Page 276 - Modern Analytical Chemistry

1400-CH08 9/9/99 2:18 PM Page 259

Chapter 8 Gravimetric Methods of Analysis 259

8C.2 Quantitative Applications
Unlike precipitation gravimetry, which is rarely used as a standard method of analy-
sis, gravimetric methods based on volatilization reactions continue to play an im-
portant role in chemical analysis. Several important examples are discussed in the
following sections.

Inorganic Analysis Determining the inorganic ash content of organic materials,
such as polymers and paper, is an example of a direct volatilization gravimetric
analysis. The sample is weighed, placed in an appropriate crucible, and the organic
material is carefully removed by combustion. The crucible containing the residue is
then heated to a constant weight using either a burner or an oven.
Another example of volatilization gravimetry is the determination of dissolved
solids in water and wastewater. In this method a sample of the water is transferred
to a weighed dish and dried to a constant weight at either 103–105 °C, or at 180 °C.
Samples dried at the lower temperature retain some occluded water and lose some
carbonate as CO 2 . The loss of organic material, however, is minimal. At the higher
temperature, the residue is free from occluded water, but losses of carbonate are
greater. In addition, some chloride, nitrate, and organic material are lost through
thermal decomposition. The residue remaining after drying at either temperature
can be ignited to constant weight at 500 °C. The loss in weight on ignition provides
an indirect measure of the amount of volatile solids in the sample, and the weight of
the remaining residue gives the amount of fixed solids.
Indirect analyses based on the weight of the residue remaining after volatiliza-
tion are commonly used in determining moisture in a variety of products and in de-
termining silica in water, wastewater, and rocks. Moisture is determined by drying a
preweighed sample with an infrared lamp or in a low-temperature oven. The differ-
ence between the original weight and the weight after drying equals the mass of
water lost.
The determination of silicon is commonly encountered in metallurgical and
mining laboratories responsible for the analysis of ores, slags, and alloys. The
volatilization gravimetric method, which is appropriate for samples containing high
concentrations of silicon, was described earlier in Method 8.2.
As a final example, the determination of carbon in steels and other metal alloys
can be determined by heating the sample. The carbon is converted to CO 2 , which is
collected in an appropriate absorbent trap, providing a direct measure of the
amount of C in the original sample.

Organic Analysis The most important application of volatilization gravimetry to
the analysis of organic materials is an elemental analysis. When burned in a stream
of pure O 2 , many elements, such as carbon and hydrogen, are released as gaseous
combustion products, such as CO 2 and H 2 O. The combustion products are passed
through preweighed tubes containing appropriate absorbents. The increase in the
mass of these tubes provides a direct indication of the mass percent of carbon and
hydrogen in the organic material.
Alkaline metals and earths in organic materials can be determined by adding
H 2 SO 4 to the sample before combustion. Following combustion, the metal remains
behind as a solid residue of metal sulfate. Silver, gold, and platinum can be deter-
mined by burning the organic sample, leaving a metallic residue of Ag, Au, or Pt.
Other metals are determined by adding HNO 3 before combustion, leaving a residue
of the metal oxide.