Question

What species must be present in a buffer? Write a chemical reaction equation starting with the components in the buffer (acetic acid and acetate anion). Explain how the buffer reacts with acid and how it reacts with base, and how the pH is affected in both cases.

Answer 1

Buffer consists of weak acid and its conjugate base in large proportion or a weak base and its conjugate acid.

Let us consider weak acid be HA that ionizes in water to form hydronium ion and conjugate base of the acid.

HA(aq) + H₂O(l) ---> H₃O⁺(aq) + A^-

   weak acid water    hydronium ion conjugate base

Acetic acid is a weak acid (Ac - CH₃COOH) and its salt that contain conjugate base (Ac^- - CH₃COO^-) ions. These buffers can resist change in pH.

  CH₃COOH(aq) + H₂O(l) ----> H₃O⁺(aq) + CH₃COO^-

Acetic acid    water hydronium ion acetate ion

When buffer reacts with the acid then the conjugate base reacts with the acid and forms weak acid

   A^- (aq) + H₃O⁺(aq) ----> HA(aq) + H₂O(l)

conjugate base hydronium ion weak acid water

(acid)

When an acid (H₃O⁺) is added to a buffer solution then the conjugate base reacts as above. The decrease in the pH after addition of acid is balanced by the reaction of conjugate base with the acid added.

But this results in decrease of pH very slightly i.e. very less change in pH when buffer is nor present and it is insignificant.

When a base (OH^-) ions are added to a buffer solution then the weak acid reacts with the hydroxide ions or base added to form water and conjugate base.

HA(aq) + OH^- (aq) -----> A^- (aq) + H₂O(aq)

   weak acid hydroxide ions conjugate base water

   (base)

This increase in concentration of conjugate acid and decrease in weak acid, the change in the pH (increase ) after addition of base is balanced by the reaction of weak acid with the base added and maintains the pH.

So, pH is not much affected in the both the cases.