The Influence of the Calibration Procedure of a Volumetric Adsorption Apparatus on the Form of the Isotherm and its BET Parameters

Abstract

An up-to-date piston-operated automatic adsorption apparatus has been tested. Statistical analysis of the results has shown that the adsorbate must be considered as a real gas if a precision of 1% in the adsorption isotherm is to be aimed for at points around 1 bar for gases having a second virial coefficient higher than about 30 cm 3 /mol. A quadratic calibration is recommended and the influence of operational variables demonstrated on a few examples. Results can be applied, mutatis mutandis, to any volumetric adsorption apparatus. It has then been shown that the generally applied linear calibration procedure gives deformed isotherms, their evaluation resulting in BET surface areas 2–12% lower than those obtained by subtracting the non-adsorbed part of the adsorbate with the quadratic, real gas, calibration curve. Finally, it has been shown that isotherms determined by the gravimetric method are the same as those determined with the real gas calibration and give the same BET surface areas.