Question

For a lab to determine the change in oxidation state of Cl when KClO3 oxidizes I, NaOH was used to neutralize the solution before the Na2S2O3 titration. The solution was cooled in an ice bath until the air above the solution in the stoppered flask was "no longer purple". Why would it be expected that the solution is no longer purple after cooling? During the next step, the exothermic neutralization reaction may have caused the loss of I2. Why?

Answer 1

In this particular experiment cooling of solution was done to prevent the vaporization of I₂. I₂ in water is purple while I^- colorless. So if we make sure that the air is no longer purple, we can ensure that no more I₂ is loosing by vapourisation.

The nuetralisation step is exothermic in nature. So during the reaction heat energy will be liberated, which will inturn increase the rate of vapourisation of I₂. Rate of vapourisation is directly proportional to temperature.