Function of HCI

The function of HCI (a strong electrolyte) is to suppress the degree of ionization of H₂S (a weak electrolyte) by common ion effect. Hence, the concentration of S^2- ions in the solution is very low as :

HCI      →       H⁺   +  CI ^-
H₂S    <==>     2H⁺  +  S^{2 -}
                                       ( very low concentration )

Consequently, the ionic product of S^2- ions and the cations of the group II have low values. However, even this low values exceed the solubility product of the sulphides of the cations of group II but fails to exceed the solubility product of the sulphides of the cations of subsequent groups. As such only the sulphide of group II cations gets precipitated.

Since the sulphides of As³⁺ , Sb³⁺ and Sn²⁺ are soluble in yellow ammonium sulphide while those of the remaining metal ions are practically insoluble, it is usual to divide this group into two sub-groups. The sulphides of Hg²⁺ ,Pb²⁺ , Bi³⁺ , Cu²⁺ and Cd²⁺ which are insoluble in yellow ammonium sulphide belong to group IIA whereas sulphides of As³⁺ , Sb³⁺ and  Sn²⁺ which are soluble in yellow ammonium sulphide belong to group IIB. Thus

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