Equilibrium :  Buffers

Buffer - A mixture of a conjugate acid-base pair that can resist changes in pH when small amounts of strong acids or bases are added

Conjugate Acid-Base Pairs

The part of the acid remaining when an acid donates a H+ ion is called the conjugate base.  The acid formed when a base accepts a H+ ion is called the conjugate acid.  For the generic acid HA:

For the generic base A-:

More examples of conjugate acid-base pairs:

Four ways to prepare a buffer solution

  • The most common preparation method for a buffer solution is combining a weak acid with its conjugate base.  The conjugate base comes from an aqueous salt which dissociates in water to give the base.
  • Though less common, the exact opposite of the first method can be done by combinding a weak base with its conjugate acid.
  • A third way to make a buffer solution is to start with a weak acid and add half as many moles of strong base.
  • Conversely, a buffer solution can also be prepared by starting with a weak base and adding half as many moles of a strong acid.

The main thing with any of the above preparation methods is that the starting solution is a WEAK acid or base or else the starting acid or base would already dissociate 100%.  Both components of a conjugate acid-base pair must remain in the solution to be able to neutralize any added acid or base.

  • The ACID neutralizes the OH- ions formed if a strong base is added.
  • The BASE will neutralize the H3O+ ions formed if a strong acid is added.

Example of a buffer solution

Suppose there is a buffer solution with acetic acid and its conjugate base, the acetate ion.

[Image]    
When strong
base is added
  
When strong
acid is added