The product of ionic concentration of a sparingly soluble salt in its saturated solution at a fixed temperature is called its solubility product. It is denoted by Ksp, The value of Solubility Product is constant for a given temperature.
Consider a slightly soluble ionic salt, AgCL, is dissolved in water. The dissociation of
following salt is as follows:
Agcl ====== Ag- + Cl-
These ions from solid phase pass into solution till the solution becomes saturated.
Now there an equilibrium exists between undissolved and dissolved particles.
Applying the Law of Mass Action:
Kc = [Product] / [Reactant]
Kc = [Ag+] [Cl-] / [Agcl]
Kc x [Agcl] = [Ag+] [Cl-]
Since the concentration of solid AgCl in the solid phase is fixed:
Kc X [AgCl] = constant = Ksp
Ksp = [Ag+] [Cl-]
Ksp = [+ve ionl [-ve ion]
Where Ksp is known as Solubility Product.
Application of Solubility Product:
The value of solubility product, Ksp, represent the equilibrium condition between the dissolved and undissolved solid phase of solute when the solution is saturated. The value of Ksp is used to determine whether a precipitation should form from a solution of known ionic concentration.
For example, the solubility product of calcium sulphate solution is 2.4 x 1O-5 mole/dm³. Its dissociation will be as follows:
CuSO4 ============= Cu+2 + SO4‾²
If the ionic product is less than the Solubility Product.
[Cu+²][SO4‾²] < Ksp
Then the solution is not saturated and more amount of solute can be dissolved in the solution till the product becomes equal to 2.4 x 10-5 mole/dm³.
If ionic product is larger than the Solubility Product.
[Cu+²][SO4‾²] > Ksp
The solution is set to be super saturated and excess of solute should precipitate so as to restore the equilibrium condition.
Solubility Product plays a vital role in salt analysis by common effect. Chemical Yields in manufacturing processes can be controlled and greatly increased by making use of common ion effect.