Volumetric methods for high surface areas

In all the volumetric methods the basic principles are the same. The adsorbate is degassed under vacuum to remove surface contamination. Helium is next admitted into a burette of known volume and its pressure and temperature measured so that the amount at STP can be calculated. The sample tube is immersed in liquid nitrogen and helium admitted. The residual amount in the burette is determined and the amount expanded into the sample tube determined. Since helium does not adsorb on to the solid, this volume is termed the dead space volume and it is found to be linearly dependent on pressure, l e helium is removed and the procedure repeated with nitrogen. When the nitrogen expands into the sample tube, it splits into thi parts, residual in the burette, dead space which can be calculated from the previously found dead space factor, and adsorbed. The process is repeated at increasing pressures and the amount adsorbed determined as a function of relative pressure. 

The accurate determination of the amount of gas adsorbed depends upon a precise knowledge of the dead space. Estimation of the quantity of unadsorbed gas in the dead space is complicated by the fact that part of the dead space is at room temperature and part is at liquid nitrogen temperature. 

Since the amount adsorbed represents the difference between the amount admitted to the burette and the amount in the dead space at equilibrium, it can only be evaluated with confldence if the two quantities are of unlike magnitude. To achieve this the apparatus is designed to minimize dead space volume. In practice it is found convenient to fix the volume and temperature and measure the pressure. 

A great variety of volumetric apparatus have been described in the literature and the earlier ones have been reviewed by Joy [112]. Conventional types of nitrogen adsorption apparatus follow the design described by Emmett [113]. 

The main disadvantage of the original design is that the sample tube is not directly connected to the vacuum line and hence any powder flying from the tube is likely to contaminate the whole apparatus. Improved designs have been described by Emmett (1944) [114], Harvey (1947) [115], Joyner (1949) [116], Schubert and Koppelman (1952) [117], and Vance and Pattison (1954) [118]. 

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