Question

Why does an acid-base titration curve (pH versus volume of titrant) have an abrupt change at the equivalence point?

Short answer

In an acid-base titration curve, there is an abrupt change at the equivalence point due to the absence of base and a small amount of excess acid.

Step-by-step solution

Define equivalent point.

The stage of titration where the concentrations of titrate and titrant are chemically equivalent is known as the equivalence point. The equivalence point occurs a few milliseconds before or virtually simultaneously with the endpoint.

Reason for the abrupt change of titration curve at the equivalence point.

• The number of moles of titrant added so far as an equivalency point correlates to the number of moles of the chemical in question titrated according to the stoichiometry of the reaction.
• Because the pH scale is logarithmic, the pH rises slowly at initially.
• The initial addition of the titrant to the acid (in the burette) does not result in significant modifications.
• A buffering action occurs in this relatively flat section of the pH curve.
• As the titration progresses and more base is introduced, some of the acid is reacted with the new base, but some excess acid will remain anywhere before the equivalence point, keeping the pH low.
• With the addition of a few more drops around the equivalence point, a tiny excess of acid becomes a small excess of base, and the pH quickly shifts.

Hence the pH changes abruptly at the equivalence point because the base in the solution has been titrated completely.