JEE Main 2025 Jan 24 Shift 1, Chemistry Q24: Principles Of Qualitative Analysis

Potassium ferrocyanide ${\mathrm{K}}_4[\mathrm{Fe}(\mathrm{CN}{)}_6]$ is used as a confirmatory reagent because it forms insoluble, characteristically coloured ferrocyanide salts with certain cations.

Going through the list:

$\mathrm{C}{\mathrm{u}}^{2+}$ $\to$ chocolate-brown $\mathrm{C}{\mathrm{u}}_2[\mathrm{Fe}(\mathrm{CN}{)}_6]$. $✓$

$\mathrm{F}{\mathrm{e}}^{3+}$ $\to$ Prussian-blue $\mathrm{F}{\mathrm{e}}_4[\mathrm{Fe}(\mathrm{CN}{)}_6{]}_3$. $✓$

$\mathrm{B}{\mathrm{a}}^{2+}$ $\to$ no characteristic precipitate under standard test conditions.

$\mathrm{C}{\mathrm{a}}^{2+}$ $\to$ no sharp, diagnostic precipitate (this is the disputed cation; see note).

$\mathrm{N}{\mathrm{H}}_4^{+}$ $\to$ no precipitate.

$\mathrm{M}{\mathrm{g}}^{2+}$ $\to$ no characteristic precipitate.

$\mathrm{Z}{\mathrm{n}}^{2+}$ $\to$ white/bluish-white $\mathrm{Z}{\mathrm{n}}_2[\mathrm{Fe}(\mathrm{CN}{)}_6]$. $✓$

Counting the cations that give the standard, well-recognised characteristic ferrocyanide precipitates: $\mathrm{C}{\mathrm{u}}^{2+}$, $\mathrm{F}{\mathrm{e}}^{3+}$ and $\mathrm{Z}{\mathrm{n}}^{2+}$