Controlled-atmosphere studies, choice

The choice of materials of construction for controlled-atmosphere studies is also crucial. Most metals adsorb, desorb, and react with sulfur-containing compounds depending upon the experimental conditions. For experiments at very low h2s /Ph2 values (<1-10 ppm), adsorption, desorption, and even generation of H2S from metal- and glass-containing systems result in serious problems, particularly at reaction temperatures 196). Quartz and teflon are the preferred materials of construction for studies at low H2S concentrations 99, 100, 114, 140, 196, 197). 

The technique of choice for the study of solvates is thermogravimetric analysis (TGA), where the weight loss of a sample undergoing thermal treatment (in air or in a controlled atmosphere) allows the estimation, often with great precision, the... 

The primary influence of the use of the self-generated atmosphere technique is to increase the pressure of the gas evolution to I atm, which gives rise to favorable thermodynamic, physical, and kinetic effects. The control is not exact, and some investigators consider the use of the technique to be a makeshift or last resort (57). In many instances, however, precise atmosphere control is not available, or is difficult or impossible, because the reaction products are complex or unknown. The technique has a sound theoretical basis and, in such instances, would seem to be a good choice for the initial TG study of a complex solid-gas system. 

The choice of method of study depends very much upon the objectives, which may be to establish one or more of the following threshold temperature for breakdown, nature of the evolved volatile products, production of a convenient fingerprint for identification or purity control, establishment of kinetics, or ultimate understanding of the mechanism of decomposition. For some purposes, heating in air may be satisfactory, but for fundamental mechanistic studies, it is customary to reduce the complexity of the problem by use of an inert atmosphere or vacuum. 

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