Ideality of water vapour and its adsorption on the glass surfaces of a conventional glass vacuum apparatus at 295 K between 0 and 10 Torr

Abstract

The properties of water vapour have been investigated quantitatively between 0 and 10 Torr and at ambient temperature (295 K). Provided a satisfactory correction for surface adsorption is made, the deviation from ideal gas behaviour is not detectable within the accuracy of the corrected pressures (±3%). This is consistent with theoretical modelling calculations and also with experiments made at higher temperatures when they are extrapolated to ambient temperature. There was considerable adsorption of water vapour onto the glass of the apparatus which had previously been in constant use, up to ca. 20 monolayers and taking place in two distinct rate processes that were completed in ca. 5 and ca. 30 min. Very much less adsorption occurred on new Pyrex glass (ca. 1 monolayer). It was also shown that the measured equilibrium vapour pressures in the range 0–10 Torr were directly proportional (±8%) to the amount of water admitted.

Both results are important in confirming the validity of the vacuum apparatus measurements when used to study rates of water release such as the dehydrations of solids which depend on this relationship. However as a consequence of water vapour adsorption, apparent yields were less by 32% than the values calculated using the ideal gas equation. Yield determinations therefore cannot be based on pressure and apparatus volume but must be obtained by direct calibration.