What is the function of ammonium hydroxide in group IV?

Solution

Cations of group IV (Zn²⁺, Mn²⁺, Co²⁺, Ni²⁺) are precipitated as their sulphides by passing H₂S gas through their solution containing NH₄OH. This is explained on the basis of common ion effect.

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H⁺ ions react with OH^– ions to give water Due to decrease in the concentration of H⁺ions, the equilibrium shifts towards right and S^2– ions concentration increases. As a result ionic product increases and becomes sufficient to exceed the solubility products of sulphides of group IV cations. Hence radicals of group IV get precipitated.
In the absence of NH₄OH, the concentration of S² ions is not sufficient. Hence the sulphides of group IV cations may not get precipitated due to their high K_sp value .