Back Titration of Vitamin C

[angelique10112]

I have three major questions that I am stuck on if anyone can help offer any insight that would be great!

In the experiment, a Vitamin C tablet was extracted into dilute H2SO4 and an aliquot of standard KIO3 solution was added. This was followed by addition of an excess of KI. The liberated I2 reacted with the ascorbic acid, and the amount of unreacted I2 was determined by a back titration with the previously standardised Na2S2O3 solution.


The following questions I have about this experiment is listed below:

1.By knowing the amount of I2 originally liberated and determining the amount of I2 that did not react with ascorbic acid, it is possible to determine the amount of ascorbic acid in the tablet.
Vitamin C back titration analysis can also be performed by adding starch to the reaction mixture once the color of the tri-iodide in solution fades to golden yellow. Starch combines with free tri-iodide to make a blue-black complex; once all the tri-iodide is titrated, the solution turns colorless. If starch indicator had been added to the reaction mixture during a potentiometric titration, at what volume of added thiosulfate would the color end-point be observed, relative to the equivalence point determined with the redox electrode?

2.Vitamin C is a powerful reducing agent, easily oxidized even by exposure to air.
a. Why does this fact make it ideally suited for back-titration?
b. Why was sodium bicarbonate added to the acidic reaction mixture? How does this aspect of the experimental design limit the air-oxidation of all the reactants?
c. Consider the question: Once the dissolved tablet or juice sample is added to the reaction mixture, does any unoxidized Vitamin C remain to be oxidized by air?

3.Identify the most-likely sources of error in each of the three steps of the experiment: For the standardization, tablet analysis, and fruit analysis, explain how one of those errors would alter the calculated result relative to the “true” analysis (ie. Will the error cause the result to be less than or greater than true)?



Thanks so much for your help, I REALLY appreciate it!

Angelique