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266 Modern Analytical Chemistry

8F SUMMARY

In a gravimetric analysis a measurement of mass or change in In volatilization gravimetry, thermal or chemical energy is used
mass provides quantitative information about the amount of ana- to decompose the sample containing the analyte. The mass of
lyte in a sample. The most common form of gravimetry uses a pre- residue remaining after decomposition, the mass of volatile prod-
cipitation reaction to generate a product whose mass is propor- ucts collected with a suitable trap, or a change in mass due to the
tional to the analyte. In many cases the precipitate includes the loss of volatile material are all gravimetric measurements.
analyte; however, an indirect analysis in which the analyte causes When the analyte is already present in a particulate form that is
the precipitation of another compound also is possible. Precipita- easily separated from its matrix, then a particulate gravimetric
tion gravimetric procedures must be carefully controlled to pro- analysis may be feasible. Examples include the determination of
duce precipitates that are easily filterable, free from impurities, dissolved solids and the determination of fat in foods.
and of known stoichiometry.

8G Suggested EXPERIMENTS –

Experiments Burrows, H. D.; Ellis, H. A.; Odilora, C. A. “The Henrickson, C. H.; Robinson, P. R. “Gravimetric
A number of gravimetric methods, such as the determination of Cl in a soluble salt, have been part of the
“standard” repertoire of experiments for introductory courses in analytical chemistry. Listed here are additional
experiments that may be used to provide practical examples of gravimetry.

Dehydrochlorination of PVC,” J. Chem. Educ. 1995, 72,
Determination of Calcium as CaC 2 O 4× H 2 O,” J. Chem. Educ.
1979, 56, 341–342.
448–450.
This experiment describes a simple gravimetric procedure
A procedure is provided for the analysis of calcium in
for determining the %w/w Cl in samples of poly(vinyl
using urea and acidified ammonium oxalate. By acidifying
chloride).
the ammonium oxalate, the oxalate is introduced as oxalic
Carmosini, N.; Ghoreshy, S.; Koether, M. C. “The samples of CaCO 3 . Precipitation is done homogeneously
2+
acid and does not precipitate the Ca . Heating the solution
Gravimetric Analysis of Nickel Using a Microwave Oven,”
hydrolyzes the urea, forming NH 3 . As the NH 3 neutralizes the
J. Chem. Educ. 1997, 74, 986–987.
acid in solution, oxalate acid is converted to oxalate and
A procedure for using a microwave oven to digest samples CaC 2 O 4× H 2 O precipitates. The acid–base indicator methyl
of Ni ore and to dry precipitates of nickel–dimethylgloxime is red is used to signal the completion of the precipitation.
described in this experiment.
Snow, N. H.; Dunn, M.; Patel, S. “Determination of Crude
Harris, T. M. “Revitalizing the Gravimetric Determination in Fat in Food Products by Supercritical Fluid Extraction and
Quantitative Analysis Laboratory,” J. Chem. Educ. 1995, 72, Gravimetric Analysis,” J. Chem. Educ. 1997, 74, 1108–1111.
355–356.
The %w/w fat in candy bars is determined by an indirect
This experiment investigates the accuracy of a gravimetric particulate gravimetric analysis. Supercritical CO 2 is used to
analysis when modifying a standard procedure. The extract the fat from the sample, and the change in the
2+
gravimetric procedure is the determination of Ba as BaSO 4 . sample’s weight is used to determine the fat content.
Modifications that are investigated include the addition of a
2+
potential interferent (Ca ), changing the pH at which Thompson, R. Q.; Ghadiali, M. “Microwave Drying of
precipitation occurs, changing the rate at which the Precipitates for Gravimetric Analysis,” J. Chem. Educ. 1993,
precipitant is added, changing the conditions for digesting 70, 170–171.
the precipitate, and changing the procedure for filtering and This article describes conditions for using a household
drying the precipitate. Errors introduced by modifying the microwave oven to dry precipitates for the determination of
2–
–
standard procedure can be explained by considering the Cl as AgCl, the determination of SO 4 as BaSO 4 , and the
2+
process by which precipitation occurs. determination of Ca as CaC 2O 4× H 2O.