If we consider the titration of an acid by a base, the pH will increase. However, if we titrate a base by an acid, the pH will decrease.
Example:
Let us consider the titration of #1M##HCl# by #1M##NaOH#.
The initial pH of the acidic solution #HCl(aq)# is around #1#. This is resulting from the high concentration of #[H^+]#.
When #NaOH# is added to the solution, the #OH^-# ions from #NaOH# will react with the #H^+# ions from #HCl# to form water #H_2O# according to the following reaction:
#H^+ +OH^(-)->H_2O#
Since the #[H^+]# is decreasing and knowing that the #pH=-log[H^+]# the #pH# will increase until neutralization where the #pH=7#. If we continue adding #NaOH# the #pH# will continue increasing until reaching a maximum value of #pH=14#.
Here is a video explaining titration of an acid by a base, including the stoichiometry.
