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Solubility Techniques for Precipitation Titration

In precipitation titration, the titrant -- the chemical you add -- combines with ions or molecules in the sample to form an insoluble compound, which precipitates, or falls out of, the solution. Typically, the sample turns cloudy as the titration progresses. Precipitation titration is somewhat less straightforward than acid-base titration.

Salts
- Precipitation titration often is applied with salts. Sodium sulfate and barium nitrate, for example, are both soluble in water, but when they are mixed, the barium combines with the sulfate to form insoluble barium sulfate, and the solution turns cloudy as the precipitate forms. A popular example is silver nitrate plus sodium chloride; both starting reagents are soluble in water, but the silver chloride that forms when they are combined is not.
Endpoint
- The goal of titration is to determine the concentration of the salt in the sample. If working with sodium chloride, for example, you might want to figure out how many moles of NaCl were originally present. Just as with acid-base titration, you need an indicator chemical in the solution to see when you've reached the endpoint, but finding the right indicator and determining the endpoint is trickier with precipitation titration.
Indicators
- Generally, the indicator is a compound that reacts with the excess titrant. In the case of silver nitrate and sodium chloride, for instance, you could employ chromate ions as an indicator, as these combine with silver to form a blood-red precipitate. Unfortunately, spotting the blood-red color in the cloudy solution isn't easy, and you can't use as much chromate as you'd like because at higher concentrations, the chromate gives the solution a bright yellow color.
Back Titration
- Another approach is called back titration. You might, for example, add excess silver nitrate to precipitate the chloride. At this point, you can add iron (III) ions as an indicator, then titrate with potassium thiocyanate. The silver ions combines with the thiocyanate to form a silver-colored precipitate. When you have reached the point of adding excess thiocyanate, the thiocyanate combines with the iron and turns your solution dark red. Use the amount of thiocyanate you added to calculate the excess silver present, and the amount of excess silver tells you how much silver combined with the chloride to make the initial precipitate.

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