STANDARDISATION OF IODINE SOLUTION

Principle:

Iodine reacts with sodium thiosulphate forming sodium iodine and sodium tetrathionate. The end point of the reaction is indicated by the disappearance of the blue colour produced by the solution with the starch indicator

            2Na₂S₂O₃ + I_2­                  Na₂S₄O₆ + 2NaI

Procedure:

1.      Rinse a clean burette with the N/10 sodium thiosulphate solution.

2.      Fill the burette with thiosulphate solution and fix it to the stand.

3.      Pipette out given 25 ml of Iodine solution to an Erlenmeyer flask and add 100 ml distilled water with a measuring cylinder.

4.      Run down the thiosulphate solution to the Iodine solution in the flask till the colour of the solution turns to straw yellow.

5.      At this stage add about 1 ml of freshly prepared starch indicator into the flask when an intense blue colour is formed. Mix well.

6.      Continue the addition of the thiosulphate solution drop wise till the blue colour disappears (the first disappearance of the blue colour should be considered the correct end point since the blue colour reappears after some time).

7.      Note the volume of the thiosulphate required to affect the disappearance of the blue colour.

8.      Repeat the experiment till at least two concordant results are obtained.

9.      Record the burette readings in the tabular form.

Observation:

Burette reading	Titration I	Titration II	Titration III
Final reading
Initial reading
Volume delivered in ml.

Calculation:

Volume of N/10 Na₂SO₃ required to react with 25 ml of Iodine solution = V₁ ml

Volume of Na₂S₂O₃  X  Normality = Volume of Iodine (V₂)  X  Normality (N₂)

                                    V₁ X N₁

Therefore   N₂ =

                                         V₂

N₂ X Equivalent weight of Iodine (126.9) = g / l

Result: The given iodine solution contains ____ g. of iodine per liter