21 - Buffers and Neutralisation

21 - Buffers and Neutralisation

1. Buffer solutions

   • = a system that minimises pH changes when small amounts of an acid or a base is added
   • weak acid (HA) removes added alkali
   • conjugate base (A-) removes added acid

2. Preparing weak acid buffer solutions

   • Preparation from a weak acid and its salt
     • CH3COOH (aq) <---> H+ (aq) + CH3COO- (aq)
       • Acid partially dissociates in water + amount of ethanoate ions v. small
     • CH3COONa (s) + aq <---> CH3COO- + Na+
       • Salt of weak acid dissolves in water to form A-
     • CH3COOH = HA CH3COO- = A-
   • Preparation by partial neutralisation of weak acid
     • Alkali (aq) + excess HA, partially neutralises HA, forming A-, left over acid is unreacted
   • Two reservoirs to remove added acid and alkali
     • CH3COOH (aq) <---> H+ (aq) + CH3COO- (aq)
     • Equilibrium can shift to left or right

3. Action of buffer solution

   • A- removes added acid
     • Addition of H+ (aq): 1. [H+ (aq)] increases; 2. H+ (aq) react with A-; 3. Equilibrium shifts to left, removing most H+ (aq)
   • HA (aq) <---> H+ (aq) + A- (aq)
   • HA removes added alkali
     • Addition of OH- (aq): 1. [OH- (aq)] increase; 2. H+ (aq) + OH- (aq) ---> H20 (l); 3. HA dissociates, shifting equilibrium to right, restoring most H+ (aq)

4. Calculating pH of a buffer solution

   • Equilibrium + Ka for HA:
     • HA (aq) <---> H+ (aq) + A- (aq)
     • Ka = ( [H+ (aq)][A- (aq)] ) / [HA (aq)]
   • to work out pH of buffer solution:
     • [H+ (aq)] = Ka x [HA (aq)] / [A- (aq)]
   • If [HA (aq)] = [A- (aq)]
     • Ka = [H+ (aq)]
     • pKa = pH

5. Controlling blood pH

   • Acidosis = blood pH below 7.35
   • Alkalosis = blood pH above 7.45
   • Carbonic acid - hydrogen carbonate buffer system
     • H2CO3 (aq) <---> H+ (aq) + HCO3- (aq)
     • A- (aq) = HCO3- (aq)

6. pH titration curves

   • Equivalence point = The point in a titration at which the volume of one solution has reacted exactly with the volume of a second solution
   • End point = The point in a titration where the indicator changes colour; the end point indicates when the reaction is just complete
   • Indicator colour changes work in similar way to buffers
   • At end point: [HA (aq)] = [A- (aq)], Ka = [H+ (aq)], pKa = pH

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