A Different Way to Make an Analyte Disappear
In complexometry, you made a metal disappear by caging it. There is a second, even older way to make an ion vanish from solution: turn it into a solid. When certain pairs of ions meet, they clump together so tightly that they fall out of the water as fine particles — a process called precipitation. If we add a counter-ion drop by drop until all of the analyte has settled out as a solid, then the amount we added counts the analyte. This is a precipitation titration.
For this to count cleanly, the solid must be genuinely insoluble — once formed, it must stay formed, not dissolve back. How insoluble a solid is gets measured by its solubility product: a small solubility product means almost none of the solid redissolves, so the ions are effectively removed for good. Just as a huge stability constant made caging sharp, a tiny solubility product makes precipitation sharp. The two ideas are cousins: both describe a reaction that goes essentially to completion.
Silver: The Reagent That Names the Family
When the titrant is a silver solution, the method gets a special name: an argentometric titration, from the Latin "argentum" for silver (the same root as the chemical symbol Ag). Silver ions are prized here because they form beautifully insoluble solids with the halide ions — chloride, bromide, and iodide. Add silver to a chloride solution and a milky-white solid of silver chloride clouds the liquid instantly, pulling chloride out of solution.
The counting logic is identical to every titration you have met. Each silver ion pairs with exactly one chloride ion to make the solid, a clean one-to-one ratio. So if you slowly add silver of known concentration until every chloride has been pulled into solid, the volume of silver you used directly counts the chloride. The only genuinely new problem is the same one as before — the disappearance is invisible in the sense that needs a signal. How do you see the moment the last chloride is gone?
Three Ways to Spot the End Point
Chemists invented three elegant tricks to mark the end point of a silver titration, each named after its inventor. The first is the Mohr method. You add a second ion that forms a brightly coloured solid with silver — but one that is slightly more soluble than silver chloride. As long as any chloride remains, every silver ion is snapped up by chloride first. Only when chloride runs out does the next silver finally react with the indicator ion, and a sudden rusty-red solid appears against the white. That splash of colour is your end point.
The second trick is the Volhard method, which is really a back-titration in disguise. Instead of titrating chloride directly, you add a known, deliberate excess of silver — more than enough to precipitate every chloride. Then you titrate the leftover silver with a second reagent that forms a blood-red colour the instant silver is in excess. Total silver added, minus the leftover you titrated back, equals the silver that the chloride consumed. Volhard shines for samples where a direct colour change would be hard to see.
The third trick, the Fajans method, is the most subtle and beautiful. It uses a dye that sticks to the surface of the growing precipitate particles. Before the end point, the solid particles carry a slight surplus of the analyte ion on their surface and the dye stays in solution. The instant the first tiny excess of silver appears, the particle surfaces flip their surface charge, the dye is suddenly attracted onto them, and the whole cloudy suspension changes colour at once. The end point is signalled by the precipitate itself blushing.
Choosing the Right Method
Why keep three methods instead of just one? Because each suits different conditions. The Mohr method needs a near-neutral solution — too acidic and the coloured indicator solid will not form; too alkaline and silver forms an unwanted solid of its own. The Volhard method, by contrast, is run in acid, which makes it ideal for samples that are already acidic or where Mohr's conditions cannot be met. The Fajans method often gives the sharpest end point of all but depends on choosing a dye matched to the particular precipitate.
Step back and notice the unifying picture across both halves of this rung. Whether you cage a metal with EDTA or settle an ion out as a solid with silver, the strategy is the same: make the analyte vanish through a reaction that goes essentially to completion, in a clean fixed ratio, and find a signal that fires the instant the last bit is gone. Master that one idea and every titration in this rung — complexometric and precipitation alike — becomes a variation on a single, satisfying theme.