1.

Which of the following indicators is used in the titration of Na_(2)S_(2)O_(3) against CuSO_(4) solution?

Answer»

Starch
Barium diphenylamine sulphonate
Cresol red
Iodine solution

Solution :At the end point of an iodine TITRATION with sodium THIOSUPHATE, the iodine is all converted to iodide ions. Thus the bluish colour produced by the starch indicator with iodine disappears and the solution becomes colourless.
If the indicater turns BLUE again. This is because some atmospheric oxidation has taken place forming `I_(2)`, which reacts with the starch to give the blue colour again.
`2I^(-)+1/2 O_(2)+2H^(+) rarr I_(2)+H_(2)O`
The end point of the titration is usually taken as being when the colour disappears and the solution remains colourless for half a minute.


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