Solubility rules (section 28.1) and concept of solubility product help us to predict the precipitation of insoluble salts from the solution. For precipitation of the sparingly soluble salt from the solution, the ionic product of the salt must be made to exceed the value· of its solubility product constant at that temperature. For example, suppose we have an aqueous solution of lead (II) salt (say Pb (NO3)2) and we want to precipitate Pb2+ ions from the solution. Now solubility rules provides us the clue that lead (II) chloride, PbCl2 is insoluble. So addition of any solution containing Cl- ions such as HCl (aq) or NaCl (aq) will cause the precipitation of Pb2+ ions.
Solubility product constant further helps us to control the concentration of Cl- ions in such way that the value of ionic product [Pb2+] [Cl-2] must become more than the value of Ksp (solubility product constant) of PdC12 at that temperature.
Chemical reactions involving the formation of precipitate of insoluble salt by mixing aqueous solutions of soluble salts are called precipitation reactions.
Use of Precipitation Reactions in Qualitative Analysis
Precipitation reactions are quite often used in the qualitative analysis for identification of some cations as well anions in the aqueous solutions. The regent used for the precipitation of insoluble salt of the concerned ion is called precipitating agent. Let us study the precipitation reactions used for identification of some cations and anions.
Precipitation Reactions of Some Cations
The cation, its precipitating agent and the precipitation reactions are given in tabular form as follows:
Precipitation Reactions of some Anions