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Function of NH4OH

The function of ammonium hydroxide is to increase the concentration of sulphide ions by removing H+ ions produced due to the dissociation of H2S as

      H2S   <==>       2H +  S2-
NH4OH   <==>      NH4 +  OH -

The OH - ions furnished by NH4OH combine with H+ ions generated by H2S to form practically unionized H2O.

      H+            +        OH -                 →     H2O
( formH2S )       ( form NH4OH )

Consequently, concentration of sulphide ions increases to such an extent that the ionic product of the concentrations of cations of group IV and S2- ions exceeds the solubility product of the sulphides of cations of group IV which get precipitated.

Nickel sulhide and cobalt sulphide are insoluble in dil. HCI while manganese sulhide and zinc sulphide react with dil. HCI and get dissolved. In this way nickel and cobalt get separated form manganese and zinc. However, nickel sulhide and cobalt sulphide get dissolved in aqua regia as

NiS + 2HCI + 3(O)      →       NiCI2 + SO2 + H2O
CoS + 2HCI + 3(O)     →       CoCI2 + SO2 + H2O

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