(205a) Absolute, Excess and Net Adsorption and Helium Adsorption

The aim of this contribution is to highlight some inconsistencies often encountered in the literature. We will discuss the relative advantages and disadvantages of reporting absolute, excess and net adsorption with particular reference to the determination of adsorption of helium on HISIV 3000. Simple relationships will be derived to show the limiting behaviour of the three approaches at low pressures and in the saturation limit. The discussion will highlight the difficulties that lead to issues with the definition of the excess adsorbed amount and the approaches of Sircar (2001) and Gumma and Talu (2003) to determine the skeletal density of microporous solids. Classical thermodynamic approaches and molecular simulations will be used to identify the correct representation of the temperature dependence of the Henry law constant. This will be used to identify the correct temperature at which to carry out helium pycnometry experiments.