pycnometer

A pycnometer is a laboratory instrument used to determine the density of a material by measuring the volume it occupies. It typically consists of a small, sealed flask or chamber with a precisely known volume and a stopper that allows complete sealing and filling with a reference fluid or gas. The device enables calculation of density as mass divided by volume, with the volume determined through displacement of a liquid or gas or by applying gas laws.

Gas pycnometers use a reference gas, commonly helium, and rely on pressure measurements to infer the volume of a solid sample. A sample is placed in the known-volume chamber, the chamber is connected to a pressure reservoir, and pressure changes are analyzed (often using Boyle’s law) to obtain the sealed volume of the solid. Gas pycnometry is especially suitable for porous materials because small gas molecules penetrate pores, yielding a true skeletal density that excludes pore volume.

Liquid pycnometers determine density by weighing a known-volume flask filled with a liquid. For liquids, the density is the mass of the liquid divided by the known volume of the pycnometer. For solids, the method can be adapted to measure the volume of a solid by replacing part of the liquid with the solid and measuring the resulting change in mass and volume, often yielding the apparent density or true density depending on the liquid choice.

In practice, pycnometers are used to determine true or skeletal densities of powders, ceramics, metals, polymers, and porous materials, as well as to assess porosity and porosity-related properties. They are valued for precision, simplicity, and compatibility with a range of materials, though care must be taken to avoid air bubbles and to select appropriate liquids or gases for the sample.