ORGANIC cHEMISTRY - III

Experiment 2

Aim of the Experiment

Estimation of % purity of ascorbic acid.

Principle

Percentage purity of ascorbic acid can be determined by using iodimetric method. In this method, iodine reacts with -OH group of ascorbic acid. Since iodine is not a primary standard, its solution is standardized using solution of sodium thiosulphate. The reaction for oxidation of ascorbic acid (vitamin C) with iodine is shown below

ascorbic acid + I2 → 2 I− + dehydroascorbic acid

Materials required

Ascorbic acid, iodine solution 0.1 N, Sodium thiosulphate solution 0.1 N, Glacial Acetic Acid

Reagents: Prepare 0.1 N 100 ml Na₂S₂O₃ and 100 ml I₂ solutions.

Procedure

(1) Boil about 150 ml of distilled water and cool.

(2) Dissolve 1 gm. ascorbic acid in about 50 ml of boiled and cooled distilled water in a 100 ml measuring flask and make up the volume using the same water.

(3) Carry out standardization of iodine solution using standard Na₂S₂O₃ solution using freshly prepared starch solution as indicator.

(4) Calculate normality of iodine solution.

(5) Pipette out 10 ml ascorbic acid and 10 ml glacial acetic acid and titrate this solution with standard iodine solution using starch solution as indicator.

Preparation of 0.1 N Iodine solutions:

1000 ml 1 N I₂ solution = 254 g I₂

100 ml 0.1 N I₂ = 100 × 0.1 × 254/1000 = 2.54 gm of I₂

Dissolve I₂ in water take mixture of KI and I₂. KI should be in excess.

Calculation

Calculations:

I₂ ≡ 2Na₂S₂O₃ ≡ 176.13 gm. ascorbic acid,

[I] ≡ Na₂S₂O₃ ≡ 176.13/2 gm. ascorbic acid

X gm sample contains ………. gm. ascorbic acid

100 gm sample contains ………. gm. ascorbic acid;

I.e. percent purity =……….

Result

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

Reference Material

www.canterbury.ac.nz/media/documents/science-outreach/vitaminc_iodine.pdf

Questions

1. Difference between iodometry and iodimetry?

2. Which type of reaction involved in this titration?

3. Why starch indicatior is added near the end point of titration?

4. Structure of ascorbic acid?