In Winkler's method why is some amount of Na2S2O3 added before addition of starch solution turning the deep straw coloured solution pale straw?
Is it to generate Iā ions needed for formation of I3ā in the I2 containing solution? This I3ā inturn reacts with the starch to form blue-black complex.
However once we didn't add Na2S2O3 and the addition of starch solution did turn the solution it blue-black.