The graph (A) shows how pH changes during the titration of 50 cm3 of 0.1 M HCl with 0.1 M NaOH.
NaOH (aq) + HCl (aq) → NaCl (aq) + H2O (l)
The pH of 0.1 M solution of HCl in the beginning would be 1. As alkali is added, the pH changes slowly in the beginning. However, at the equivalence point pH changes rapidly from about 3.5 to 10. It can be shown by simple calculations that pH of the solution is 3.7 when 49.8 cm3 of 0.1 M NaOH solution have been added. The pH suddenly changes to 10 after addition of 50.1 cm3 of the NaOH solution. Thus, any indicator having pH range between 3.5 to 10 will identify the equivalence point. This means that any one of phenolphthalein, methyl orange or bromothylmol blue could be used as an indicator.
Titration curves : (A) strong base with strong acid ;
(B) weak base with strong acid ;
(C) strong base with weak acid ;
(D) weak base with weak acid.
