Alternative determination of the skeletal density of solids using a manometric gas physisorption apparatus: A systematic and methodological study

摘要

The comparison of skeletal densities determined by standard gas pycnometer and manometric gas physisorption apparatus was performed for 42 solid materials differing in density (1.4-11.3 g/cm(3)) chemical nature (metals, aluminosilicates, carbons), physical form (granules, powders) and porosity (from non-porous to highly porous). Under nearly identical experimental conditions with helium as probe gas, the use of both types of equipment provides practically the same values of densities (deviations below 8%) for metallic samples and aluminosilicates. The results differ more significantly for porous carbons especially for highly porous materials with the surface area above 400 m(2)/g (deviations up to 70%). Analysis of possible reasons of this disagreement leads to finding that in gas physisorption apparatus it is necessary to use extra purging with helium and prolong the time of equilibration of the system to achieve results comparable with values obtained by standard procedure with gas pycnometer for highly porous samples. Contrary, the detailed investigation of pressure changes during equilibration stage of measurement (available only with gas physisorption apparatus) of porous carbonaceous samples showed a gradual increase of pressure (consequently decrease of determined density) with the time of equilibration, which is expected to be due to evolution of gases from the porous system even after outgassing. The use of short equilibration time is recommended for such kind of samples to obtain realistic values of skeletal density.