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JEE Main levelSingle correct optionDifficulty 2 of 3qc143

In the thiocyanate test for $Co X^{2 +}$ (blue colour extracted into amyl alcohol), interference from $Fe X^{3 +}$ is removed by adding

Aexcess $NaOH$
Bdilute $HCl$
Csolid $NaF$ , forming colourless $[FeF X_{6}] X^{3 -}$
D $KCN$ in excess

Answer: C

Fe X^{3 +}

gives a red thiocyanate complex that masks the blue

[Co (SCN) X_{4}] X^{2 -}

. Fluoride binds

Fe X^{3 +}

as colourless

[FeF X_{6}] X^{3 -}

.

NaOH

would precipitate hydroxides and

KCN

would also complex cobalt.

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