The reaction that takes place between an acid and an alkali is called neutralisation.
It is possible to calculate the concentration of an acid or an alkali using titration:
- the concentration of an acid can be worked out by titrating it against a standard alkali solution
- the concentration of an alkaline can be worked out by titrating it against a standard acid solution
How an acid-alkali mixture behaves depends on how strong or weak the acids and bases involved are.
To test, drops of NaOH are used. The end-point is when a pH of 7 is reached. The drops can then be measured before and after this point.
pH changes during a strong acid – strong alkali titration: there is a change of approximately 4 to 10 two drops at the end-point.
pH changes during a weak acid – strong alkali titration: there is a change of approximately 7 to 10 over two drops at the end-point.
At the beginning of the titration the pH increase is quite rapid until the point is reached where enough salt has been formed for an effective buffer solution.
pH changes during a strong acid – weak alkali titration: there is a change of approximately 4 to 7 at the end-point.
pH changes during a weak acid – weak alkali titration: throughout the titration this mixture will behave like a buffer solution. Therefore, at the end-point there is no dramatic pH change. Usually the end-point is around 7 but this depends on the relative strengths of the acid and alkali involved.
Half-neutralisation
When exactly half the volume of alkali needed for neutralisation has been added, a weak acid is said to be half-neutralised.
At this stage the mole number (or concentration) of the salt and the acid will be equal: [HA] = [A–]
As: Ka = [H3O+][A–] / [HA]
Then: [H3O+] = Ka[HA] / [A–]
So: if [HA] = [A–] then [H3O+] = Ka which means pH = pKa
Therefore at half-neutralisation: [H3O+] = Ka and pH = pKa
It is possible to figure out the Ka of an acid from a graph of pH against the volume of alkali added: the pH of the mixture after exactly half of alkali amount has been added is equal to the pKa of the acid.
Titrations with polybasic acids
More than one stage occurs when a polybasic acid is titrated against a strong alkali. However, these reactions occur one after the other, each having its own end-point. Therefore, the reaction has two end-points.