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 + , Hg 2+ and Pb 2+ form insoluble chlorides since they have low values of K sp . Group-2 as insoluble sulphide in acidic medium H 2 S <========> H + + HS - ; K 1 - first ionization constant HS - <========> H + + S 2 - ; K 2 – second ionization constant [S 2 - ] = K 1 K 2 [H 2 S]/[H + ] 2 K sp values of second group sulphides (PbS, CuS, SnS, HgS, As 2 S 3 , Bi 2 S 3 , Sb 2 S 3 ) are very low. In acidic buffer, [S 2 - ] is decreased due to common ion effect and this results in the precipitation of Pb 2+ , Cu 2+ etc of second group as their sulphides. Third and fourth group sulphides have high value of K sp , hence they remain soluble. Group- 3 as insoluble hydroxide in basic buffer of NH 4 OH and NH 4 Cl The concentration of OH - in a...