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JEE Main levelSingle correct optionDifficulty 2 of 3ev133

In a COD determination, the excess dichromate that remains after oxidising the sample is determined by back titration with a standard solution of:

Asilver nitrate
Bpotassium permanganate
Csodium chloride
Dferrous ammonium sulfate

Answer: D

The unused

Cr X_{2} O X_{7} X^{2 -}

is reduced by

Fe X^{2 +}

from ferrous ammonium sulfate. The dichromate consumed by the sample is found by difference.

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