Consider acidic buffer mixture,
CH3COOH ⇌ CH3COO– + H+;
CH3COONa → CH3COO– + Na+
Here, acetic acid is a weak electrolyte, in it’s solution there exists equilibrium between it’s ions and molecules. Where as sodium acetate completely dissociates into it’s ions.
Therefore, the buffer mixture contains large number of CH3COO– ions followed by Na+ , H+ , CH3COOH.
Case i) When an acid added to this solution:
H+ ion of the acid combines with CH3COO– ion in buffer solution and makes equilibrium with acetic acid.
CH3COO– + H+ ⇌ CH3COOH
As a result pH remains constant.
Case ii) When a base is added to this solution:
OH– ion of the base combines with H+ ion in buffer solution forms water molecule.
H+ + OH– → H2O
As a result pH remains constant.