In the precipitation of the iron group (III) in qualitative analysis, ammonium chloride is added before adding ammonium hydroxide to :
Select the correct option:
A
prevent interference by phosphate ions
B
decrease concentration of ${ }^{-} \mathrm{OH}$ ions
C
increase concentration of $\mathrm{Cl}^{-}$ions
D
increase concentration of $\mathrm{NH}_4^{+}$ions
✓ Correct! Well done.
✗ Incorrect. Try again or view the solution.
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 $\mathrm{NH}_4^{+}$,
[ $\mathrm{OH}^{-}$]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 $10^{-38}$.
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