Why NH4Cl is added before adding NH4OH in group 3??

In salt analysis, NH4Cl is added in sufficient amount before adding NH4OH because otherwise the cations of higher group may get precipitated in group III.
Reason: NH4Cl is strong electrolyte, it decomposes completely. While the NH4OH is weak base it does not ionises completely. Thus due to presence of common ion NH4 + in NH4Cl, it supresses the ionisation of weak base NH4OH in order to decrease the OH- concentration so that higher group cations will not get precipitated.
Common ion effect: 
NH4Cl  NH4 + + Cl- 
NH4OH   NH4 + + OH-  

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