Instant Notes: Analytical Chemistry

Section G – Thermal methods

G2 DIFFERENTIAL THERMAL

ANALYSIS AND DIFFERENTIAL

SCANNING CALORIMETRY

Principles If an inert sample, such as alumina, is heated at a constant rate of 10°C min−^1 ,
the temperature-against-time curve is practically a straight line. A sample that
reacts or melts within the temperature range studied will give small changes
on its temperature-time curve. By heating both a reactive sample and an inert
reference together at the same rate, these small differences may be detected
and amplified as a function of temperature. The simplest example is the
melting of a crystalline solid. If 10 mg of metallic indium are heated as sample
and a similar amount of alumina as reference, both heat at nearly the same rate
until around 156°C the indium starts to melt. This absorbs energy and the
temperature of the indium rises less fast. This goes on until all the indium has

Key Notes

Both of these methods relate to the monitoring of the heat absorbed or

evolved during the heating of a sample and a reference in equivalent

environments. Differential thermal analysis (DTA) monitors temperature

difference, while differential scanning calorimetry (DSC) measures the

power supplied.

The measurement unit has a matched pair of temperature sensors placed

in or near the sample and reference pans and is heated in a temperature-

controlled furnace.

Since heat capacity relates to the quantity of heat required to raise the

sample temperature by one Kelvin it may be studied by DTA and DSC.

Physical changes such as melting and vaporization as well as crystal

structure changes give peaks, and some may be used to calibrate the

system.

Heating chemical substances may cause decompositions, oxidations or

other reactions. The temperatures at which these occur and the nature

and rates of the reactions are studied by these methods.

Both DTA and DSC are used to study pure chemicals, mixtures such as

clay minerals and coal, biological samples, pharmaceuticals and

especially polymers and materials.

Related topics Thermogravimetry (G1) Evolved gas analysis (G4)

G3 Thermomechanical analysis

Principles

Instrumentation

Physical properties

and changes

Chemical reactions

Applications