Instant Notes: Analytical Chemistry

Section G – Thermal methods

G1 THERMOGRAVIMETRY

Principles One of the simplest tests that may be applied to an analytical sample is to heat
it and observe the changes that occur. These may be color changes, burning,
melting or a variety of other reactions. The group of techniques that has been
developed to make analytical measurements during heating is given the general
name thermal analysis. Any property change may be monitored, and Table 1
lists the more important thermal methods.
While some analytical methods, such as spectrometry, give results that are
very specific for the particular sample, thermal methods will respond to the
totality of the effects. Anything that changes the mass at a particular tempera-
ture: evaporation, reaction or oxidation will affect the thermogravimetric
measurement. It is sometimes an advantage to combine techniques, or to run
two simultaneously (see Section F) to extract the maximum benefit from the
analysis.
In thermogravimetry, the sample is heated, often at about 10°C min−^1 in a
thermobalance instrument as described below. Only those changes that affect
the massof the sample will affect the measurements, so that condensed phase
changes such as melting or crystalline transitions cause no mass change. The
rate of changeof mass, dm/dtdepends on the amount of sample present, and
the reaction rate constant at the experimental temperature. With solids it is

Key Notes

Thermal methods investigate changes that occur upon heating a sample.

Thermogravimetry measures changes in the mass of a sample that occur

when it is heated. These changes relate to the reactions during

decomposition, the loss of volatile material and the reactions with the

surrounding atmosphere.

A crucible containing the sample is heated in a furnace at a controlled

rate and weighed continuously on a balance. Temperature and mass data

are collected and processed by a computer dedicated to the system.

Control of the atmosphere surrounding the sample is important.

There are many factors that affect the results obtained by thermal

methods. These must be carefully controlled and recorded.

Any physical or chemical change involving mass may be studied.

Evaporation of volatile material, oxidation, and particularly

decompositions of inorganic salts, organic and polymeric samples are

investigated analytically.

Related topics Gravimetry (C8) Thermomechanical analysis (G3)

G4 Evolved gas analysis

and differential scanning

calorimetry (G2)

Principles

Instrumentation

Proper practice

Applications