Gaseous substances highly volatile

In the previous sections, we indicated how, under certain conditions, pressure may be used to induce immiscibility in liquid and gaseous binary mixtures which at normal pressures are completely miscible. We now want to consider how the introduction of a third component can bring about immiscibility in a binary liquid that is completely miscible in the absence of the third component. Specifically, we are concerned with the case where the added component is a gas in this case, elevated pressures are required in order to dissolve an appreciable amount of the added component in the binary liquid solvent. For the situation to be discussed, it should be clear that phase instability is not a consequence of the effect of pressure on the chemical potentials, as was the case in the previous sections, but results instead from the presence of an additional component which affects the chemical potentials of the components to be separated. High pressure enters into our discussion only indirectly, because we want to use a highly volatile substance for the additional component. 

A low explosion temperature together with a great amount of gaseous products and a high specific pressure suggested the used of ammonium azide as a propellent explosive. In practice the use of the substance, however, is prevented by its high volatility. 

Many substances react in the gas phase rather than in solution. An important example is the process thought to deplete the ozone layer the reaction between gaseous ozone, O3, and chlorine radicals, high up in the stratosphere. Ultimately, the chlorine derives from volatile halocarbon compounds, such as die refrigerant Freon-12 or the methyl chloroform thinner in correction fluid. 

High-temperature treatment systems involve destruction of contaminant(s) through complete oxidation, whereas low-temperature systems increase the rate of phase transfer (e.g., liquid phase to gaseous phase), and thus encourage contaminant partitioning from soil. Some of the disadvantages of heat treatment include its high cost and its ineffectiveness with some contaminants (e.g., low volatilization potential or incineration actually produces more toxic substances). 

Sublimation is the transfer of a substance from the solid to the gaseous state without formation of an intermediate liquid phase, usually at a relatively high vacuum. Major applications have been in the removal of a volatile component from an essentially nonvolatile one separation of sulfur from impurities, purification of benzoic acid, and freeze drying of foods, for example. The reverse process, desublimation (16), is also practiced, for example in the recovery of phthalic anhydride from reactor effluent. The most common application of sublimation in everyday life is the use of dry ice as a refrigerant for storing ice cream, vegetables and other perishables. The sublimed gas, unlike water, does not puddle and spoil the frozen materials. 

In order to preclude any misunderstanding, it has to be pointed out that the term partition coefficient here and in the following stands for the volume-related ratio of the concentration of the substance under consideration in the liquid phase to that in the gaseous phase (and not for the reciprocal value, as was often used in former times). This means that a high value of the partition coefficient indicates low volatility of the relevant substance and vice versa. The term apparent indicates that the values mentioned are related to non-equilibriiun conditions. 

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