why can't starch be used as an indicator during a titration of sodiumthiosulphate/iodine using an oxidant X?

Question

it is a redox titration. using a measuring cylinder to add iodide ions in excess (gotten from potassium iodide) and sulphuric acid to get iodine in a conical flask, it is titrated with an oxidant X

Answer 1

Because part of your oxidizer (not oxident ;o) would also react with the starch as well as your sodium thiosulphate.

This would make it "appear" like you had more sodium thiosulphate than was accually present. Plus depending on the stregth of the oxidizer, there may not be any starch around at the end point. Remember you need that starch for the Iodine to bind to to make that nice deep blue color.

Answer 2

A deep blue complex is formed between iodine and starch, and it is much easier to detect the absence of that distinctive blue colour than to try and decide when a yellow solution is no longer yellow.