Thermogravimetric Analysis

Abstract

The formal definition of thermogravimetry (TG is the preferred abbreviation, although TGA is also used) has been given by the Nomenclature Committee of the International Confederation for Thermal Analysis and Calorimetry (ICTAC) as “a technique in which the mass of a substance is measured as a function of temperature whilst the substance is subjected to a controlled temperature programme”. In the above definition, a controlled temperature program means heating or cooling the sample at some predetermined and defined rate. Although it is common to have just one constant heating or cooling rate, it is also advantageous in some cases to have different rates over different temperature ranges and in some cases even a varying rate over a specific temperature range. An apparatus called a thermobalance is used to obtain a thermogravimetric curve. By means of a thermobalance the temperature range over which a reaction involving mass change occurs may be determined. The sample is usually a solid, or more rarely a liquid. Thermogravimetric data can be presented in two ways. The TG curve is a plot of the mass against time or temperature, with the mass loss on the ordinate plotted downward and mass gains plotted upward relative to a baseline. Alternatively, data can be presented as a derivative thermogravimetric (DTG) curve, which is a plot of the rate of change of mass with respect to time or temperature against time or temperature. The DTG mass losses should also be plotted downward and the gains upward. Conventionally, thermal analysis experiments are carried out at a constant heating rate, and a property change is measured as a function of time. An alternative approach is to keep the change in property constant by varying the heating rate. For TG, the rate of mass loss is kept constant by variation in heating rate. To achieve this, the mass change is monitored and the heating rate decreased as the mass loss increases, and vice versa. At the maximum rate of mass loss, the heating rate is a minimum. This gives mass losses over very narrow temperature ranges and sometimes enables two close reactions to be resolved. This method has the advantage of using fast heating rates when no thermal event is taking place and then slowing down the heating rate when a mass change is in progress. Thermogravimetry does not give information about reactions that do not involve mass change, such as polymorphic transformations and double‐decomposition reactions. Also, it is not useful for identification of a substance or mixture of substances unless the temperature range of the reaction has already been established and there are no interfering reactions. However, when a positive identification has been made, TG by its very nature is a quantitative technique and can frequently be used to estimate the amount of a particular substance present in a mixture or the purity of a single substance. The types of processes that can be investigated by TG are given.