How do buffers change pH?

1 Answer

Answer:

Buffers do not change pH, but minimize changes in pH. Buffer solutions provide a chemical mechanism by which excess acid or excess base is removed from solution.

Explanation:

A buffer solution is defined as a weak electrolyte + the salt of the weak electrolyte.

  • Acidic buffers would then be composed of a weak acid and the salt of the weak acid.

Example: acetic acid and sodium acetate.

  • Alkaline buffers would be composed of a weak base and the salt of the weak base.

Example: ammonium hydroxide and ammonium chloride. (Ammonium hydroxide is in equilibrium with ammonia, and the equilibrium is mostly skewed toward ammonia.)

Assume an acidic buffer is composed of Acetic Acid (HOAc) and Sodium Acetate (NaOAc).

The solution would contain HOAc and NaOAc in an appropriate ratio that would define a specified pH value.

  • If excess strong acid is added, the acetate ion of the sodium acetate would combine with the excess strong acid and form acetic acid thereby removing the excess from the solution.

  • If excess strong base were added, the ionic hydrogens would combine with the hydroxide ions forming water until all of the excess strong base is removed. The change in pH is very slight.

Assume a basic buffer composed of ammonium hydroxide and ammonium chloride.

The solution would contain #NH_4OH# (really, #NH_3#) and #NH_4Cl#.

  • If excess strong acid is added, the #NH_3# combines with the #H^(+)# ions to form #NH_4^(+)# and remove the excess from the solution.
  • If excess strong base is added (assumed #"OH"^(-)#), it takes a proton from ammonium to form ammonia and thus changes the excess strong base to water.