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
Exam Tip
- 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
Worked example
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
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 = pKa of the acid/base pair)
- This is because a buffer with a pKa 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 pKa 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