Question

In this reaction:

I2 + 2S2O32- --> S4O62- + 2I-

Please explain why the immediate color change that happens when the titrant is first added is not the equivalence point. When first adding 2S2O32- it immediately turned yellow, then after a while went to blue (using starch indicator).  

Answer 1

In the reaction, I- ions are liberated. These are less efficient to estimate. Hence starch indiactor is used.

However, starch cannot produce colour if only I- ions are present. It is the I₃^- -starch complex whixh gives blue color.

I^- + I₂ = I₃^-

First, all I2 is consumed to give I- at equivalence point. Once equivalence point is attained, a slight excess of addition of I2 is added which combins with I- present in the system to give I₃^- triiodide ions. These ions then make a blue colour complex with starch, hence producing blue colur in solution.

So the delay is due to formation of triiodide ions which then produces blue coloured triiodide-starch complex.