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  4. In the titration of I 2(a q) by S 2 O 32-(a q) using the starch indicator, the end point is indicated by

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Q. In the titration of $I _{2}(a q)$ by $S _{2} O _{3}^{2-}(a q)$ using the starch indicator, the end point is indicated by

TS EAMCET 2018

Solution:

The chemical reaction involved during the titration of

$I _{2}(a q)$ by $S _{2} O _{3}^{2-}(a q)$ using starch indicator is

$I _{3}^{-}+2 S _{2} O _{3}^{2-} \longrightarrow S _{4} O _{6}^{2-}+3 I ^{-}$

At the end point deep blue colour disappears due to the decomposition of the iodine-starch complex. Thus, the colour change at end point is from blue to colourless.