PLAN
Ksp(ZnS) is very high and
Zn2+ is precipitated as ZnS by high concentration of
S2− formed when
H2S is passed in ammoniacal solution.
$\hspace20mm H_2 S \rightleftharpoons Zn^+ + S^{2-} \, (I)$$\hspace10mm H^+ + OH^- \rightleftharpoons H_2 O \, (II)$ Reaction (I) is favoured in forward side if
H+ is removed immediately by
OH−(NH4OH).
$\hspace10mm Zm^{2+} + S^{2-} \longrightarrow \, \underset{White \, ppt}{ \, ZnS\downarrow}$Fe3+andAI3+ are precipitated as hydroxide.