Thermal Analysis
Thermal analysis is probably the oldest chemical testing process known and was used extensively by the alchemists in their original primitive chemical explorations. Basically, the alchemists placed the substance to be examined in an iron or ceramic crucible, inserted it into a charcoal furnace and watched what happened. Despite their somewhat bizarre and sometimes fanciful explanations as to what exactly ensued during the heating process they can be considered the inventors of the basic technique of thermal analysis. As the world progressed and the industrial revolution became established in the early nineteenth century, so the techniques of the alchemists were developed and in a modified form became progressively more and more useful. One of the major needs of the early industrialists was a means for checking the quality of their raw materials and in due course the quality of their products. Flour, for example, could be contaminated with significant quantities of water or chalk (that is a non toxic substance), which would improve the profit to the miller but probably denigrate the product of the food manufacturer. Now flour normally has a water content of about 13% w/w and an ash value of about 0.3% w/w and so by determining the loss of weight by first heating a sample of flour to 105oC and weighing the sample and then heating the same sample again but this time to 750oC (thus ‘burning-off’ all the carbon based material) and re-weighing, the moisture and the ash contents could be obtained. Consequently, in a simple manner and with the minimum of equipment, any water or chalk contamination (chalk is not combustible and leaves a residue of lime on heating) could be quickly identified.
In the industrial revolution the most important energy source was coal and so the quality of the coal became a critical economic factor in all types of industrial production. The analytical demands made by coal had a significant impact on the development of thermal analysis. There are a number of different types of coal such as lignite, bitumous, anthracite, graphite, etc. Each type has a different optimum use (such as use in steam engines, power production, smelting metals etc.) In addition, each coal type has different water contents, volatile contents, carbon contents and ash. A thermal analytical procedure was well established by the end of the nineteenth century, which was roughly as follows.
1/ A weighed sample of the coal is heated for about an hour at 105oC and the loss of weight recorded giving the water content of the coal.
2/ The sample is re-heated to 900oC in a muffle furnace for seven minutes, cooled under anhydrous conditions and reweighed. The extra loss in weight is attributed to the volatile content of the coal (mostly aliphatic and aromatic hydrocarbons), which is important for the efficient combustion of the coal in relatively small furnaces such as steam locomotives or small power plants.
3/ The sample is again heated for two hours in the muffle furnace at 1000oC cooled under anhydrous conditions and reweighed. The extra loss in weight is attributed to the ‘fixed carbon’ of the coal, which gives an indication of the heating capacity of the coal and thus the maximum temperatures that could be reached by efficient combustion (an important quality in smelting).
4/ Finally the weight of the residue gives a value for the ash content of the coal which is important in Portland Cement manufacture (the ash content effects the chemical composition of the ‘clinker’ thus, the quality of the resulting cement) and the level of waste disposal from coal employed in power plants.
It is clear that thermal analysis was developing rapidly but was still a manual technique using the traditional chemical balances; the age of automatic temperature programming and simultaneous mass measurement with microbalances was still a ‘long way away’.
The thermal analysis techniques were, however, still basically thermo-gravimetric analysis (the measure of change in weight with temperature) but in the late Victorian era another form thermal analysis was introduced which was the measurement of the calorific value of materials, particularly coal. The calorific value of coal is of prime importance as the real purchase of coal is the purchase of ‘energy’, the purchase of heat on which the whole of the industrial revolution (in fact also modern industry) was/is based. It was essential to know the available energy from any particular supply of coal. Thus the technique of calorimetry moved from the primitive scientific laboratories of the universities to the plant ‘test sheds’ (the forerunners of the modern plant or quality control laboratories).
The apparatus consisted of a
water bath containing a strong high-pressure steel calorimeter in the bottom of
which was placed a little water and a sample of the coal on a small platform
above the water. In the coal was dipped a short length of fuse wire that was
connected (through high-pressure connections) to a low voltage power supply.
The water bath temperature was monitored with an accurate thermometer. The
calorimeter was sealed, then filled to a high pressure
with oxygen and the coal ignited electrically. The temperature of the water
bath was measured continuously over about fifteen minutes and, by applying
The effect of heat on the physical properties of substances is now one of the most important analytical procedures available to the scientist as it not only helps to identify the substances (which perhaps is its major industrial use) but is also used extensively in research to identify the nature and extent of a range of different chemical and physical processes. In due course, this book will give a detailed account of how thermal analysis has been employed in a variety of ways to identify the exact physical and chemical nature of the surface of silica gel.