Question

Why do we add ${NH}_{4}Cl$ before adding ${NH}_{4}OH$ for precipation of group $3$ cations ?

Answer 1

During salt analysis, $N{H}_{4}Cl$ is added in sufficient quantity before adding $N{H}_{4}OH$ because otherwise the cations of higher group may get precipitated in group III.

$N{H}_{4}Cl$ is strong electrolyte, it decomposes completely. While the $N{H}_{4}OH$ is weak base it does not ionises completely. Thus due to presence of common ion $N{H}^{4+}$ in $N{H}_{4}Cl$, it supresses the ionisation of weak base $N{H}_{4}OH$ in order to decrease the $O{H}^{-}$ concentration so that higher group cations will not get precipitated.This process is called common ion effect.