Buffer Solutions (HL) (DP IB Chemistry)

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

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

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) 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) 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) NH₃ (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