The sorption of water vapour at low pressures by activated charcoals.–Part I

Previous work has led to the accumulation of many data on the sorption of water vapour by activated charcoals. Comparatively few measurements at pressures less than 1mm. have, however, been carried out, and the purpose of the present experiments, of which the first results are described below, is to fill this gap. The nature of charcoal–water vapour isothermals at low pressures is indeed of considerable interest, as in their form at higher pressures they differ widely from the isothermals given by the great majority of gases and vapours.Experimental Method. This was very similar to that worked out by Chaplin in connection with low pressure carbon tetrachloride isothermals. The water supply was contained in a small bulb sealed on to the apparatus. It was freed as far as possible from dissolved carbon dioxide and permanent gases by the continued application of the cycle (i) freezing at the temperature of solid carbon dioxide and ether; (ii) evacuation; (III) melting–until the residual pressure over the ice at – 78°C. was practically unchanged by further treatment. Owing to the solubility of carbon dioxide in ice, this gas could not be completely removed and, in the actual experiments, a charging pressure of water vapour of about 10-1 mm. contained about 10-4 mm. pressure of carbon dioxide.