The Thermal Absolute Method

Harkins and Jura [7] described a method of obtaining the surface area in an absolute way from a calorimetric measurement. They addressed many of the concerns regarding the method [8] but one must still qualify the method as being very limited. Porosity of any type would significantly alter the answer. 

The apparatus is schematically represented in Fig. 28. The powdered sample, which is known to be non-porous, is allowed to equilibrate over liquid water. (In principle this should work for any liquid.) It is assumed that a film of water is adsorbed about the particles as envisioned in Fig. 29. The powder is then lowered into liquid water. In the process of doing this the outer film of the adsorbed water is destroyed thus releasing the surface energy of this film. Since the surface tension of water is known, then the surface area may be calculated from the heat evolved, or by the simple equation, 

There is not much work performed using this method. Possibly the reason for this is the uncertainty in the interpretation and the difficulty of controlling the experiment. Bed porosity should be a large problem, although one could find samples for experimentation that would minimize this problem. An example of these latter adsorbents would be the rare earth plasters. The measurement of the surface area in this case is at the very high-pressure region versus the BET, which is at the low-pressure region. Thus a comparison between the BET and this absolute method is somewhat questionable. 

More importantly, for most researchers and engineers, this technique is very limited to special types of powders. With an unknown sample it does not seem to have much utility, as ingenious as it is. 

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