Instant Notes: Analytical Chemistry

● the crucibleor container, its size, shape and material;

● the rate of heatingand any special program;

● the atmosphere, and whether static or flowing;

● the massof the sample, and its properties, e.g. particle size.

One example may illustrate this. Several thermogravimetric experiments were

carried out on a polymeric sample. One was very different from the others. This

was traced to the use of a copper crucible in an oxidizing atmosphere and the

consequent formation of copper oxide above 500°C.

Applications The analysis of the thermal decomposition of inorganic salts and complexes is
an important part of the study of catalysts, semiconductors and fine chemicals.
The decomposition of barium perchlorate, Ba(ClO 4 ) 2. 3H 2 O was investigated by
thermogravimetry and other techniques. The TG curve is shown in Figure 3.

G1 – Thermogravimetry 309

100

0

4.6% loss

13.8% loss

46.6% loss

TG

Mass %

100 200 300 400 500 600

Temperature (°C)

Fig. 3. Thermal decomposition of barium perchlorate, powder 30 mg, platinum dish,

10 °C min−^1 , nitrogen, 10 cm^3 min−^1.

There are clearly two losses, both occurring in two steps. The first, around

100 °C, might well be loss of hydrate water, which is substantiated by the calcu-

lated loss of 13.8%, agreeing with that found by experiment. The major loss of a

further 32.8% (of the original mass) near 450°C agrees with the formation of

barium chloride:

Ba(ClO 4 ) 2 =BaCl 2 +4O 2

Polymer stability is very important, and the decomposition temperatures of

commodity plastics are often investigated by TG. The mechanism of decomposi-

tion varies between different polymers, as does the temperature at which this

occurs. For example, polyethylene and polypropylene degrade completely in a

single step between 150 and 450°C, but polyvinyl chloride (PVC) shows two

steps, the first with a loss of 60%, by about 400°C. Cellulose and polyacrylamide

both show several steps.

The analysis of rubber may be carried out by TG, as shown in Figure 4. The

analysis is started in a nitrogen atmosphere. The low temperature loss to 200°C

is due to the oil content of the rubber, whereas the polymer rubber is lost before