Every titration comes down to one moment: the moment you stop adding liquid and read the number. Stop too early and you have undercounted. Stop too late and you have overcounted. The chemistry has already decided where the right moment is, but you cannot see the chemistry. You can only see a signal, and you have to judge from it.
The sudden jump
Chemists measure how acidic a solution is with a scale called pH. If you measured the pH of a sample all the way through a titration and drew it on a graph, you would get a stretched S-shape. For a long time, the pH changes only slowly. Then, right near the end, it shoots up. A single drop can swing it by a huge amount. After the jump, it flattens out again.
That jump is where the right answer lives. The exact point you are looking for sits at the steepest part of it. Every way of finding the endpoint is really aiming at that steep stretch.
Colours that change
The oldest way is a colour indicator, a dye that is one colour in acid and another in base. The famous one is phenolphthalein: colourless in acid, turning pink once the solution tips just past neutral into alkaline. The trick is choosing a dye whose colour change happens right inside the jump. Then the colour flips within a drop or two of the right answer.
Pick the wrong dye and it changes too early or too late. A common mistake is assuming every titration ends at "neutral". Many do not. When a weak acid like vinegar is titrated, the right answer lands on the alkaline side, so you need a dye that changes there.
Letting a probe find it
The other way is an electrode, a probe that reads the solution continuously. Instead of a colour, it gives a stream of numbers, and the right answer is where those numbers are climbing fastest. Modern automatic titrators do this arithmetic themselves and report the volume. That is convenient, but it is worth knowing that they decide what counts as "the jump" using settings someone chose. Settings that work well for one sample can miss a small jump in another, or mistake a noisy blip for the real thing. When a result looks odd, look at the curve, not just the number.
When there is no clear jump
Some titrations barely jump at all. Weak acids and very dilute samples give gentle curves, because the solution resists change for most of the titration. The weaker the acid, the softer the jump, until there is nothing sharp left to find. In those cases, a colour is a poor guide and even a probe struggles. The answer is to change the conditions so the jump gets bigger, for example a more concentrated sample or a different solvent, not to trust a curve that barely bends.
Why this matters
It is easy to think of the endpoint as the boring last step. It is actually the step where the whole measurement is won or lost. A well-chosen signal turns a slow, invisible reaction into a moment you can see, and lets you stop exactly when the chemistry says you should.
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References
- D. Harvey, Analytical Chemistry 2.1, 9.2 Acid-Base Titrations (LibreTexts).
- University of Calgary, Chemistry Open Textbook, "Acid-Base Colour Indicators".
- Metrohm, tiamo 2.4 manual, endpoint-recognition parameters.