculate the grams of vitamin C tablets and the absorbic acid per 1g of vitamin C.

3. Calculations Molarity of KIO3 solution
1.0148g KIO3 x 1mol KIO3 x 1= MKIO3
214.0gKIO3 0.5L
IO3- + 8I- + 6H I3- + 3H2O
mol x 0.05L x 3mol I3- = mol I3-
1mol IO3-
Moles of S2032-
mol x 2mol S2032- = mol S2032-
1mol I3-
Na2S2O3 concentration
molS2O32-x = MS2O3- L

4. Analysis of Vitamin C Moles of I3- moles formed
mol KIO3x 3mol I3- = mol I3-
1mol KIO3
Moles of S2O3-
mol x L = mol S2O3- L
Moles of I3- reacted with S2O3-
molS2O3- x 1mol I3- = 6.935X10-4mol I3-
2mol S2O3-
Moles of Absorbic Acid
mol I X10-4mol I3- = x 10-4 mol I3-

5. Final solution Mass of Absorbic Acid
7.295 x 10-4 mol I3- x g I3- = g I3-
1mol I3-
Absorbic Acid per 1g of Vitamin C
0.1285g I3- = g
0.9968g

6. Question and Answer
Why is iodine almost always used in a solution containing excess I-?
I- reacts with I2 to give I3-. This reaction increases the solubility of I2 and decreases volatility.

7. Bonus Question
Does it matter whether starch indicator is added at the beginning or near the end point in the titration ?

8. Bonus Answer
Starch solution should not be added until just before the end point because I3- is present throughout the titration and will irreversibly bind to starch if starch is added too early.