Combining Skeletal and Envelope Volume Measurements to Determine Total Pore Volume and Percent Porosity
The skeletal volume of a solid body excludes pores as well as the void spaces between particles. Envelope volume includes all pores and voids. While these measurements are each important on their own merit, their combination also allows one to accurately calculate percent porosity and total pore volume of a body.
Micromeritics’ AccuPyc II 1340 Gas Pycnometer provides skeletal volume and density measurements on a wide variety of materials having volumes from 0.01 to 350 cm3. Micromeritics’ GeoPyc 1360 Pycnometer determines the envelope volume and density of monolithic samples as well as the bulk volume and density of powdered materials. Both instruments are based on Archimedes’ displacement principle. The AccuPyc is a fully automated instrument that utilizes a gaseous displacement medium, typically helium. The pressures observed upon filling the sample chamber, then discharging the displaced gas into a second empty chamber allow computation of the sample solid phase volume. The GeoPyc uses a quasi-fluid displacement medium composed of microspheres having a high degree of flowability that do not wet the sample or fill its external or internal pores. A variety of sample chambers is available to accommodate a wide range of sample sizes.
Pharmaceutical scientists realize that many of the physical, mechanical, and pharmacokinetics properties of tablets are influenced by the basic settings of a tablet press. Using the AccuPyc coupled with the GeoPyc, scientists are now able to determine quickly and easily the skeletal density, envelope density, total pore volume, percent porosity, and closed-cell pore volume of tablets produced with varying press settings.
These two instruments have also been used to determine a variety of pharmaceutical ribbon physical properties. Using the AccuPyc, the skeletal or true density of the blend of materials making up the ribbon can be measured. When the skeletal density is included in the setup parameters for the envelope density, the GeoPyc will calculate and report the percent porosity and total pore volume of the ribbon. This information has proven to be useful in controlling the mechanical properties of the material, compression force settings on the roller compactor, and subsequent tablet press settings.