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Buffer Solutions (HL) (HL IB Chemistry)

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Philippa Platt

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Buffer Solutions

  • 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.

Adding H+ ions to an acidic buffer solution

  • The equilibrium position shifts to the left as H+ ions react with CH3COO– ions to form more CH3COOH until equilibrium is re-established
  • As there is a large reserve supply of CH3COO–, the concentration of CH3COO– in solution doesn’t change much as it reacts with the added H+ ions
  • As there is a large reserve supply of CH3COOH, the concentration of CH3COOH in solution doesn’t change much as CH3COOH is formed from the reaction of CH3COO– with H+
  • As a result, the pH remains reasonably constant

Ethanoate ions reacting with hydrogen ions

Ethanoate ions reacting with hydrogen ions

When hydrogen ions are added to the solution the pH of the solution would decrease. However, the ethanoate ions in the buffer solution react with the hydrogen ions to prevent this and keep the pH constant

Adding OH– ions to an acidic buffer solution

  • The OH– reacts with H+ to form water

OH– (aq) + H+  (aq) → H2O (l)

  • The H+ concentration decreases
  • The equilibrium position shifts to the right and more CH3COOH molecules ionise to form more H+ and CH3COO– until equilibrium is re-established

CH3COOH (aq) ⇌ H+ (aq) + CH3COO– (aq)

  • As there is a large reserve supply of CH3COOH, the concentration of CH3COOH in solution doesn’t change much when CH3COOH dissociates to form more H+ ions
  • As there is a large reserve supply of CH3COO–, the concentration of CH3COO– in solution doesn’t change much
  • As a result, the pH remains reasonably constant

Ethanoic acid dissociating into hydrogen ions and ethanoate ions

ethanoic-acid-dissociating-into-ethanoate-ions-and-hydrogen-ions

When hydroxide ions are added to the solution, the hydrogen ions react with them to form water; The decrease in hydrogen ions would mean that the pH would increase however the equilibrium moves to the right to replace the removed hydrogen ions and keep the pH constant

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)rightwards harpoon over leftwards harpoon 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

Adding acid to an basic buffer

  • If H+ is added
    • NH3 (aq) + H+ (aq)rightwards harpoon over leftwards harpoon NH4+ (aq) 
    • H+ will combine with NH3 to form NH4+ so removing any added H+

Adding base to a basic buffer

  • If OH– is added
    • NH4+ (aq) + OH– (aq) rightwards harpoon over leftwards harpoonNH3 (aq) + H2O (l)
    • OH– will combine with the acid NH4+ and form NH3 and H2O so removing any added OH–
  • Therefore there is no overall change is pH if there are small amounts of acid or base are added

Examiner 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

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    Philippa Platt

    Author: Philippa Platt

    Expertise: Chemistry

    Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.