Diffusion through with sealed edges

A catalyst bed contains a uniform mixture of equivolume porous spheres and cubes with two pairs of opposite faces sealed to diffusion. Accordingly the cubes may be regarded as slabs with diffusion through two faces. The spheres have a diameter of 0.02 ft, so cubes of the same volume have edges of 0.01614 ft. 

The most numerous researches have been carried out upon water-membrane systems, for which a summarising paper by Carson (1) has outlined the main methods of measurement. To prevent lateral diffusion of water through the edges of the specimen, or between it and its supports, various devices have been adopted. The edges have been moulded in wax, or compressed with a mixture of beeswax and resin. Shellac, petrolatum, and rubber have been used for the same purpose or the cell may be mercury sealed. Films of lacquers and paints may be made by painting the lacquer on suitable surfaces, such as amalgamated tin-plate (2), and then peeling off the dried membrane. 

For solid shapes where the diffusion takes place from one rather than two opposite faces, the functions are calculated as if the thickness were twice the true value. For example, if diffusion occurs through only one face of the flat slab of thickness 2a, edges sealed, the calculation is made with D9f4a. The equations can also be used for diffusion into, as well as out of, the various shapes. Concentrations c may be expressed as mass solute/volume or mass solute/mass solid. 

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