pyknometriaa

Pyknometria, commonly spelled pycnometry, is a set of laboratory methods to determine the volume and density of solid materials using a pycnometer. The principal approaches are liquid pycnometry and gas pycnometry. The technique is used in material science, geology, pharmaceuticals, and ceramics to characterize powders, grains, polymers, and porous solids.

In liquid pycnometry, a calibrated pycnometer is filled with a reference liquid at a controlled temperature. The mass of the pycnometer is measured in several states: empty, filled with liquid, and filled with liquid plus the solid sample placed inside and brought to the same volume. From these masses and the known liquid density, the solid’s volume is obtained from the displaced liquid, and its density is calculated as mass divided by volume. Temperature control is essential because liquid density changes with temperature.

Gas pycnometry uses a gas, typically helium, to determine the true or skeletal density by measuring the volume occupied by the solid via gas pressure changes and Boyle’s law. Because the gas can permeate open pores, this method yields the volume of the solid’s skeleton rather than the bulk volume.

Interpretation: The resulting density can be combined with bulk density to estimate porosity and pore volume. The methods are sensitive to sample dryness, solvent selection (for liquid pycnometry), and temperature stability. The presence of closed porosity, surface roughness, or moisture can affect results.

See also: pycnometer, density, Archimedes’ principle, porosity measurement.