Question
What is the function of dilute HCl in group II analysis?
Solution
The cations of group II are precipitated as their sulphides when H2S gas is passed through their solution acidified with HCl. This is explained oh the basis of common ion effect.
Due to common ion effect of H+ ions, the degree of dissociation of H2S is suppressed, so the concentration of sulphide ions decreases. Thus the ionic product of sulphides of group II cations (Hg2+, Pb2+, Cu2+, Cd2+, Bi3+) decreases. But this is large enough to exceed the value of their solubility products and radicals of group II get precipitated.
Other groups cations are not precipitated because their Ksp are comparatively higher.
