EXPERIMENT 5
REDOX TITRATION: TITRATION USING SODIUM THIOSULPHATE
Objectives
1.

To prepare a standard solution of potassium iodate for use to determine the concentration of sodium thiosulphate solution accurately.

2.

To acquire the proper techniques of carrying out a titration.

Introduction
Redox titrations using sodium thiosulphate as a reducing agent is known as iodometric titration since it is used specifically to titrate iodine. The reaction involved is:
I2 + 2Na2S2O3

2NaI + Na2S4O6

I2 + 2S2O32-

2I- + S4O62-

In this equation I2 has been reduced to I- :2S2O32-

S4O62- + 2e

I2 + 2e

2I-

The iodine/thiosulphate titration is a general method for determining the concentration of an oxidising agent solution. A known volume of an oxidising agent is added into an excess solution of acidified potassium iodide. The reaction will release iodine:Example:
(a) With KMnO4
2MnO4- + 16H+ + 10I-

2Mn2+ + 5I2 + 8H2O

(b) With KIO3
IO3- +

5I-

+

6H+

3I2 + 3H2O

The iodine that is released is titrated against a standard thiosulphate solution.
From the stoichiometry of the reaction, the amount of iodine can be determined and from this, the concentration of the oxidising agent which released the iodine, can be calculated.
In an iodometric titration, a starch solution is used as an indicator as it can absorb the iodine that is released. This absorption will cause the solution to change to a dark blue colour. When this dark blue solution is titrated with the standardised thiosulphate solution, iodine will react with the thiosulphate solution. When all the iodine has reacted with the thiosulphate solution, the dark blue colour will disappear. So, the end point of the titration is when the dark blue colour disappears.
Iodine is usually dissolved in water by adding an excess of KI so that KI3 which has similar