ANALYTICAL cHEMISTRY - III

Experiment 6

Principle

• Percentage purity of ascorbic acid can be determined by using iodometric method. In this method, iodine reacts with -OH group of ascorbic acid.

• In this method, iodine, I₂ is getting reduced to iodide, I⁻ and the ascorbic acid is getting oxidized to the dehydroascorbic acid. The amount of iodine, I₂ consumed during this transformation is determined by titrating the unreacted iodine, I₂ against 0.1 N Na₂S₃O₃.5H₂O solution using starch indicator.

• Since iodine is not a primary standard, its solution is standardized using standardized 0.1 N Na₂S₃O₃.5H₂O solution. The 0.1 N Na₂S₃O₃.5H₂O solution itself is standardized using standard 0.1 N CuSO₄.5H₂O solution.

• The reaction for oxidation of ascorbic acid (vitamin C) with iodine is shown below:

Apparatus required

Burette, Pipette, Conical flask, Standard measuring flask (100 mL), etc.

Chemicals required

Ascorbic acid, Iodine solution (0.1 N), Na₂S₃O₃.5H₂O solution (0.1 N), Glacial acetic acid, CuSO₄.5H₂O solution (0.1 N), etc.

Preparation of reagents

Preparation of 0.1 N Na₂S₂O₃.5H₂O solution:

1000 mL 1 N Na₂S₂O₃.5H₂O solution = 248.19 g of Na₂S₂O₃.5H₂O

100 mL 0.1 N Na₂S₂O₃.5H₂O solution = ______ g Na₂S₂O₃.5H₂O

Preparation of 0.1 N Iodine solution:

1000 mL 1 N I₂ solution = 127 g I₂

100 mL 0.1 N I₂ = 100 × 0.1 × 127/1000 = ______ g I₂

Note: Dissolve I₂ in in the water having excess of KI dissolved. KI should be ~ 5.2 times excess.

Result

Purity of the given sample of ascorbic acid is ___________ %.

References

1. G H Jeffery, J Bassett, J Mendham and R C Denney, Vogel's Textbook of Quantitative Chemical Analysis, 5th Edition

2. S. Suzanne Nielsen, Food analysis, 4th Edition

Questions