Buffers

  • A buffer solution is one which resists changes in pH when small quantities of an acid or alkali are added to it.
  • Buffers are commonly made up of a weak acid and its conjugate base or a weak base and its conjugate acid.
  • They are used when the pH of a system needs to be kept constant, e.g. in the human body
  • Acid buffers have a pH less than 7; alkaline buffers have a pH greater than 7

A buffer solution contains substances that will remove any hydrogen ions or hydroxide ions put into the system, or replenish them should they be removed. This is because it reacts with them.

Therefore, the concentration of hydrogen ions would change only minimally, and thus the pH too.

Buffer capacity is the effectiveness of a buffer to resist change.

  • If you keep on adding acid to a buffer, eventually its pH will change
  • This is because the substances that buffer the solution are all used up
  • Most effective buffers are made up of stoichiometric ratios of each solution.
Example

A solution of ethanoic acid and sodium ethanoate is a typical buffer

CH3COOH(aq) ⇌ CH3COO (aq) + H+(aq)

It has excess ethanoic acid AND excess ethanoate ions.

Adding an acid
  • The buffer must remove hydrogen ions, otherwise pH would drop.
  • If more H+ ion is added, by LCP, the equilibrium will shift to the left.
  • Hence, much of the added H+ ion will be removed, creating more ethanoic acid.
  • However, the change will not be completely opposed, so the pH will drop slightly.
Adding a base

There are two process where the buffer gets rid of hydroxide ions

Reaction with ethanoic acid
  • CH3COOH(aq) + OH(aq) ⇌ CH3COO(aq) + H2O(l)
  • A lot of the hydroxide ions will be removed, since the Kc in this reaction is far to the right.
Reaction with hydrogen ions
  • H+ + OH –> H2O
  • This removes both hydroxide ions and hydrogen ions from the system, so the equilibrium shifts right to replace the hydrogen ions.
  • Not all of the hydroxide ions are removed though, so the pH would increase slightly.

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