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Acid Base and Buffer solution


  • Acid are the substance which converts a blue litmus paper to red.
  • Having the PH less than seven.
  • Acid usually has a sour taste.
  • When the acid react with base to form a salts and water.
  • Example is hydrochloric acid.


  • Bass are the substance which convert red litmus paper to blue.
  • Having the PH More than seven.
  • Base usually has a bitter taste.
  • When the base react with acid to form a salt and water.
  • Example. Sodium hydroxide. 

Arrhenius theory

The concept of acid and base was first presented by the ARRENIUS in 1884. It is also known as the ARRENIUS theory of Ionization and the Electronic Association.


Acid is a substance which when dissolved in water, it gives the hydrogen ion (H+) is known as the acid.
Example is hydrochloric acid. 
HCl-------------> H+ + Cl-


Base is the substance which when dissolved in water, it gives the hydroxyl ion (OH-) is known as the base.
Example sodium hydroxide
NaOH-------------> Na+ + OH-

Limitation of Arrhenius theory:

  • The definition of acid and base are only applicable for aqueous solution, not for the substances.
  • The theory does not explain acidic and basic properties of substance in non-aqueous solvent.
  • The neutralization of acid and base in absence of solvent is not explained.
  • The basic substance which do not contain hydroxide ion not explained by the theory.

Bronsted and lowery theory:


Acid is a substance which donates proton ions (H+) in chemical reaction.
HCl  ---------> H+ + CL-


Base is a substance which accept the proton ions (H+) in chemical reaction.
OH- +H+---------------> H2O

Buffer solution:

A solution that maintain its PH when a small amount of a strong acid or a strong base is added to it. It contains a weak acid or a weak base with salt that has its conjugate pair.

Types of Buffer solution:
  • Acidic buffer solution
  • Basic buffer solution
  • Neutral buffer solution.

 Acidic buffer solution:

Acidic buffer solution has less than 7 PH (PH< 7)
An acidic buffer solution is made up of a weak acid and its salt (containing its conjugate base)

Acidic buffer system:

A solution of CH3COOH and CH3COONa
                      (Weak acid)            (Salt)

 CH3COOH----> CH3COO- + H+

(Weak acid)        (Conjugate base)                                                          

The dissociation reaction is:

CH3COOH----> H+  + CH3COO-

CH3COONa --> CH3COO-  +  Na+

CH3COONa dissociates completely and produces high concentration of CH3COO-
The high concentration of CH3COO- will disturb the equilibrium of the dissociation of ethanoic acid, CH3COOH
Equilibrium of acid shift back word less CH3COOH dissociates
Solution now has high concentration of CH3COOH (From weak base) and its conjugate base ion CH3COO-  (From salt).

Basic buffer solution:

Basic buffer solution has more than 7 PH (PH> 7)
A buffer which contains weak base and it's salts of strong acid is called basic buffer.

Basic buffer system:
  • Basic buffer system contain ammonium hydroxide/ammonia and ammonium chloride (NH4OH & NH4Cl)
  • In basic buffer solution, if small amount of HCl is added, H Ion combine with ammonium hydroxide and form ammonium ion.
H  + NH4OH--------> NH4+   + H2O
  • In basic buffer solution, if small amount of NaOH is added, Hydroxide ion (OH-) combine with ammonium hydroxide.
OH + NH4+--------> NH4OH

Neutral buffer solution:

It is  compound of salts of weak acid and weak base is called as neutral buffer.

Role of buffer's In pharmacy:
  1. Specific compound are soluble in specific medium of suitable PH the required PH adjust by buffer.
  2. Color of synthetic compound like phenolphthalein is called PH dependent.
  3. Stability of the various compounds depend upon PH for e.g. ascorbic acid and penicillin are unstable in alkaline PH.
  4. Internal and external use of preparation with appropriate PH, otherwise their will be possibility of tissue damage.


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