Most people who remember titration from school remember acids and bases and a flask turning pink. But the same idea, counting something you cannot see by reacting it with a measured amount of something you know, works with other kinds of chemistry too. Two of the most useful are redox titrations and a type built around a molecule called EDTA.
Counting with electrons
In a redox titration, the liquid you add takes electrons from the thing you are measuring, or gives them to it. The counting rule comes from the electrons. A solution of potassium permanganate, deep purple, takes five electrons when it reacts in acid; dissolved iron (in the form chemists call iron(II)) gives up one. So one permanganate reacts with five of those iron ions. Know that ratio and you know how much iron was there.
Permanganate has a lovely trick: it is its own indicator. It is intensely purple, and what it turns into is almost colourless. As long as there is something left to react with, each drop loses its colour. The first drop whose faint pink colour stays tells you you are done. The catch is that permanganate slowly breaks down on the shelf, so it has to be checked against a standard regularly.
Its cousin, dichromate, is the opposite: very stable and reliable to weigh out, but its own colour change, orange to green, is too gradual to mark the end, so it needs a separate indicator. And iodine titrations, often finished with starch that turns deep blue, are a whole family of their own.
Counting with a claw
EDTA is a molecule that wraps around a dissolved metal ion and holds it tightly, like a claw, one EDTA for each metal ion. That one-to-one grip makes counting simple.
Its classic use is measuring water hardness, the calcium and magnesium that leave scale in your kettle. The method uses a dye that turns wine red when it holds those metals. As EDTA is added, it pulls the metals away from the dye, because it grips them more tightly. When the last metal is taken, the dye is free and the solution turns blue. The answer is reported as an equivalent amount of calcium carbonate, so that two different metals share one number.
Conditions decide whether it works
The hardness test also shows how carefully chemists set the stage. Too acidic and the colour change is not sharp; too alkaline and the calcium or magnesium can start to fall out of solution as a solid. So the method holds the solution at a fixed level of alkalinity with a buffer, and asks for the titration to be finished within five minutes of adding it. Other metals in the water, like iron or copper, would also grab EDTA and spoil the count, so they are tied up first with a reagent that hides them.
Why this matters to you
The water report from your utility, the iron in a supplement, the bleach strength on a label: many of these numbers come from titrations like these. Different chemistry, same honest idea: write the counting rule correctly, choose a signal you can trust, and set the conditions so the reaction goes the one way you expect.
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References
- D. Harvey, Analytical Chemistry 2.1, 9.4 Redox Titrations (LibreTexts).
- D. Harvey, Analytical Chemistry 2.1, 9.3 Complexation Titrations (LibreTexts).
- Standard Methods for the Examination of Water and Wastewater, 2340 C, EDTA Titrimetric Method.