$\begin{aligned}
& \mathrm{NH}_4 \mathrm{OH} \rightleftharpoons \mathrm{NH}_4^{+}+\mathrm{OH}^{-} \
& \mathrm{NH}_4 \mathrm{Cl} \rightarrow \mathrm{NH}_4^{+}+\mathrm{Cl}
\end{aligned}$
Due to common ion effect of NH4+, [OH−]decreases in such extent that only group-III cation can be precipitated, due to their very low Ksp in the range of 10−38.