22.3    Buffer Solutions

What is a buffer solution?

• A buffer solution is a solution whose pH does not change significantly when small amount of acid or base is added to it

 

• Buffer solutions should therefore contain:
  1. large amount of acid to react with the base added
  2. large amount of base to react with the acid added

• Since the added acid and base can be removed, the pH does not change significantly

 

• These conditions can be achieved if the solution contains a mixture of a weak acid and its conjugate base, or a weak base with its conjugate acid.

 

• There are two types of buffer solutions, named acidic buffer solution and alkaline buffer solution

 

Acidic buffer solution

• An acidic buffer solution is acidic and can be made by mixing a weak acid and its conjugate base together. Take ethanoic acid and ethanoate ion as an example

 

• Sodium ethanoate is added to ethanoic acid. Sodium ethanoate dissociates completely according to the equation:
  CH3COONa → CH3COO⁻ + Na⁺
  …while ethanoic acid dissociates partially according to the equation:
  CH3COOH ⇌ CH3COO⁻ + H⁺

• The presence of ethanoate ions causes the equilibrium position to shift to the left. The solution now contains:
  • large reservoir of  ethanoic acid molecules.
  • large reservoir of ethanoate ions.
  • enough hydrogen ions to make the solution acidic
• When acid is added:
  1. The hydrogen ions from the acid added will react with the large reservoir of ethanoate ions.
     H⁺ + CH3COO⁻ → CH3COOH
  2. The extra hydrogen ions are removed, and the pH drops very little
• When base is added:
  1. The hydroxide ions from the base added will react with the large reservoir of ethanoic acid molecules.
     OH⁻ + CH3COOH → CH3COO⁻ + H2O
  2. The extra hydroxide ions are removed, and the pH increases very little

 

Alkaline buffer solution

• An alkaline buffer solution is alkaline and can be made by mixing a weak base and its conjugate acid together. Take ammonia and ammonium ion as an example

 

• Ammonium chloride is added to Ammonium chloride dissociates completely according to the equation:
  NH4Cl → NH4⁺ + Cl⁻
  …while ammonia ionises partially according to the equation:
  NH3 + H2O ⇌ NH4⁺ + OH⁻

• The presence of ammonium ions causes the equilibrium position to shift to the left. The solution now contains:
• large reservoir of  ammonia molecules.
• large reservoir of ammonium ions.
• enough hydroxide ions to make the solution
• When acid is added:
  1. The hydrogen ions from the acid added will react with the large reservoir of ammonia molecules.
     H⁺ + NH3 → NH4⁺
  2. The extra hydrogen ions are removed, and the pH drops very little
• When base is added:
  1. The hydroxide ions from the base added will react with the large reservoir of ammonium ions.
     OH⁻ + NH4⁺ → NH3 + H2O
  2. The extra hydroxide ions are removed, and the pH increases very little

 

Calculating the pH of  buffer  solutions

• For an acidic buffer solution:
  CH3COOH ⇌ CH3COO⁻ + H⁺

1. Make the following assumptions:
   • Concentration of ethanoate ions is equal to the concentration of sodium ethanoate added. This is done by ignoring the negligible amount of ethanoate ions coming from the dissociation of ethanoic acid
   • Concentration of ethanoic acid is equal to its original concentration. This is done by ignoring the negligible amount of ethanoic acid dissociated
2. The fta expression of ethanoic acid now becomes:
3. Given the value of fta of ethanoic acid, concentration of hydrogen ions and hence the pH of the buffer solution can be found

• For an alkaline buffer solution:
  NH3 + H2O ⇌ NH4⁺ + OH⁻

1. Adjust the point view to the ammonium ion rather than ammonia. This is possible because ammonium ions are weak acids. Therefore:
   NH4⁺ ⇌ NH3 + H⁺
2. Make the following assumptions:
   • Concentration of ammonium ions is equal to the concentration of ammonium chloride added. This is done by ignoring the negligible amount of ammonium ions coming from the ionisation of ammonia
   • Concentration of ammonia is equal to its original concentration. This is done by ignoring the negligible amount of ammonia ionised.
3. The fta expression of ammonium ion now becomes:
4. Given the value of fta of ammonium ion, concentration of hydrogen ions and hence the pH of the buffer solution can be found

 

Importance of buffer solutions in biological systems

• Most biological reactions(especially those involving enzymes) are very sensitive to pH, since enzymes can only function within a narrow range of pH

 

• For example, if the pH of human blood(pH 7.4) is changed by as little as 0.4 unit, it could prove fatal

• There are two important buffer systems in the human blood:
  1. The carbonic acid/hydrogen carbonate system
  2. The phosphate system

• The carbonic acid/hydrogen carbonate system:
  1. The following equilibrium is involved:
     H2CO3 ⇌ H⁺ + HCO3⁻
  2. When a little acid is added:
     H⁺ + HCO3⁻ → H2CO3
  3. When a little base is added:
     OH⁻ + H2CO3 → HCO3⁻ + H2O

• The phosphate system:
  1. The following equilibrium is involved:
     H2PO4⁻ ⇌ HPO4²⁻ + H⁺
  2. When a little acid is added:
     H⁺ + HPO4²⁻ → H2PO4⁻
  3. When a little base is added:
     OH⁻ + H2PO4⁻ → HPO4²⁻ + H2O