Exp 13: Iodimetric Titration of Vitamin C by Oladoyin Ogunbayo

A Sample Calculation A thiosulfate solution was standardized by dissolving 1.0148g 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, 0.9968g 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.

Calculations Molarity of KIO3 solution 1.0148g KIO3 x 1mol KIO3 x 1= 0.00984MKIO3 214.0gKIO3 0.5L IO3- + 8I- + 6H ---- 3I3- + 3H2O 0.009484mol x 0.05L x 3mol I3- = 0.001423mol I3- 1mol IO3- Moles of S2032- 0.001433mol x 2mol S2032- = 0.002845mol S2032- 1mol I3- Na2S2O3 concentration 0.002845molS2O32-x 1 = 0.06583MS2O3- 0.04322L

Analysis of Vitamin C Moles of I3- moles formed 0.009484mol KIO3x 3mol I3- = 0.001423mol I3- 1mol KIO3 Moles of S2O3- 0.06568mol x 0.02112L = 0.001387mol S2O3- L Moles of I3- reacted with S2O3- 0.001387molS2O3- x 1mol I3- = 6.935X10-4mol I3- 2mol S2O3- Moles of Absorbic Acid 0.001423mol I3- - 6.935X10-4mol I3- = 7.295 x 10-4 mol I3-

Final solution Mass of Absorbic Acid 7.295 x 10-4 mol I3- x 176.12g I3- = 0.1285g I3- 1mol I3- Absorbic Acid per 1g of Vitamin C 0.1285g I3- = 0.1289g 0.9968g

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.

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

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.