Buffer solution
A system that minimises pH changes when small amounts of an acid or base are added. They contain 2 components: a weak acid and a conjugate base
Preparing weak acid buffer solutions
- from a weak acid and it’s salt
- by partial neutralisation of the weak acid
- 2 reservoirs to remove added acid and alkali
Conjugate base removed added acid
1) [H+] increases
2) H+ ions react with conjugate base
3) equilibrium shifts left (removes H+ ions)
Weak acid removes added alkali
1) [OH-] increases
2) small concentration of H+ reacts with OH- ions
H+(aq) + OH-(aq) -> H2O(l)
3) HA dissociates = equilibrium shifts right to restore H+ ions
Most effective buffers
When [HA(aq)] = [A-(aq)]
- pH of buffer solution is the same as the pKa value of HA
- operating pH is over about 2pH units, centred at the pH of the pKa value
[H+] calculation for buffers
[H+(aq)] = Ka x [HA(aq)]/ [A-(aq)]
Equivalence point
The volume of one solution that exactly reacts with the volume of the other solution. The solutions have reacted exactly and the amounts used match the Stoichiometry of the reaction
End point
Indicator contains equal concentrations of acid and conjugate base = colour shows inbetween the 2 extremes
Indicator
Weak acid- equilibrium shifts towards the weak acid in acidic conditions and towards the conjugate base is basic conditions
On addition of a basic solution containing OH- ions to an indicator
-OH- reacts with H+ in indicator
H+ + OH- -> H2O
- weak acid dissociates = equilibrium shifts right
- colour change to end point colour then conjugate base colour
On addition of an acid to an indicator
- H+ reacts with conjugate base
- equilibrium shifts left
- colour change to end point colour and then acid colour
What indicator is suitable for a weak acid-weak base titration
No indicator
Strong acid/strong base titration
- starts at pH0
- ends at pH 14
- steep in the middle
- pH at equivalence is very close to pH 7
Strong acid/weak base titration
- starts at pH 0
- ends at pH 9-10
- slight incline at the end (when buffer is set up)
- pH at equivalence is acidic
Weak acid/ weak base titration
- starts at pH 3-4
- ends at pH 9-10
- steep in the middle
- pH at equivalence point is close to pH 7
Weak acid/ strong base titration
- starts at pH 3-4
- ends at pH 14
- slight incline at the start then flattening out
- steep in the middle
- pH at equivalence is alkaline
