Question

Titration between iodine and thiosulphate.

Why starch solution has to be added when the solution just turned pale yellow, but not at the beginning of reaction?

Answer 1

If the concn of I2 is too large, the starch-I2 complex that forms is NOT the same as that formed with small I2 concns and the complex does not change color properly (sometimes not at all) at the equivalence point. I'm sure your text goes into this.