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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...

The solubilityof ionic solids in water varies depending on a number of factors like lattice enthalpy of the salt and tha solvation enthalpy of the ions in a solution. As a general rule, for a salt to be able to dissolve in a particular solvent, its solvation enthalpy must be greater than its lattice enthalpy. Each salt has its characteristic solubility, which depends on temperature. We can classify salts on the basis of their solubility in three categories. Soluble - Solubility > 0.1 M Slightly soluble - 0.01 M < solubility < 0.1M Sparingly soluble – solubility < 0.01M We have now consider the equilibrium between the sparingly soluble ionic salt and its saturated aqueous solution. A solution which remains in contact with excess of the solute is said to be saturated . The amount of a solvent () in 100 ml or 1L) to form a saturated solution at a given temperature is termed as the solubility of the solute in the solvent at that temperature. For a sparingly soluble salts like ...