Discuss the role of `NH_(4)OH` in group III of basic redicals.

Explanation

In group III, the cations `Fe^(3+), Cr^(3+) " and "AI^(3+)` are precipitated as hydroxides by the addition of `NH_(4)OH` to the aqueous solution of the mixture. But a small amount of `NH_(4)CI ("common " NH_(4)^(+) " ion")` is added before the addition of `NH_(4)OH.` its purpose is to suppress the `NH_(4)^(+)` ion concentration in solution due to common ion effect.

Thus only the hydroxides of cations of group III `("low " K_(sp)" values")` will be precipiteted while the hydroxides of cations of higher groups will remain in solution since their `K_(sp)` value are higher and the products will remain less than their solubility proudct `(K_(sp))` values.