Picture a jar of marbles and someone asks you how many are inside. You could pour them out and count them one by one. Or, if you knew exactly how much one marble weighs, you could weigh the jar and do a little arithmetic. Chemists face the same problem every day, except the marbles are molecules, far too small and far too many to count. A titration is one of the oldest and most reliable ways we have of counting them anyway.
How it works
You take a measured amount of your sample, say a little vinegar, and you slowly add a second liquid whose strength you know exactly. In the case of vinegar, that liquid is a base, and it reacts with the acid in the vinegar. Every bit of base you add uses up a matching bit of acid (for vinegar, one for one; other reactions have their own fixed ratio). At some point the acid is all gone. You stop, and you read how much base you added.
Because you know how much vinegar you started with, how strong the base was and how much of it you used, you know how much acid there was. That is the whole idea. A titration counts the molecules of one substance by reacting them with a measured amount of another. The reaction is the counting rule; the volume you add is the count.
Knowing when to stop
The tricky part is knowing the exact moment the acid runs out, because you cannot see molecules. So we use a signal. Sometimes it is a colour: a few drops of an indicator that stays colourless while there is acid left and turns pink once there is a drop too much base. Sometimes it is an electrode, a small probe that reads the solution and jumps sharply when the acid is used up.
Chemists have two names for this. The point where the amounts match exactly is the equivalence point; it is a fact about the chemistry. The point where you see the signal and stop is the endpoint; it is a fact about your detector. A good titration puts them so close together that the difference does not matter. A careless one does not, and the result is off by however far apart they landed.
Three ways to set it up
Most titrations are direct: you add your known liquid straight to the sample until it is used up. But sometimes the thing you want to count reacts too slowly, or does not dissolve, or has no clean signal of its own. Then chemists get creative. They might add a known excess of something and count what is left over afterwards, which is called a back titration. Or they might let the sample release something else in exact proportion and count that instead. Different routes, same idea: an exact count of what reacted.
Why it still matters
Titration sounds old-fashioned next to the instruments in a modern lab, but it is everywhere. It is how labs check the acid in a used engine oil, the water in a jet fuel, the hardness of the water coming out of your tap, and the strength of medicines. When it is done carefully, it is a very accurate measurement, which is why it is still used to check other methods.
And it rests on a few things you can check: a known amount of sample, a reaction that goes one known way to the end, a liquid whose strength you really know, and a signal that lands where the chemistry says it should. Get those right and you have counted something you can never see.
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References
- IUPAC Compendium of Chemical Terminology (Gold Book), "titration error", term T06389.
- D. Harvey, Analytical Chemistry 2.1, 9.1 Overview of Titrimetry (LibreTexts).
- D. Harvey, Analytical Chemistry 2.1, Appendix 16.1, Normality (LibreTexts).