heliumpycnometria

Heliumpycnometria is a term used in materials science to describe the use of helium-based pycnometry to determine the true density and specific volume of solid materials. The name combines helium, the inert gas used as the probe, with pycnometry, the method of volume measurement by gas displacement.

Principle and procedure: A sample is weighed and placed in a calibrated pycnometer. The chamber is evacuated and then filled with helium at controlled pressure. Helium penetrates pores that are accessible to gas, allowing the instrument to deduce the sample’s true volume from pressure changes. From the measured volume and the sample mass, the true density is calculated. Measurements are typically performed at controlled temperature, and samples are often degassed beforehand to remove adsorbed moisture.

Applications: Heliumpycnometria is widely applied to porous materials such as powders, ceramics, polymers, catalysts, and pharmaceutical excipients, where it provides density and porosity information that complements other methods such as mercury intrusion porosimetry or Archimedes-type liquid displacement.

Advantages and limitations: The method is non-destructive, fast, and sensitive to accessible porosity, yielding a true solid density that is independent of open porosity. It requires careful sample preparation and calibration, assumes ideal gas behavior with corrections for non-ideality, and only measures pores accessible to helium. Moisture, surface adsorption, and gas purity can affect results.

History and usage: The technique evolved as an extension of classical pycnometry in the mid-20th century, with helium gas replacing liquids to avoid sample damage and solvent effects. The term heliumpycnometria appears in some sources to denote this helium-based approach.

See also: Pycnometer, Gas pycnometry, Porosity.