titration curve
/ ty-TRAY-shun KURV /
Picture filling a bathtub one cup at a time and tracking the water level. For a long while each cup barely changes things, then suddenly the tub overflows and the level jumps. A titration curve is the same kind of story told for pH: a graph of how the pH of a solution changes as you steadily add the other reagent.
Specifically, a titration curve plots a measured property — usually pH for an acid-base titration — against the volume of titrant added. Its shape tells the tale: gentle slopes where the solution is buffered and resists change, then a steep, near-vertical jump at the equivalence point where almost all the analyte has just reacted. The midpoint of a weak-acid curve even reveals the acid's pKa directly.
The curve matters because its steep section is exactly where you read the endpoint and judge whether an indicator will fire at the right moment. The honest caveat is that the jump is sharp only when the reaction is reasonably complete and the species reasonably concentrated — for very weak acids or very dilute solutions the curve flattens, and the equivalence point becomes hard to locate by eye.
Titrating strong hydrochloric acid with strong sodium hydroxide gives a curve that idles near pH 1, then leaps almost vertically through pH 7 at the equivalence point, and settles near pH 13 — the steep middle spanning several pH units in a single drop.
The near-vertical leap marks the equivalence point where reaction is just complete.
The equivalence point is not always at pH 7. Titrating a weak acid with strong base leaves a basic salt behind, so the equivalence point sits above 7; a weak base with strong acid lands below 7.