Emanation Thermal Analysis

 

Emanation thermal analysis involves the measurement and determination of gasses that are thermally evolved from solid samples. This type of analysis is usually carried out in a thermal gravimetric apparatus with facilities for collecting the evolved gas and passing it to a gas chromatograph or a mass spectrometer (as already discussed). If the gas is radioactive, the evolved gas can be passed though a radioactive counter to assess its radioactivity.

 

Employing this technique, the nature of the gas or gases evolved can be determined, the evolution temperature can be recognized, different sources of gas can be identified and the yields calculated. In general, solids do not absorb gasses and the gases are usually occluded in lattice faults or are adsorbed on the surface, so unless decomposition occurs, only small quantities of gas are usually produced. The technique is used to determine the presence of radioactive gases in minerals, the characteristics of adsorption on solid samples and in some cases the identification of phase changes.

 

The Application of Different forms of Thermal Analysis to Solve a Specific Physical Chemistry Problem

 

The purpose of this section of the book is to show how different thermal techniques can be employed in a variety of ways to solve a basic physical chemistry problem, namely to identify the nature of the surface of silica gel. Silica gel is an extremely important commercial substance. It is used as a base for catalysts, fillers for many types of plastics, and pharmaceuticals and extensively in liquid chromatography.

 

Silica gel is an amorphous, highly porous, partially hydrated form of silica, a substance made from silicon and oxygen, the two most abundant elements in the earth's crust. Actually, more than 55%w/w of the earth's surface consists of either silica (silicon dioxide) or silicates (metallic silicate salts made from silica combined with metal oxides). The majority of silica (as opposed to silicates) found naturally is not significantly hydrated and, although it can exist in both crystalline and amorphous forms, it usually occurs naturally as quartz, cristobalite or tridymite crystals.

 

Quartz is sometimes found clear and colorless but more frequently in an opaque form often colored yellow by traces of iron. When crushed by earth movement and weathered by air and water, it forms sand. Each silicon atom in both quartz and cristobalite is associated with four oxygen atoms. However, in cristobalite the silicon atoms are thought to be orientated in a similar manner to the carbon atoms in diamond where the oxygen atoms would be considered to be situated half way between the 'carbon' atoms. In quartz, the silicon atoms adopt a helical configuration, and consequently, the crystals are enantiomorphic and, thus, optically active.

 

Quartz, tridymite and cristobalite are all stable at atmospheric pressure and ambient temperature, but at higher temperatures quartz is converted into tridymite and at even higher temperatures tridymite is converted to cristobalite.

 

 

 

Despite each change being reversible, some of the changes are extremely slow. For example, the change from vitreous silica to crystalline cristobalite may take many thousands of years at ambient temperature. It is interesting to note that at elevated pressures other, different forms of silica can exist. At a pressure of approximately one thousand atmospheres, and between 400 and 500^oC, a form of silica called keatite is formed and between 15 and 40 thousand atmospheres pressure, and at temperatures between 300 and 1700^oC, a substance called coesite has been found to be stable. At even higher pressures (160 thousand atmospheres) and between 1200 and 1400^oC a form of silica called stishovite  is thought to exist. 

 

Silica, (SiO₂) can also be considered as the anhydride of silicic acid, and in its naturally occurring crystalline form, does slowly hydrate to silicic acid and, as a consequence, is very slightly soluble in water. However, the process is very slow and it is not practical to manufacture silica gel by directly hydrating quartz.