It is evident that there is no serious disagreement between the two methods when applied to finely porous substances. It is recognized that the concept of a meniscus in equilibrium with a vapor ceases to have precise meaning as the capillary radius approaches the diameter of the adsorbate molecules. Consequently extension of the use of the Kelvin equation to pores of very small radius constitutes a sort of extrapolation which cannot be rigorously justified. However, it will be noted that, in general, only a small fraction of the total volume is contained in pores of such small radius as to cast doubt on the applicability of the Kelvin relationship. For example, it seems gratuitous to assume the likelihood of a sudden break in a smooth curve such as that of Fig. 6, occurring due to a sudden failure of the Kelvin equation to apply, at 15 or 10 Á. radius.Further confirmation of the accuracy of both this method and that of Oulton is given by the high pressure mercury porosimeter work of Drake11 in which he finds by a more direct approach that a silica-alumina gel of the cracking catalyst type has a sharp pore volume maximum in the neighborhood of 20 Á. radius. An exact comparison between the (11) L. C. Drake, Ind. Eng. Chem., 41, 780 (1919). results of Drake, Oulton and this paper is, of course, impossible since there is no certainty that the same kind of catalyst was used in each case. However, the catalysts were of the same general type and the similarity of the results are to that extent confirmatory. High pressure mercury porosimeter measurements are being made on a selection of the adsorbents discussed in this paper so as to obtain such a direct comparison. These data will be published in the second paper of this series.Acknowledgment.-The authors wish to acknowledge their indebtedness to Dr. A. Wheeler for his helpful suggestions on the theoretical aspects of the work. They are grateful to Baugh and Sons Company for permission to publish this paper.
SummaryAn analysis of the nitrogen desorption isotherms based upon the Wheeler theory of combined physical adsorption and capillary condensation has been developed.A method for computing the pore volume and area distribution directly from the desorption isotherm of porous substances is presented. The method has been successfully applied to adsorbents covering a wide range of pore volume maxima. Pittsburgh, Pa.
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