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 NH4ions.

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 + H2sign of equilibrium reactionCH3COO‾ + 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 H3Oions.

When a few drops of an acid are added to the above mixture solution ,the H3Oions given by the acid combine with CH3COO‾ ions to form weakly ionized molecules of CH3COOH.

CH3COO‾ + H3O———–> CH3COOH + H2O

The  H3Oion 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 H3Oion. Hence the H3Oion 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 sign of equilibrium reaction  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 H3Oion concentration or pH of the solution remains fairly constant.

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