pycnometric

Pycnometric refers to a set of methods used to determine the true (skeletal) density and volume of a solid by measuring how much space the material occupies. The core instrument is the pycnometer, a precisely known-volume container used with a balance. Two common variants are liquid pycnometry and gas pycnometry. In liquid pycnometry, a liquid of known density fills the pycnometer, the sample is added, and the resulting displaced volume is inferred from mass changes using Archimedes’ principle. The solid’s volume is then used with its mass to calculate true density. Temperature control is important because density and buoyancy depend on temperature and the liquid must be compatible with the sample.

Liquid pycnometry procedures typically involve weighing the empty pycnometer, filling it with liquid, weighing, placing the sample in the liquid-filled pycnometer, and reweighing. Corrections for buoyancy and liquid density are applied. The method is widely used for materials that are stable in the chosen liquid, such as ceramics and metals, but unsuitable for volatile or reactive substances.

Gas pycnometry, often using helium, measures volume by analyzing how the sample confines gas within a known reference space and applying the ideal gas law to pressure measurements. This approach avoids liquids and is particularly advantageous for powders and porous materials where liquid immersion is impractical. Gas pycnometry provides rapid, repeatable measurements of skeletal volume and true density, with the caveat that trapped air or closed porosity can affect results.

Applications span ceramics, metals, polymers, pharmaceuticals, and soils. Pycnometry is commonly used alongside bulk density measurements to assess porosity and material quality. Limitations include sample preparation, temperature sensitivity, and potential interactions with liquids or gases.