Chemistry
Understanding Acid-Base Titration Curves and Endpoints
Quick fact
The steepest part of a titration curve occurs near the equivalence point, and a single drop of titrant can change the pH by several units—this is why indicator selection is so crucial.
Why this is interesting
You've probably seen a pH meter jump dramatically during a titration—but why does the pH stay almost constant for a while and then suddenly change? What's really happening inside the flask at that moment?
Read the full explanation
Understanding Understanding Acid-Base Titration Curves and Endpoints
Imagine you are slowly adding a strong base (like NaOH) to a strong acid (like HCl). Initially, the solution is very acidic. As you add base, it neutralizes some acid, forming water and salt. The pH rises only slightly at first because the solution resists change—this is the buffer region. But once nearly all the acid is neutralized, the pH surges upward with just a tiny extra drop of base. If you plot pH on the vertical axis and volume of added base on the horizontal axis, you get an S-shaped curve. The point where the curve is steepest is the equivalence point, where the moles of acid and base are equal. The endpoint is the point where an indicator changes color, which we try to make coincide with the equivalence point by choosing an indicator that changes color in the pH range of that steep rise.
A deeper explanation
The shape of a titration curve is governed by the equilibrium between the acid and its conjugate base. For a strong acid-strong base titration, the curve is symmetric because both dissociate completely. The initial pH is low, and the pH change is gradual until near the equivalence point, where the concentration of H⁺ or OH⁻ becomes extremely small and a tiny addition of titrant causes a huge relative change. For weak acid-strong base titrations, the curve starts at a higher pH and has a more gradual initial rise due to buffering by the undissociated acid and its conjugate base. The equivalence point in that case is above pH 7 because the conjugate base is slightly basic. The endpoint is detected using an indicator that changes color within the pH range of the vertical part of the curve. Understanding this mechanism allows chemists to accurately determine unknown concentrations and to choose the correct indicator or instrument for precise titrations.