In III group precipitation, NH_4 Cl is added before adding NH_4 OH...

In III group precipitation, $\mathrm{NH}_4 \mathrm{Cl}$ is added before adding $\mathrm{NH}_4 \mathrm{OH}$ to decrease the conc. of$\mathrm{OH}^{-}$, as $\mathrm{NH}_4 \mathrm{Cl}$ and $\mathrm{NH}_4 \mathrm{OH}$ dissociates as follows. $$\begin{aligned} & \mathrm{NH}_4 \mathrm{Cl} \rightleftharpoons \mathrm{NH}_4^{+}+\mathrm{Cl}^{-} \\ & \mathrm{NH}_4 \mathrm{OH} \rightleftharpoons \underset{\substack{\text{ common } \\ \text{ ion }}}{\mathrm{NH}_4^{+}+\mathrm{OH}^{-}} \end{aligned}$$ Due to presence of common ion, conc. of $\mathrm{OH}^{-}$ decreases and for III group precipitation lesser conc. of $\mathrm{OH}^{-}$is required.