Theories of Adsorption and the Technique of its Measurement

Abstract

SEVERAL years of measurement of sorption in the University of Bristol laboratory by the simple and searching method of the sorption balance (for description see J. Am. Chem. Soc., 1924, 46, 2781, and March 1926, 48, 690) have taught us two things: First, most of the published data on the sorption of gases and vapours possess no quantitative significance because they refer to insufficiently cleaned surfaces. Second, the behaviour of a surface when cleaned from pre-existing impurities assumes a different and highly significant form. It is usually supposed that mere heating of a substance like charcoal to, say 400°, with exposure to an ordinary vacuum, will completely remove previously sorbed gases. This is by no means the case. For example, carefully prepared sugar charcoal, which had been activated by superficial oxidation at about 1200°, was given an exposure for five hours at 450° to a vacuum of 10^-5 mm., and was afterwards found to sorb a maximum of 17.5 per cent, of its own weight of nitrous oxide in a protracted series of experiments. The amount of nitrous oxide sorbed was raised to no less than 26.5 per cent, by the simple expedient of re-evacuating the charcoal at 450° after its saturation with nitrous oxide, and then repeating the measurements. Similar behaviour has been observed in many other cases; for example, the observed sorption of such inert substances as hexane or decane may be increased several fold by repeating the evacuation after the charcoal has been saturated with the hexane or decane to displace other impurities from the surface.