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.