Possible Standard States for the Adsorbed Material

The most useful type of standard state is one defined in terms of a small number of molecules per unit area of adsorbent surface. In an attempt to have a definition analogous to that for three-dimensional matter—one atmosphere at any temperature—Kemball and Rideal (12) defined a standard state with an area per molecule of 22.53T A.2 where T is the absolute temperature. This corresponds to the same volume per molecule as the three-dimensional state if the thickness of the surface layer is 6A. In terms of surface pressure it corresponds to 0.0608 dynes/cm. for a perfect two-dimensional gas at all temperatures, and as such the definition may be extended to cover condensed films. 

However, in practice it is frequently difficult to obtain results at such low surface concentrations either by direct measurement or by extrapolation. Consequently it is convenient to have an alternative definition in terms of surface covered, e.g., Barrer (3) and Foster (4) take 8 = as the standard state. A knowledge of the amount of material required to complete a monolayer is needed to apply this definition. Where such information is not available, one has to fall back on a definition in terms of so much adsorbate per cubic centimeter or per gram of adsorbent, and this makes interpretation more difficult. The accuracy of the determination will be greater when the heat of adsorption is known for adsorption directly to the standard state rather than as an average over the isotherm as a whole. 

The recent investigation by Damkohler and Edse (5) virtually consists of the calculation of the entropy of CO adsorbed on copper oxide. They calculated the theoretical value of the constant bo in the Langmuir equation 

They showed that b0 may be expressed statistically in terms of the partition function of the molecule in the adsorbed state Sa less the Boltzmann factor, cx/ST, the partition function of the molecules in the 

The values of 60 ranged from 8.5 X 10 7 (mm. Hg) 1 for complete three-dimensional translational freedom and rotational freedom to 6.6 X 10-u (mm. Hg) 1 for the replacement of all modes by vibrations. If these 

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