Buffer Action

Buffer action, in general, is defined as the ability of the buffer solution to resist the changes in pH value when a small amount of an acid or a base is added to it.

Mechanism of Buffering Action

To understand the mechanism of buffer action, we can take the example of an acidic buffer that is made up of a weak acid like acetic acid and its sodium salt, sodium acetate. In this acidic buffer, the solution contains equimolar amounts of acetic acid and sodium acetate. Usually, a large number of sodium ions (Na⁺), acetate ions (CH₃COO^–) and undissociated acetic acid molecules are present.

The salt exists completely as ions.

Here, the buffer will consist of both acid (CH₃COOH) and its conjugate base (CH₃COO^–). If we add a small quantity of acid, the hydrogen ions will be removed by the conjugate base (CH₃COO^–). It is represented as follows:

H+ (aq) + CH3COO– (aq) ↔ CH3COOH (aq)

Here, the ethanoic acid will only be slightly dissociated in the form of CH₃COOH, which means that it will not contribute any H+ ions. Therefore, the pH of the resulting solution will remain more or less constant. The added H+ ions are also removed, due to which there is no appreciable decrease in pH.

Also Read: Buffer Solutions

The reaction comes to a completion as CH₃COOH is a weak acid whose ions have a strong tendency to form non-ionised CH₃COOH molecules. On the other hand, if we add a strong base, the OH^– ion gets neutralised by the reaction with the acid in the buffer,

CH₃COOH (aq) + OH– (aq) → CH₃COO– (aq) + H₂O (l).

We can also consider that the OH^– ion can react with the H+ ion in order to form water. The OH^– ions that are added are removed, wherein the acid equilibrium shifts to the right to replace the H+ ions that are exhausted. This results in a minor change in the pH value.

Alternatively, if we add a drop of NaOH, the OH^– ions react with the free acid to give undissociated water molecules. The extra OH^– ions of the base are neutralised. As a result, the pH of the solution remains the same. This condition, or the resistance offered by the pH when a base is added, is known as reserve acidity. This is mainly due to CH₃COOH.

If we add a strong base, the acid present in the buffer neutralises the hydroxide ions (OH^–).

So, we can say that when acid or base is added, its effect is practically balanced, and the pH of the solution is always constant.

Key Points to Remember

If we take a solution, the salt will be completely ionised, and the weak acid will be partly ionised.

• CH₃COONa ⇌ Na+ + CH₃COO–
• CH₃COOH ⇌ H+ + CH₃COO–

On Addition of Acid and Base

1. When acid is added, the protons of acid that are released will be removed by the acetate ions to form an acetic acid molecule.

H⁺ + CH₃COO^– (from added acid) ⇌ CH₃COOH (from buffer solution)

2. When a base is added, the hydroxide that is released by the base will be removed by the hydrogen ions to form water.

HO– + H+ (from added base) ⇌ H₂O (from buffer solution)

Buffer Action – Video Lesson