Permeability of Glasses to Gases

Methods of measurement of gas flow through glasses and crystals 

The experiments of Wiistner(24) were made at very high pressures and temperatures, so that special requirements had to be met. The diffusion cells were contained in a platinum oven which in turn was in the interior of a steel bomb. The pressure was transmitted to the bomb from the pump (with oil as transmitting fluid), through the separator which was a U-tube containing mercury, to water, which filled the bomb. The requisite temperature within the bomb was reached by the use of the platinum oven. The silica diffusion cells inside 

Rayleigh (i6) succeeded in the technically difficult task of mounting a mica plate for diffusion experiments at temperatures up to 415° C. The principle of his method was to keep an outer annulus of the plate cold and thermally insulated as far 

The main features observed when one studies the passage of gases through membranes are  

A linear pressure dependence of the permeation rate is indicated by the results of various observers (24,30,28,31,32). It was immaterial whether the gas was monatomic (He) or diatomic (Hg, Og, Ng), the same linear relation was obtained. There is one contrary result (33), data on the velocity of permeation of helium through pyrex, soda, lead, and Jena 16 ii glasses being reported as conforming to an equation dpjdt = ccp. The values of n were given as follows  

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It must, however, be remembered that permeabilities towards certain of the gases vary from specimen to specimen of glass thus much smaller permeabilities both to argon and to helium have been reported (28). In yet another series may be given the permeability of a number of glasses to helium ... 

Bottles range in quality from cheap to pretty sophisticated, depending upon the use to which they are put. The polymer(s) used in the construction of the bottle is chosen based on strength, compatibility with the contents, clarity, and cost. We would prefer that the bottle not break if dropped on the floor. The marketing people tell us we prefer certain products, especially foods, in optically clear bottles. Plastic bottles are, after all, primarily a replacement for glass. Often a bottle not only must contain its contents but must also prevent the passage of gases from one side of the bottle to the other. In other words, the permeability of the bottle and of the polymer(s) from which it is made need to be taken into consideration. Finally, only certain polymers are approved for contact with food. 

The use of glass in glass-metal seals is justified by its transparency, its relatively low permeability to gases, its desirable electrical properties, and the relative ease with which it can be fused to metal to make vacuum-tight seals. Glass, unlike many crystalline materials, can be altered to control expansion coefficient, viscosity, electrical resistivity. 

Storage of water samples to detect trace metals is normally carried out in plastic containers fluorinated plastic materials i.e.. Teflon, fluorinated ethylene propylene (FEP), perfluoroalkoxy polymers (PFA) or polyethylene are used because if opportunely treated and conditioned they guarantee contamination-free samples. However, FEP or Teflon bottles are preferable to those made of polyethylene when speciation studies are carried out. In fact, polyethylene over long periods of storage can release plasticizers (above all phthalates or amines), which behave as ligands and modify the complexation equilibria of the solution. When Hg is to be determined plastic materials must be avoided because they are permeable to gases and vapours glass or quartz are therefore used. 

It is possible to measure the partial pressure of any gas in a mixture directly if there is a membrane that is permeable to that gas alone for example, palladium is permeable to hydrogen and certain types of glass are permeable to helium. The fact that at present only a few such membranes are known does not destroy the physical reality of the concept of partial pressure. Later it will be shown that in chemical equilibria involving gases and in physical equilibria such as solubility of gases in liquids and solids, it is the partial pressures of the gases in the mixture that are significant (further confirmation of the physical content of the concept). 

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