How to Read a Titration Curve

A titration curve is one of the most information-dense graphs in all of chemistry. Every region tells you something different about what's happening in the flask, and once you know what to look for, these curves become one of the easiest sources of exam points. I've seen students go from "I have no idea what this graph means" to "Oh, that's a weak acid with a pKa around 4.7" in about fifteen minutes of focused instruction. The trick is understanding the four regions and what each one tells you.

Region 1: Initial pH (before any titrant is added).

This first data point is diagnostic. For a strong acid, the initial pH is very low, typically around 1–2, depending on concentration. For a weak acid, it's higher, usually in the 3–5 range depending on Ka and initial concentration. If you can classify the analyte as strong or weak from the starting pH alone, you've already narrowed down what the rest of the curve should look like. Strong acids start low and have no buffer plateau. Weak acids start higher and show a gradual slope before equivalence.

Region 2: The buffer region.

This is the gradual slope between the initial point and the equivalence point. The pH changes slowly here because the solution contains significant amounts of both the weak acid (HA) and its conjugate base (A⁻), creating a buffer system that resists pH change. The more titrant you add, the more HA converts to A⁻, but the pH shifts only gradually because the buffer absorbs the added base.

The midpoint of the buffer region is the single most useful feature on any weak acid titration curve. At that exact point, exactly half the acid has been neutralized, so [HA] = [A⁻]. Plug that into the Henderson–Hasselbalch equation (pH = pKa + log([A⁻]/[HA])) and the log term becomes log(1) = 0, which means pH = pKa. If someone hands you a weak acid titration curve on an exam and asks for pKa or Ka, find the midpoint of the buffer region and read the pH. That's pKa. Free points, every time.

Region 3: The equivalence point.

The steep, nearly vertical section where pH changes rapidly over a very small volume of added titrant. At equivalence, moles of acid exactly equal moles of base: all the original acid has been neutralized. For a strong acid/strong base titration, the equivalence pH is 7.00. For a weak acid/strong base titration, the equivalence pH is above 7, typically around 8–9, because the conjugate base of the weak acid undergoes hydrolysis with water, producing OH⁻.

This is one of the most commonly tested concepts in acid–base chemistry: "Why isn't the equivalence point at pH 7 for a weak acid/strong base titration?" The answer is always hydrolysis. At equivalence, the flask contains only the conjugate base (e.g., acetate from acetic acid), which is a weak base and reacts with water to produce a basic solution.

Region 4: Post-equivalence.

After the steep rise, the curve flattens again. The excess titrant now controls the pH. For a strong base titrant, the pH approaches 13–14 with continued addition. Nothing chemically interesting happens in this region; it's simply a dilution of excess strong base. It's still useful as visual confirmation that you've passed the equivalence point.

The at-a-glance classification.

Strong acid/strong base: the curve starts very low (pH 1–2), has no buffer plateau, features a steep equivalence region centered at pH 7, and the transition is sharp and symmetric. Weak acid/strong base: the curve starts higher (pH 3–5), has a visible buffer plateau before equivalence, the equivalence point sits above pH 7, and the initial slope is more gradual. Once you've seen a few of each, you can classify a curve in seconds, and that classification immediately tells you what questions to expect and what equations to reach for.

A note on polyprotic acids.

Polyprotic acid titration curves (like H₃PO₄ or H₂CO₃) show multiple equivalence points, one for each proton that can be donated. A diprotic acid produces two equivalence points and two buffer regions, each with its own half-equivalence midpoint where pH = pKa for that specific deprotonation step. The curve looks like a staircase, with each step corresponding to the neutralization of one proton. If you see a titration curve on an exam with two steep vertical sections instead of one, don't panic; it's a polyprotic acid, and the same four-region analysis applies to each segment individually.