F A Q s of Determination of iodate and iodide in a mixture

I. How could you determine the concentration of potassium dichromate in a solution quantitatively by an iodometric titration?

 II. Why cannot you titrate the oxidizing agent directly with the thiosulphate solution?

The oxidizing agent reduces thiosulphate to sulphur.

III. Explain why you do not add starch at the beginning of the titration.

In the beginning iodine concentration is too high and starch form a stable complex with iodine and blue color do not fades wawy even at the end point.
IV. Why is it necessary to do iodometric titrations in acidic medium as quickly as possible?
Acidic medium: Many weak oxidizing anions are completely reduced by iodide if their reduction potentials are raised considerably by the presence of large amount of acid in the medium.