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.