Gravimetric Analysis

Topics: Solubility, Precipitation, Solid Pages: 2 (631 words) Published: July 21, 2010
Gravimetric analysis, by definition, includes all methods of analysis in which the final stage of the analysis involves weighing. In the most basic case, this could involve simply heating a sample to dryness and weighing to determine the amount of volatile components. In this account, however, shall limit to gravimetric methods which rely on the use of precipitation reactions.

The quantitative determination of a substance by precipitation followed by isolation and weighing of the precipitate is called gravimetric analysis.

The basic method of gravimetric analysis is fairly straightforward. A weighed sample is dissolved after which an excess of a precipitating agent is added. The precipitate which forms is filtered, dried or ignited and weighed. From the mass and known composition of the precipitate, the amount of the original ion can be determined.

For successful determinations the following criteria must be met;

1. The desired substance must be completely precipitated. In most determinations the precipitate is of such low solubility that losses from dissolution are negligible. An additional factor is the "common ion" effect, this further reduces the solubility of the precipitate. When Ag+ is precipitated out by addition of Cl-

Ag+ + Cl- =<-> AgCl(s)

the (low) solubility of AgCl is reduced still further by the excess of Cl- which is added, pushing the equilibrium to the right. 2. The weighed form of the product should be of known composition. 3. The product should be "pure" and easily filtered. It is usually difficult to obtain a product which is "pure", i.e. one which is free from impurities but careful precipitation and sufficient washing helps reduce the level of impurity.

Notes:

Another good example for this method is illustrated in this diagram. In this experiment, the goal is to separate the lead with other chemical species. So the best technique to perform is gravimetric analysis. We start with a solid that contains some...
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