Buffer solution and Buffer Action

Buffer Solution

A buffer solution is defined as a solution which resist any change in its pH value even when small amount of acid or base are added to it.

Types of the buffer solution

1) Solution of single substance

The solution of the salt of weak acid and weak base eg : ammonium acetate ( CH₃COOH) act as a buffer.

2) Solution of Mixture:

These are of 2 types:

Acidic buffer : It is the solution of mixture of the weak acid and a salt of this weak acid with a strong base.

For Ex: CH₃COOH + CH₃COONa or HCOOH + HCOONa

Basic buffer : It is the solution of a mixture of weak base and a salt of this  weak base with a strong acid.

For Ex: NH₄OH + NH₄Cl  or NH₄OH + NH₄NO₃

Buffer Action

The property of a buffer solution to resist any change in its pH value even when small amount of the acid or the base are added to it is called Buffer action.

 

Buffer action of ammonium acetate solution

Ammonium acetate is almost completely dissociated in the aqueous solutions as follow:

CH₃COONH₄ ————> CH₃COO‾ + NH₄⁺

In the solution , there is excess of CH₃COO‾  ions and NH₄⁺ ions.

When a few drops of an acid are added to the above solution, the H₃O⁺ ions given by the acid combine with the CH₃COO‾  ions to form weakly ionized molecule of CH₃COOH.

CH₃COO‾ + H₃O⁺—————>CH₃COOH + H₂O

The H₃O⁺ ion concentration of the solution does not change practically and hence the pH of the solution remains almost constant.

When a few drops of base are added to the above solution the OH‾ ions given by the base combine with NH₄⁺  ions to form some weakly ionized molecules of NH₄OH.

NH₄⁺ + OH‾ ———-> NH₄OH

The OH‾  ion concentration and hence the H₃O⁺  concentration or the pH of the solution remains almost constant.

Buffer Action of acidic buffer

Let us consider the buffer action of acidic buffer containing CH₃COOH and CH₃COONa. Acetic acid dissociates to a small extent whereas sodium acetate is almost completely dissociated in aqueous solution as:

CH₃COOH + H₂O CH₃COO‾ + H₃O⁺          —————-1

CH₃COONa ———-> CH₃COO‾ + Na⁺

By common ion effect, the ionization of CH₃COOH is further suppressed. Thus in the solution, there are excess of acetate ions and a small amount of H₃O⁺ ions.

 

When a few drops of an acid are added to the above mixture solution ,the H₃O⁺ ions given by the acid combine with CH₃COO‾ ions to form weakly ionized molecules of CH₃COOH.

CH₃COO‾ + H₃O⁺ ———–> CH₃COOH + H₂O

The  H₃O⁺ ion concentration and hence the pH of the solution remains almost constant.

When a few drops of a base are added, the OH‾  ions given by the base combine with the H₃O⁺ ions already present to form weakly ionized molecules of H₂O.

H₃O⁺ + OH‾ ———–>2H₂O

As H₃O⁺ ions are consumed ,the equilibrium 1 shifts towards right. Thus more of CH₃COOH dissociates to make up the loss of H₃O⁺ ion. Hence the H₃O⁺ ion concentration or pH of the solution does not change.

Buffer action of basic buffer

The buffer action of a basic buffer i.e. NH₄OH + NH₄Cl may be explained as:

NH₄OH dissociates to a small extent whereas NH₄Cl dossociates completely in aqueous solution as:

NH₄OH   NH₄⁺ + OH‾  —————— 1

NH₄Cl ————-> NH₄⁺ + Cl‾

By common ion effect the ionization of NH₄OH is further suppressed. Thus, in the solution ,there are excess of NH₄⁺ ions and a small amount of OH‾ ions.

When a few drop of a base are added the OH‾ ions given by it immediately combine with NH₄⁺ ions to for weakly ionized NH₄OH.

NH₄ ⁺ + OH‾ ———> NH₄OH

The H₃O⁺ ion concentration or pH of the solution remain unaffected.

When a small amount of acid is added ,the H₃O⁺ ions  given by its combine with OH‾ ions already produced by NH₄OH in equilibrium 1.

As the OH‾ ions are consumed,  the equilibrium 1 shifts in forward direction. More of NH₄OH dissociates  to produce more of OH‾ ions which makes up the loss of OH‾ ion and concentration therefore, the H₃O⁺ ion concentration or pH of the solution remains fairly constant.