1. Strong acid – strong base titrations
Imagine we are titrating a strong acid such as hydrochloric acid against a strong base such as sodium hydroxide
HCl (aq) + NaOH (aq) ⇾ H2O (l) + NaCl (aq)
And removing the spectator ions leaves us with the ionic equation for neutralisation
H+ (aq) + OH– (aq) ⇾ H2O (l)
When [H+(aq)] = [OH–(aq)] neutralisation is complete – this is known as the equivalence point and in the case of a titration between a strong acid and and a strong base, this will also mean we have a solution with a pH of 7.
A graph showing the change in pH during a titration is called a titration curve.
2. Weak acid – strong base titrations
For example, titrating sodium hydroxide against ethanoic acid
CH3COOH (aq) + NaOH (aq) ⇾ CH3COO–Na+ (aq) + H2O (l)
Why has the first part of the curve changed?
Weak acids are not fully dissociated in solution so to begin with we have a lower [H+], a higher starting pH and the presence of both ethanoic acid molecule and ethanoate ions in an equilibrium system.
CH3COOH (aq) ⇌ CH3COO– (aq) + H+ (aq)
As both the acid (HA) and the conjugate base (A–) are present, the solution acts as a buffer initially when the OH– ions are added. These OH– ions react with the H+ ions causing the equilibrium position to shift to the right, generating more H+. As a result, the H+ ion concentration and hence the pH changes very slowly at the start of the titration.
Why is the equivalence point greater than pH7?
CH3COOH (aq) + NaOH (aq) ⇾ CH3COO–Na+ (aq) + H2O (l)
At neutralisation, [H+(aq)] = [OH–(aq)] and the reaction is complete, but the ethanoate ion hydrolyses / reacts with the water
CH3COO– (aq) + H2O (l) ⇌ CH3COOH (aq) + OH– (aq)
The presence of hydroxide ions means the neutralised solution (and equivalence point) is alkaline.
3. Strong acid – weak base titrations
In these titrations, the acid is added to the base …
HCl (aq) + NH3 (aq) ⇾ NH4Cl (aq)
Why has the first part of the curve changed?
At the beginning of the titration pH changes more slowly as the strong acid is added (compared with a strong acid – strong base titration curve). Once again, the initial weak base solution is actually a buffer system containing the weak base and its conjugate acid.
NH3 (aq) + H2O (l) ⇌ NH4+ (aq) + OH- (aq)
As H+ are added they react with the OH– present, shifting the equilibrium position to the right: pH is maintained as the H+ concentration hardly changes.
Why is the equivalence point lower than pH7?
The equivalence point is lower than pH7 because although once the neutralisation reaction is complete and [H+(aq)] = [OH–(aq)], the ammonium chloride salt formed hydrolyses / reacts with water producing acidic H3O+ ions.
NH4+ (aq) + H2O (l) ⇌ NH3 (aq) + H3O+ (aq)