1.

When I_(2) is estimated by Na2S2O, iodometrically, blue colour (due to starch), changes to colourless when 12 has completely reacted 2S_(2)O_(2)^(2-)+I_(2)toS_(4)O_(6)^(2-)+2I but after sometimes blue colour reappears. Explain?

Answer»

Solution :This is because some atmospheric oxidation has taken PLACE, forming a which reacts with starch to GIVE the BLUE colour again.
`2I^(-)+(1)/(2)O_(2)+2H^(+)toI_(2)+H_(2)O`


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