# Use of Na2S2O3 before starch in Winkler's method

In Winkler's method why is some amount of $\ce{Na2S2O3}$ added before addition of starch solution turning the deep straw coloured solution pale straw?

Is it to generate $\ce{I-}$ ions needed for formation of $\ce{I3-}$ in the $\ce{I2}$ containing solution? This $\ce{I3-}$ inturn reacts with the starch to form blue-black complex.

However once we didn't add $\ce{Na2S2O3}$ and the addition of starch solution did turn the solution it blue-black.

• – Klaus-Dieter Warzecha Nov 29 '16 at 15:43
• @KlausWarzecha thanks but it doesn't answer the question. – Tyto alba Nov 29 '16 at 15:45
• I know :D Therefore it is just a comment, not an answer ;) – Klaus-Dieter Warzecha Nov 29 '16 at 15:47