Theory and Practice
Whether the analysis is direct or indirect, volatilization gravimetry requires that the products of the decomposition reaction be known. This requirement is rarely a problem for organic compounds for which volatilization is usually accomplished by combustion and the products are gases such as CO2, H2O, and N2. For inorganic compounds, however, the identity of the volatilization products may depend on the temperature at which the decomposition is conducted.
The products of a thermal decomposition can be deduced by monitoring the sample’s mass as a function of applied temperature. (Figure 8.9). The loss of a volatile gas on thermal decomposition is indicated by a step in the thermogram. As shown in Example 8.4, the change in mass at each step in a ther- mogram can be used to identify both the volatilized species and the solid residue.
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 CaC2O4 H2O must be heated at temperatures above 250 °C, but below 400 °C if it is to be isolated as CaC2O4. 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 which one or more of the gases are trapped. Thus, a sample of CaC2O4 H2O could be analyzed by heating to 1000 °C and passing the volatilized gases through a trap that selectively retains H2O, CO, or CO2.
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.
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 SiF4 provides an instructive example of a typical procedure.