Page 274 - Modern Analytical Chemistry

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Chapter 8 Gravimetric Methods of Analysis 257

The product’s molar mass, coupled with the temperature range, suggests that
this represents the loss of H 2 O. The residue is CaC 2 O 4.
The loss of 4.72 mg from 400–500 °C represents a 19.19% decrease in the
original mass of 24.60 g, or a loss of
0.1919 ´146.11 g/mol = 28.04 g/mol

This loss is consistent with CO as the volatile product, leaving a residue of
CaCO 3 .
Finally, the loss of 7.41 mg from 700–850 °C is a 30.12% decrease in the
original mass of 24.60 g. This is equivalent to a loss of
0.3012 ´146.11 g/mol = 44.01 g/mol

suggesting the loss of CO 2 . The final residue is CaO.

Once the products of thermal decomposition have been determined, an analyt-
ical procedure can be developed. For example, the thermogram in Figure 8.9 shows
that a precipitate of CaC 2 O 4×2 O must be heated at temperatures above 250 °C,
H
but below 400 °C if it is to be isolated as CaC 2 O 4 . Alternatively, by heating the sam-
ple to 1000 °C, the precipitate can be isolated as CaO. Knowing the identity of the
volatilization products also makes it possible to design an analytical method in
H
which one or more of the gases are trapped. Thus, a sample of CaC 2 O 4×2 O could
be analyzed by heating to 1000 °C and passing the volatilized gases through a trap
that selectively retains H 2 O, CO, or CO 2 .

Equipment Depending on the method, the equipment for volatilization gravime-
try may be simple or complex. In the simplest experimental design, the weight of a
solid residue is determined following either thermal decomposition at a fixed tem-
perature or combustion. Thermal decomposition or combustion is accomplished
using a Bunsen or Meker burner, a laboratory oven or a muffle furnace, with the
volatile products vented to the atmosphere. The weight of the sample and the solid
residue are determined using an analytical balance.
Constant-temperature decomposition or combustion, followed by trapping
and weighing the volatilized gases, requires more specialized equipment. Decom-
position of the sample is conducted in a closed container, and the volatilized
gases are carried by a purge-gas stream through one or more selective absorbent
traps.
In a thermogravimetric analysis, the sample is placed in a small weighing
boat attached to one arm of a specially designed electromagnetic balance and
placed inside an electric furnace. The temperature of the electric furnace is
slowly increased at a fixed rate of a few degrees per minute, and the sample’s
weight is monitored.

Representative Method Although each volatilization gravimetric procedure has its
own unique characteristics, the following indirect method for the determination of
Si in ores and alloys by formation of volatile SiF 4 provides an instructive example of
a typical procedure.