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Chapter 8 Gravimetric Methods of Analysis 235

Specific details, including worked examples, are found in the sections of this chapter
covering individual gravimetric methods.

8A.4 Why Gravimetry Is Important
Except for particulate gravimetry, which is the most trivial form of gravimetry, it is
entirely possible that you will never use gravimetry after you are finished with this
course. Why, then, is familiarity with gravimetry still important? The answer is that
gravimetry is one of only a small number of techniques whose measurements re-
quire only base SI units, such as mass and moles, and defined constants, such as
12
Avogadro’s number and the mass of C.* The result of an analysis must ultimately
be traceable to methods, such as gravimetry, that can be related to fundamental
1
physical properties. Most analysts never use gravimetry to validate their methods.
Verifying a method by analyzing a standard reference material, however, is com-
mon. Estimating the composition of these materials often involves a gravimetric
analysis. 2

8B Precipitation Gravimetry
Precipitation gravimetry is based on the formation of an insoluble compound fol-
lowing the addition of a precipitating reagent, or precipitant, to a solution of the precipitant
analyte. In most methods the precipitate is the product of a simple metathesis reac- A reagent that causes the precipitation of
tion between the analyte and precipitant; however, any reaction generating a pre- a soluble species.
cipitate can potentially serve as a gravimetric method. Most precipitation gravimet-
ric methods were developed in the nineteenth century as a means for analyzing ores.
Many of these methods continue to serve as standard methods of analysis.

8B.1 Theory and Practice
A precipitation gravimetric analysis must have several important attributes. First,
the precipitate must be of low solubility, high purity, and of known composition if
its mass is to accurately reflect the analyte’s mass. Second, the precipitate must be in
a form that is easy to separate from the reaction mixture. The theoretical and exper-
imental details of precipitation gravimetry are reviewed in this section.

Solubility Considerations An accurate precipitation gravimetric method requires
that the precipitate’s solubility be minimal. Many total analysis techniques can rou-
tinely be performed with an accuracy of better than ±0.1%. To obtain this level of
accuracy, the isolated precipitate must account for at least 99.9% of the analyte. By
extending this requirement to 99.99% we ensure that accuracy is not limited by the
precipitate’s solubility.
Solubility losses are minimized by carefully controlling the composition of the
solution in which the precipitate forms. This, in turn, requires an understanding of
the relevant equilibrium reactions affecting the precipitate’s solubility. For example,
+
–
Ag can be determined gravimetrically by adding Cl as a precipitant, forming a
precipitate of AgCl.
+
–
Ag (aq)+Cl (aq) t AgCl(s) 8.1
*Two other techniques that depend only on base SI units are coulometry and isotope-dilution mass spectrometry.
Coulometry is discussed in Chapter 11. Isotope-dilution mass spectroscopy is beyond the scope of an introductory text,
however, the list of suggested readings includes a useful reference.