How to perform a titration

  

Iodometry

An aqueous solution containing diiode I2 is orange-brown, whereas when it contains iodide ions, it is colorless.
Thiosulfate S2O32- is often used to titrate I2 but I- can also be dosed by the tetrathionate ion S4O62- .

Starch can be added to the solution to make it easier to observe the color change. When the diiode comes into contact with the starch, a dark blue colored I2 – starch complex is formed.

The diiodine is poorly soluble in aqueous solution so that it is most often the triiodide ion I3-. The reaction equations are as follows:

I2(aq)  + 2 S2O32- (aq) = 2 I-(aq) + S4O62-(aq)


Disappearance of a brown or black color (in the presence of starch)

2I-(aq) + S4O62-(aq) = I2(aq) + 2 S2O32- (aq)


Appearance of a brown or black color (in the presence of starch)
An example of iodometric colorimetric titration is the Winkler method.
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