Properties of Buffers (College Board AP® Chemistry)

Properties of Buffers

• A buffer solution is a solution which resists changes in pH when small amounts of acid or base are added

  • A buffer solution is used to keep the pH almost constant

  • A buffer can consist of weak acid–conjugate base or weak base–conjugate acid

Acidic Buffers

• A common acidic buffer solution is an aqueous mixture of ethanoic acid and sodium ethanoate

• Ethanoic acid is a weak acid and partially ionises in solution to form a relatively low concentration of ethanoate ions

CH₃COOH (aq) ⇌ H⁺  (aq) + CH₃COO^- (aq) 

ethanoic acid        ⇌                ethanoate

high conc              ⇌                 low conc

• Sodium ethanoate is a salt which fully ionises in solution

CH₃COONa + aq → Na⁺ (aq) + CH₃COO^- (aq) 

sodium ethanoate      →                 ethanoate ion

low conc.                  →                   high conc.

• There are reserve supplies of the acid (CH₃COOH) and its conjugate base (CH₃COO^–)

  • The buffer solution contains relatively high concentrations of CH₃COOH (due to the partial ionisation of ethanoic acid) and CH₃COO^– (due to the full ionisation of sodium ethanoate)

• In the buffer solution, the ethanoic acid is in equilibrium with hydrogen and ethanoate ions

CH₃COOH (aq) ⇌ H⁺  (aq) + CH₃COO^- (aq) 

high conc.                               high conc.

Basic buffers

• A basic buffer is made by mixing a solution of a weak base with its salt

  • E.g. NH₃ (aq) and NH₄Cl (aq)

  • In solution

    • NH₃ (aq) + H₂O (l) NH₄⁺ (aq) + OH^– (aq)

    • The equilibrium lies to the left as NH₃ is a weak base

  • And

    • NH₄Cl (aq) → NH₄⁺ (aq) + Cl^– (aq)

    • NH₄Cl is a soluble salt so fully dissociated in solution

• Therefore the mixture contains high concentrations of NH₃ (aq) and NH₄⁺ (aq) which will be able to react with any H⁺ and OH^– added

What should a buffer solution contain?

• Buffers are best able to resist changes in pH when there is a near-equal concentration of the conjugate acid and base (i.e. when the pH = pK_a of the acid/base pair)

  • This is because a buffer with a pK_a closest to the desired pH would be the best choice because it has the least adjusting to do in the base/acid ratio in order to reach the desired pH

  • For example, to create a buffer with a pH of 5.5 using pyridine/pyridine chloride with a pK_a value of 5.2 would be suitable 

• The ratio of [HA] to [A^–] should be close to 1:1 but buffers are considered to be effective when the ratio is anywhere between 10:1 and 1:10