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
Frequently Asked Questions (FAQs)
What is the buffer action of blood?
What are the characteristics of a buffer?
On dilution, there is no change in the pH value of the buffer.
A buffer has a definite pH value.
The pH of a buffer does not change with the addition of a small amount of base or acid.