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

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

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.

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

Remember that buffer solutions cannot cope with excessive addition of acids or alkalis as their pH will change significantly
The pH will only remain relatively constant if small amounts of acids or alkalis are added

Which of the following combinations would form a buffer solution

A. KI and PbNO₃

B. NH₃ and NH₄NO₃

C. HCl and HNO₃

D. NaOH and KOH

Answer

The correct answer is B, because:
- This pair does form a buffer
- NH₃ is a weak base and NH₄⁺ is a significant acid (and its conjugate acid) meaning this pair forms a buffer.
- The NO₃^– ion is a spectator ion
A is incorrect because these are both salts and no acids or bases are present
C is incorrect because HCl and HNO₃ are both strong acids
D is incorrect because NaOH and KOH are both strong bases

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

Properties of Buffers (College Board AP Chemistry)