why is it necessary to add NH4Cl prior to NH4OH to precipitate third group cations ?

NH4Cl is added before NH4OH in detection of Fe 3+ ion and Al3+ ion to ensure low concentration of OH- ions .This is due to the common ion effect. This mantains the pH required for the reaction.

Common ion effect in detection of Third group cations helps in precipitating the Iron(III)hydroxide to avoid the little/scarlet formation of Nickel dimethylglyoxime.

Thus it is treated with small amount of ammonia in water which furnishes small amount of hydroxide ions which are sufficient to precipitate iron(III)hydroxide