The Same Burette Reading Gives Two Different Concentrations

The endpoint is not the equivalence point

The equivalence point is where the reaction is stoichiometrically complete; the endpoint is where the indicator changes colour, and they are never quite the same place. A good indicator changes within a fraction of a drop of equivalence and the gap is smaller than the burette can resolve — a badly matched one changes half a millilitre early or late, which on a 24.80 mL titration is a two per cent error that no amount of careful reading will recover. Choosing the indicator to match the pH at equivalence is therefore part of the measurement rather than a detail of procedure, and running a blank titration on the solvent alone is how you find out whether the two points have drifted apart.

The titrant's own concentration is a measurement too

Typing 0.1 M for the titrant treats it as exact, and a bottle labelled 0.1 M rarely is. Sodium hydroxide absorbs carbon dioxide from the air and drifts down over weeks; a solution made up by weighing a hygroscopic solid starts off wrong. That is why analytical work standardises the titrant against a primary standard first — a solid that can be weighed accurately, does not take up water and reacts cleanly — and then uses the standardised figure rather than the label. Every per cent of error in that number passes straight through to the answer, undiminished, because the calculation multiplies by it.

Frequently asked questions

How precisely can a burette be read?

A standard 50 mL burette is graduated every 0.1 mL and is read to half a graduation by estimating between marks, which puts the reading uncertainty around 0.05 mL. Two readings are needed — before and after — so the delivered volume carries roughly 0.07 mL of uncertainty, about 0.3 per cent on a 24.80 mL titration. That is comfortably better than the errors introduced by an ill-chosen indicator or an unstandardised titrant, which is why those two dominate the result and the eye at the meniscus rarely does.

Why titrate at all instead of weighing?

Because you usually cannot weigh what you want to know. The acid in a bottle of vinegar, the free chlorine in a pool, the hardness of tap water and the acidity of a wine are all dissolved in something else and cannot be isolated onto a balance without changing them. A titration measures a quantity in place, by counting how much of a known reagent it consumes, and it does so with equipment that costs almost nothing and calibration that comes from a single weighed standard. That combination is why the technique survives in industry a century after more elaborate instruments arrived.