Instant Notes: Analytical Chemistry

● the sample and its container;

● the sensors for measuring temperature and the sample property;

● the computer, data collection and processing equipment and a display device

for the results.

An electrical furnace is often used and the furnace enclosure is purged with a

suitable gas, either to provide an inert atmosphere, or to provide a reactive

environment where the sample may be burnt, or reacted. The temperature is

measured by a thermocouple or resistance sensor and the heating is controlled

by the system computer (Fig. 2).

308 Section G – Thermal methods

Gas in

Gas out

Furnace

Sample

Thermocouple

Electrobalance Balance control unit

Programmer

Recorder and/or

computer

Fig. 2. Schematic of a thermogravimetric instrument.

The analytical sample is contained in a suitably inert crucible. This may be

made of alumina, platinum or ceramic. A sample size of around 10 mg is fairly

typical. The sensor for measuring temperature is most often a thermocouple

suitable for the temperature range to be studied. For measuring the mass, a

thermobalanceis used. This is a sensitive electronic balance sufficiently far

away from the furnace to avoid all effects of heat and any corrosive gases

produced by the sample and capable of detecting changes as small as 1 μg and

of weighing samples of 10−100 mg. The balance is purged with dry nitrogen for

protection.

Temperature calibration cannot be carried out by the normal IPTS standards

since these involve no mass change. A method has been devised using the Curie

point (TCi) of metals. This is the temperature above which they have no ferro-

magnetism. For example, for nickel metal, the Curie point temperature is 353°C.

By placing a magnet near a calibration sample of nickel, the mass is increased at

temperatures below TCi, but not above that temperature. A step occurs in the TG

trace that allows calibration.

Proper practice Since the experimental conditions have a profound effect on the results obtained
by thermogravimetry, and also other thermal methods, it is as well to establish a
set of rules to follow in order to obtain the most reproducible results, or to
recognize why runs differ.
The acronym SCRAM, standing for Sample, Crucible, Rate of heating,
Atmosphere, and Mass of sample is a useful reminder of the things to remember
and report:

● the sample, its source, history and chemical nature;