Precipitation Reaction


Precipitation gravimetry is an analytical technique that uses a precipitation reaction to separate ions from a solution. The chemical that is added to cause the precipitation is called the precipitant or precipitating agent. The solid precipitate can be separated from the liquid components using filtration, and the mass of the solid can be used along with the balanced chemical equation to calculate the amount or concentration of ionic compounds in solution. Sometimes you might hear people referring to precipitation gravimetry simply as gravimetric analysis, which is a broader class of analytical techniques that includes precipitation gravimetry and volatilization gravimetry. If you want to read more about gravimetric analysis in general, see this article on gravimetric analysis and volatilization gravimetry.

A precipitation reaction is when two aqueous ionic compounds react to form a new ionic compound that is insoluble in water. The insoluble compound is called the precipitate.

MgCl2(aq) +2AgNO3(aq)2AgCl(s) +Mg (NO3) 2(aq)

We add an excess of our precipitating agent silver (I) nitrate, AgNO (aq) and observe the formation of a precipitate, AgCl(s). Once the precipitate is filtered and dried, we find that the mass of the solid.

Of the many methods that may be employed for isolating the desired constituent from a solution of a sample, the most common is precipitation—that is, transformation into a substance not soluble in the solution. A reagent is added that forms an insoluble compound with the desired constituent but will not precipitate other constituents of the sample. The precipitate obtained is separated by filtration, washed free of soluble impurities, dried or ignited to remove water, and weighed. Certain substances can be separated by virtue of their easy convertibility into gaseous compounds, as in the determination of carbonate in a mineral analysis.

The sample is treated with an acid, and carbon dioxide is evolved as a gas. The gas is absorbed on a weighed quantity of a solid alkaline reagent, and the amount of carbon dioxide is determined from the gain in weight of the absorbent. Electrodeposition is used in order to separate certain metals that can be plated out by passing an electric current through a solution of their salts. Copper in alloys may be determined by this method as long as the sample is free from other metals that plate out under the same conditions.

Errors made in gravimetric analyses usually relate to the purity of the isolated constituent. In general, the compounds that are precipitated are very insoluble and negligible error results from the incompleteness of precipitation. Obtaining a precipitate that is 100 percent pure and is exactly of the composition represented by a chemical formula is, however, considerably more difficult. All gravimetric methods are subject to some degree of error because of this difficulty.


Angelina Matthew,

Managing Editor,

Pharmaceutical Analytical Chemistry

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