Precipitation Titration Techniques

If you have ever put a solid (like sand) into a clear liquid (such as water), you have probably seen a solid sitting undissolved in a solution before. However, in some reactions, the gradual addition of liquid results in the appearance and accumulation of a solid, as if by magic.

Obviously, most everyday liquids don't combine to form solids. But when they do, and the chemical reaction responsible is known, the amount of product formed can be used to determine the amount of reactant that was present in the solution before a second reactant was added, setting in motion the reaction resulting in the solid.

This kind of solid-from-liquid phenomenon is known as precipitation (not to be confused with the weather phenomenon), and precipitation titrations are a kind of reaction used around the world in various industries. The reason they are helpful is because they allow for a very precise reaction end point determination, one that using now-antiquated methods would require a visual determination of this point.

If you have a solution of known concentration and chemical composition, you know how many total molecules it has available to react with the molecules of another substance.

If you don't know the concentration of the second substance in solution, but know its volume, then you can figure out how much of the substance is present by slowly adding the substance of known concentration (the titrant) until the second substance (the analyte) is gone.

This is made possible by knowing the mathematical way molecules combine in specific reactions (i.e, the reaction's stoichiometry). The amount of titrant consumed translates into the amount of product formed and hence the amount of analyte that was present in the pre-reaction solution. Dividing this amount by the volume gives the molar concentration of the second substance.

In reactions that result in a precipitate rather than products that remain dissolved in the solution, it can be difficult to visually determine the reaction end point, which is why numerous precipitation titration techniques exist to more precisely signal this point.

These involve indicators that form a second precipitate when the amount of titrant added exceeds the amount of analyte present and reacts with the indicator substance.

Three broad classes of indicators exist for precipitation titrations. Two "look for"chloride ions; one looks for the cation silver.

Mohr's Method: Mohr's method, pioneered in 1855 by Karl Mohr, is one of the concentration-of-chloride determination methods used in precipitation titration. It is one of the applications of argentometric titration (meaning the silver(1) ion is used), which uses silver nitrate, AgNO₃, as a titrant.

• The Latin name for silver is argentum; it has the symbol Ag on the periodic table.

Potassium chromate, K₂CrO₄ is added to the titrant-analyte solution. The end point of the titration is signaled when a reddish-brown precipitate of silver chromate, Ag₂CrO₄, forms.

Volhard method: This method for determining silver ions uses potassium thiocyanate, KSCN, as the titrant. In one type, iron ions (Fe³⁺) are added to the solution, and the end point is heralded by the formation of red-brown Fe(SCN)²⁺(ferrous thiocyanate).

Fabans method: This is a determination of chloride, again using Ag⁺as a titrant. For example, when the greenish-yellow dye dichlorofluorescein is added to the solution, positive charges on the newly formed precipitate surface cause the negatively charged dichlorofluorescein to become attracted to silver cations and turn pink.

So now, if you ever have to perform a determination of chloride ion concentration by titration lab report, you have a great article to refer to!