Comparisons of porosimetry and gas adsorption

The useful range of the Kelvin equation is limited at the narrow pore end by the question of its applicability (see Section 8.5) and at the wide pore end, measurements are limited by the rapid change of the center core radius with relative pressure. 

Mercury porosimetry has somewhat the same constraints at the narrow pore end of its range, in that the same questions arise regarding the constancy of surface tension and wetting angle for mercury as exist for an adsorbate. Consequently, both methods have nearly the same lower limit which is about 18 A pore radius for mercury intrusion (e.g. bOOOOpsia). However, at the wide-pore end porosimetry does not have the limitation of the Kelvin equation and for example, at 1.0 psia pore volumes can be measured in pores of 107 micrometer radius or 1.07 x 10 A. 

Comparisons have been made between surface areas measured by porosimetry and gas adsorption as well as by permeametry with results ranging from excellent to poor. 

The most definitive surface area measurements are probably those made by nitrogen adsorption using the BET theory. Neither the Brunauer, Emmett and Teller (BET) theory nor equation (11.5), used to calculate surface area from mercury intrusion data makes any assumptions regarding pore shape for surface area determinations. When these two methods are compared there is often surprisingly good agreement. When 

If the porosimeter can generate 60000 psia of hydraulic pressure, the minimum radius into which intrusion can occur will be about 18 A. Assuming that pores centered about 15 A radius are present and contain a volume of 0.01 cm, an approximation of their surface area can be made by assuming cylindrical geometry. Thus, 

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