Mechanism of action of a basic buffer :

(1) A basic buffer solution is a solution containing a weak base and its salt with a strong acid. The weak base dissociates feebly, but the salt dissociates completely. 

Moreover, due to the presence of the common ion, largely supplied by the salt,the dissociation of the base is further suppressed.

(NH₄OH + NH₄Cl) :

NH₄Cl_(aq) → \(NH^+_{4(aq)}\)+ \(CI^-_{(aq)}\) (Complete)

(2) When a small quantity of a strong acid is added to the solution, the hydrogen ions combine with the base producing corresponding cations and water. 

Thus,

The addition of an acid does not change the pH of the buffer.

\(H^+_{(aq)}\) + NH4OH(aq) ⇌ \(NH^+_{4(aq)}\)+ H₂O₍₁₎

This removal of added H⁺ is called reserved basicity.

(3) When a small quantity of a strong base is added, the hydroxide ions combine with \(NH^+_4\) ions to form undissociated NH₄OH. As a result, the hydrogen or hydroxyl ion concentration does not change. 

Thus,

The pH of the solution does not change.

\(OH^-_{(aq)}\)+ \(NH^+_4\)⇌ NH₄OH_(aq)

This removal of added OH^- is called reserved acidity.