Identify the species involved in methylene blue test

The reaction described is:

• $p$-amino-$N,N$-dimethylaniline is added to a strongly acidic solution of $X$
• then a few drops of aqueous solution of $Y$ are added
• a blue colour appears due to formation of methylene blue

This is the classical test for sulfide / hydrogen sulfide. In strongly acidic medium, sulfide gives $H_2S$, which oxidatively couples with $p$-amino-$N,N$-dimethylaniline in presence of an oxidizing agent such as $Fe^{3+}$ to form methylene blue.

Hence, for methylene blue formation, $Y$ must supply $Fe^{3+}$.

Check with potassium hexacyanoferrate(II)

The question says:

• aqueous solution of $Y$ with potassium hexacyanoferrate(II) gives an intense blue precipitate
• this precipitate dissolves in excess reagent

For $Fe^{3+}$: $4Fe^{3+} + 3[Fe(CN)_6]^{4-} \rightarrow Fe_4[Fe(CN)_6]_3$ This is Prussian blue / Berlin blue, an intense blue precipitate.

So this observation supports $Y = FeCl_3$.

Check with potassium hexacyanoferrate(III)

The question further says:

• solution of $Y$ with potassium hexacyanoferrate(III) gives a brown coloration due to formation of $Z$

$Fe^{3+}$ with ferricyanide $[Fe(CN)_6]^{3-}$ gives a brown ferric ferricyanide type coloration/compound. This again matches $Fe^{3+}$.

Evaluate the options

• A: $MgCl_2$

  • $Mg^{2+}$ does not help in methylene blue formation.
  • No intense blue precipitate with potassium hexacyanoferrate(II).
  • Incorrect.

• B: $FeCl_2$

  • $Fe^{2+}$ is not the required oxidizing ion for methylene blue test.
  • With potassium hexacyanoferrate(II), $Fe^{2+}$ does not give Prussian blue.
  • Incorrect.

• C: $FeCl_3$

  • Provides $Fe^{3+}$ needed for methylene blue formation.
  • With potassium hexacyanoferrate(II), gives intense blue Prussian blue precipitate.
  • With potassium hexacyanoferrate(III), gives brown coloration.
  • Correct.

• D: $ZnCl_2$

  • $Zn^{2+}$ does not fit any of the given observations.
  • Incorrect.

Final answer

Therefore, the compound $Y$ is: $\boxed{FeCl_3}$