Buffer Solutions
- A buffer solution is a solution which resists changes in pH when small amounts of acids or alkalis are added
- A buffer solution is used to keep the pH almost constant
- A buffer can consists of weak acid - conjugate base or weak base - conjugate acid
Ethanoic acid & sodium ethanoate as a buffer
- A common 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
- Sodium ethanoate is a salt which fully ionises in solution
- There are reserve supplies of the acid (CH3COOH) and its conjugate base (CH3COO-)
- The buffer solution contains relatively high concentrations of CH3COOH (due to partial ionisation of ethanoic acid) and CH3COO- (due to full ionisation of sodium ethanoate)
- In the buffer solution, the ethanoic acid is in equilibrium with hydrogen and ethanoate ions
- When H+ ions are added:
- 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
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
- When OH- ions are added:
- The OH- reacts with H+ to form water
OH- (aq) + H+ (aq) → H2O (l)
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- 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)
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- 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
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