Procedure:

While titrating a weak acid with a strong alkali, H+ ions combine not only with OH ^- ions, but also with anions (in this case, with CH₃COO ^-  ions), as a result of which [H⁺] decreases much more rapidly at the beginning of titration, and the titration curve bends downwards much more sharply.

Such an effect of the CH₃COO ^-ion, which smoothens out the titration curve, is known as the buffering action. The value of [H⁺] in the solution of a weak acid in the presence of its salt is determined by the following formula.

[H⁺] = K _a c_acid/ c_salt

It follows in which if the acid concentration in the solution is equal to the salt concentration then

K a c acid/csalt = [H⁺]  Kacid and pH = pKacid

This means that if c_acid and c_salt have close values in such a mixture, the pH of the solution will remain constant even when considerable quantities of acid and alkali are added. Precisely such a relationship between c_acid and c_salt is observed in the case of titration of a weak acid with a strong base at the second intermediate point.