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MHT-CET : Chemistry Entrance Exam

MHT - CET : Chemistry - Ionic Equilibria Page 4

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16.

Buffer Solutions

 

Buffer Solutions:
A buffer solution is one whose pH does not change on dilution or on adding a small quantity of acid or base on storage.

Acidic Buffer Solutions: Solution containing weak acid and its salt with a strong base. Used to obtain pH lower than 7.

Basic Buffer Solutions: Solution having weak base and its salt with a strong acid.

Preparation of Acidic Buffer Solution: By adding a weak acid to a solution of its salt with a strong base.

Preparation of Basic Buffer Solution: By adding a weak base to a solution of its salt with a strong acid.

Properties:

  • pH does not change on dilution.
  • pH does not alter on storage.
  • Addition of small quantity of acid or alkali does not change pH.

Buffer Action:

  1. Acidic Buffer: The salt in the acidic buffer dissociates completely and gives common ions. The weak acid dissociates feebly.

Example: (CH3COOH + CH3COONa)
CH3COOH CH3COO
- + H+ (incomplete)
CH3COONa
CH3COO- + Na+ (complete)

  • Addition of acid, hydrogen ions from acid and acetate ions combine to form weak acetic acid. Hence, pH does not change.
    H+ + CH3COO
    - CH3-COOH
  • Addition of base, OH- ions react with acid producing anions and water. Hence concentration of H+ and OH- does not change and pH is not altered.
    CH3COOH + OH
    - CH3COO- + H2O
  1. Basic Buffer: The weak base of the buffer dissociates feebly but the salt dissociates completely.
    Example: (NH4OH + NH4Cl)
    NH4OH NH4+ + OH
    - (incomplete)
    NH4Cl
    NH4+ + Cl- (complete)
  • When small quantity of acid is added, hydrogen ions from acid combine with base producing corresponding cations and water, addition of acid does not change the pH of the buffer.

    H+ + NH4OH
    NH4+ + H2O
  • When small quantity of base is added, OH- ions combine with NH4+ ions to form NH4OH.
    Hence, H+ or OH
    - concentration does not change,
    \ pH does not change.

    OH
    - + NH4+ NH4OH.

Applications of Buffer Solution:

  • pH of human blood is maintained constant at 7.35 - 7.45.
  • pH maintained at certain level for enzymatic reaction.
  • Helps in preparation of antibiotics, alcohols by fermentation.
  • Number of food and pharmaceutical samples preserved.
  • Used in qualitative and quantitative analysis.
  • pH of soil is maintained using phosphate buffer.
  • Helps processes like electro-deposition of metals, tanning of leather, paper manufacture, etc.

 

 

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