Calibration of DTA and DSC instruments is usually carried out using stan-
dards with well-characterized and tested transition temperatures and enthalpies
of reaction; for example, the melting of indium occurs at 156.6°C and absorbs
28.7 J g−^1 , while zinc melts at 419.4°C and absorbs 111.2 J g−^1.The action of heat on samples has long been a useful ‘dry test’ to determine
some of the qualitative characteristics of the material. Dehydration occurs with
hydrates and with materials such as cellulose. Chemical decomposition and gas
evolution are observed with carbonates, sulfates and nitrates. Occasionally,
explosive reactions happen and all of these can be characterized by DTA and
DSC.
Kaolinite is the pure form of white china clay. It is a hydrated aluminosilicate
and found naturally. The small peak at low temperature (Fig. 3) is due to loss of
hydrated moisture. Around 500°C the strongly bound hydroxyl groups release
water producing a large, broad endotherm. When the temperature reaches
1000 °C, the silica and alumina react exothermically to form crystalline mullite,
3Al 2 O 3 .2SiO 2.Chemical
reactions
314 Section G – Thermal methods
0 500 1000 1300
Sample temperature (°C)DT (°C)EXOENDOFig. 3. DTA curve of kaolinite.Applications Although it must be noted that, like many thermal methods, DTA and DSC are
not compound-specific, they are still most important test methods for a wide
variety of disciplines and materials.
Inorganic materials, salts and complexes have been studied to measure their
physical properties, chemical changes and qualitative thermal behavior.
Minerals and fuel sources such as coal and oil have been examined and when
new materials (e.g. liquid crystals) are discovered, DSC is frequently used to test
them. However, by far the greatest use is made of these techniques in the phar-
maceutical and polymer industries. One special use of DSC for physical changes
is the determination of purity. While a pure substance melts sharply, perhaps
over a few tenths of a degree near its true melting point, an impure sample may
start to melt several degrees below this temperature, and will give a broad peak.
Computer analysis of the shape of this peak allows an estimation of purity, but
does not provide any information on the nature of the impurities.