Application of solubility product and common ion effect

Qualitative analysis of cations is largely based on the principle of solubility product and common ion effect. Cations are separated in to six groups depending on the solubility of their salts.
Group-1 as insoluble chlorides
Only Ag+, Hg2+ and Pb2+ form insoluble chlorides since they have low values of Ksp.
Group-2 as insoluble sulphide in acidic medium
H2S <========> H+ + HS- ;
K1 - first ionization constant
HS- <========> H+ + S2-;
K2 – second ionization constant
[S2-] = K1K2 [H2S]/[H+]2
Ksp values of second group sulphides (PbS, CuS, SnS, HgS, As2S3, Bi2S3, Sb2S3) are very low. In acidic buffer, [S2-] is decreased due to common ion effect and this results in the precipitation of Pb2+, Cu2+ etc of second group as their sulphides. Third and fourth group sulphides have high value of Ksp, hence they remain soluble.
Group- 3 as insoluble hydroxide in basic buffer of NH4OH and NH4Cl
The concentration of OH- in ammoniacal solution decreases when NH4Cl is added to it, because of the common ion effect. Thus only for least soluble hydroxide of Fe3+, Al3+, Cr3+ etc. ionic product exceeds the corresponding solubility products, hence only these ions are precipitated. Hydroxides of successive remain soluble due to high Ksp values.

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