How does the equivalence point differ from the endpoint of a titration?
Answers
The endpoint of a titration is the point where the indicator just changes colour.
The equivalence point is when the ratio of the reactants is in the amounts specified by the equation.
Ideally you would want these points to coincide.
For a strong acid and a strong base such as NaOH and HCl the final solution is neutral at pH 7:
HCL(aq)+NaOH(aq)→NaCl(aq)+H2O(l)
Most indicators will change colour at the equivalence point so can be used in a titration.
This is not always the case though. If you neutralise a weak base with a strong acid the final solution will not be neutral e.g:
NH3(aq)+HCl(aq)→NH4Cl(aq)+H2O
This is because the ammonium ion is slightly acidic:
NH+4⇌NH3(aq)+H+(aq)
The same problem occurs when a strong base is neutralised by a weak acid. The salt produced is slightly alkaline.
This is known as "salt hydrolysis".
You need to choose an indicator which will change colour at the equivalence point. In this case methyl orange is suitable but phenolphthalein is not.