Dissociation constants of acids and Bases
(1) Dissociation constant for weak acid: Consider an acid 
which, when dissolved in water ionizes as,
⇌ 
Applying the law of mass action, 
Where, 
is the dissociation constant of the acid, . It has constant value at definite temperature and does not change with the change of concentration.
Dissociation Constant for polybasic acid: Polybasic acids ionise stepwise as, for example, orthophosphoric acid ionises in three steps and each step has its own ionisation constant.
⇌ 
(I step)
⇌ 
(II step)
⇌ 
(III step)
Let 
and 
be the ionization constants of first, second and third steps respectively. Thus,
;
;
In general, 
The overall dissociation constant
is given by the relation,
(2) Dissociation constant for weak base: The equilibrium of 
(a weak base) can be represented as,
⇌ 
Applying the law of mass action, 
is constant at a definite temperature and does not change with the change of concentration.
