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.