Introduction

P. J. Haines

Oakland Analytical Services, Farnham, UK

MATERIALS, HEAT AND CHANGES

Whenever a sample of material is to be studied, one of the easiest tests to perform is to heat it. The observation of the behaviour of the sample and the quantitative measurement of the changes on heating can yield a great deal of useful information on the nature of the material.

In the simplest case, the temperature of the sample may increase, without any change of form or chemical reaction taking place. In short, it gets hotter. For many other materials, the behaviour is more complex. When ice is heated, it melts at 0°C and then boils at 100°C. When sugar is heated, it melts, and then forms brown caramel. Heating coal produces inflammable gases, tars and coke. The list is endless, since every material behaves in a characteristic way when heated.

Thermal methods of analysis have developed out of the scientific study of the changes in the properties of a sample which occur on heating. Calorimetric methods measure heat changes.

Some sample properties may be obvious to the analyst, such as colour, shape and dimensions or may be measured easily, such as mass, density and mechanical strength. There are also properties which depend on the bonding, molecular structure and nature of the material. These include the thermodynamic properties such as heat capacity, enthalpy and entropy and also the structural and molecular properties which determine the X-ray diffraction and spectrometric behaviour.

Transformations which change the materials in a system will alter one or more of these properties. The change may be physical such as melting, crystalline transition or vaporisation or it may be chemical involving a reaction which alters the chemical structure of the material. Even biological processes such as metabolism, interaction or decomposition may be included.

Sometimes a change brought about by heating may be reversed by cooling a sample afterwards. A pure organic substance melts sharply, for example benzoic acid melts at 122°C and it recrystallises sharply when cooled below this temperature. Ammonium chloride dissociates into ammonia and hydrogen chloride gases when heated, but these recombine on cooling. At high temperature, calcium carbonate splits up to yield calcium oxide and carbon dioxide gas, and these too will recombine on cooling if the carbon dioxide is not removed. The system reaches an equilibrium state at a particular temperature.

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To raise the temperature of any system heat energy must be supplied and when sufficient energy is available the system will change into a more stable state. The mechanical properties of a material change as it is heated. Often it expands and becomes more pliable well below the melting point. These are fundamental, important changes on a molecular level, and their study enables the analyst to draw valuable conclusions about the sample, its previous history, its preparation, chemical nature and the likely behaviour during its proposed use.

The temperature at which a particular event occurs, or the temperature range over which a reaction happens, are often characteristic of the nature and history of a sample, and sometimes of the methods used to study it. Sharp transitions, such as the melting of pure materials, may be used to calibrate equipment and as the "fixed points" of thermometry and of the International Practical Temperature Scale (IPTS).

For example, how does the simple, pure inorganic compound potassium nitrate, KNO3, behave when heated? At room temperature, say 20°C, this is a white, crystalline solid. To raise its temperature to 30°C at constant presssure, we must supply an amount of heat depending on the specific heat capacity, Cp approximately 1 J K-1 g-1 at this temperature, the mass m of the sample and the change in temperature. So, for 1 g heated 10°C, we must supply 10 J. To complicate matters, the heat capacity changes with temperature as well. When the temperature reaches 128 °C, the crystals change their structure, and this needs more energy, about 53 J g-1. Then the new crystals are heated, when Cp ≈ 1.2 J K-1 g-1, until the melting point of 338 °C, when more heat must be supplied to melt the sample. Raising the temperature above the melting point eventually causes the sample to decompose to form potassium nitrite, KNO2, so that the mass of the sample is decreased by around 16% and oxygen gas is given off.

This example illustrates the importance of thermal techniques and measurements. Calorimetry measures the amounts of heat, while appropriate thermal methods give the temperatures of phase changes, the temperatures of decomposition and the products of the reaction. Other methods will show the expansion, mass and colour changes on heating.

The analysis of thermal events may be approached in two ways, which overlap considerably. Either the experiment may be designed to measure thermal properties (heat capacity, enthalpy, entropy and free energy) with high precision and accuracy at particular temperatures and conditions, or we may study properties, including thermal properties, over a wider range of temperatures using a controlled heating procedure.

Which experiment is chosen depends on the sample to be analysed. There would be little point in obtaining highly accurate heat capacities on a polymeric or cement sample of complex composition, but its behaviour on heating would be informative. Theoretical work on organic structure and kinetics might require precise knowledge of equilibrium thermal properties which could not easily be obtained using variable temperature methods. Therefore, the techniques are complementary.

Since the worldwide adoption of the SI system of units it is perhaps useful to stress the symbols and units to be used for the physical quantities involved in these methods. The major quantities are given in Appendix 1A and the others may be found in the references.

DEFINITIONS OF THERMAL AND CALORIMETRIC METHODS

Formal definitions are not essential, but those accepted by the scientific community may be found in the literature.

Calorimetry is the measurement of the heat changes which occur during a process. The calorimetric experiment is conducted under particular, controlled conditions, for example, either at constant volume in a bomb calorimeter or at constant temperature in an isothermal calorimeter.

Calorimetry encompasses a very large variety of techniques, including titration, flow, reaction and sorption, and is used to study reactions of all sorts of materials from pyrotechnics to Pharmaceuticals.

Calorimetric methods may be classified either by the principle of measurement...