The function of NH₄CI (a strong electrolyte) is to suppress the degree of ionization of NH₄OH (a weak electrolyte) by common ion effect. Hence, the concentration of hydroxide ions in the solutions very low as:

NH4CI      →      NH₄⁺  +  CI ^-
NH4OH   <==>     NH₄⁺  +   OH ^-

Consequently, the ionic product of hydroxide ions and the cations of group III have low values. However, even this low value exceeds the solubility product f the hydroxides of the cations of group III but fails to exceed the solubility product of the hydroxides of the catiosn of subsequent groups. As such only the hydroxides of group III cations get precipitated.

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