In the precipitation of the iron group (III) in qualitative analysis, ammonium chloride is added before…

In the precipitation of the iron group (III) in qualitative analysis, ammonium chloride is added before adding ammonium hydroxide to :
  1. increase concentration of \(\mathrm{Cl}^{-}\)ions
  2. increase concentration of \(\mathrm{NH}_4{ }^{+}\)ions
  3. prevent interference by phosphate ions
  4. decrease concentration of \({ }^{-} \mathrm{OH}\) ions

Solution

$\begin{aligned} & \mathrm{NH}_4 \mathrm{OH} \rightleftharpoons \mathrm{NH}_4^{+}+\mathrm{OH}^{-} \\ & \mathrm{NH}_4 \mathrm{Cl} \rightarrow \mathrm{NH}_4^{+}+\mathrm{Cl} \end{aligned}$
Due to common ion effect of $\mathrm{NH}_4^{+}$, $\left[\mathrm{OH}^{-}\right]$decreases in such extent that only group-III cation can be precipitated, due to their very low $\mathrm{K}_{\mathrm{sp}}$ in the range of $10^{-38}$.

Asked in: JEE Main 2024 (04 Apr Shift 1)

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