pycnomètre

A pycnomètre is a laboratory device used to determine the density or specific gravity of liquids and small solid samples. It is a flask with a precisely defined interior volume, usually fitted with a stopper and a capillary or narrow outlet to prevent leaks and to minimize air bubbles. Some designs include two connected bulbs or chambers, enabling different measurement steps.

Principle and method: The key idea is that the pycnometer provides a known fixed volume. By measuring masses with and without the test substance, density can be calculated. For liquids, the process typically involves weighing the empty pycnometer (or the device with the stopper inserted), filling it with the test liquid to the calibration mark, and weighing again. The liquid density is then obtained from the mass of liquid divided by the known volume of the pycnometer. For solids, the sample is weighed in air, placed in the pycnometer, and a liquid of known density is introduced to fill the remaining volume; the displaced liquid volume corresponds to the volume of the solid, allowing the solid’s density to be computed from its mass and its volume. Two-volume pycnometers may be used for more complex determinations or to improve accuracy.

Applications and considerations: Pycnometers are used to determine densities of liquids, powders, polymers, ceramics, and metals, as well as to study porosity and material purity. Important factors include temperature control (density depends on temperature), complete wetting and bubble elimination, chemical compatibility of the liquid with the sample and the container, and avoidance of hazardous liquids. Calibration and cleanliness are essential to obtain accurate results.

Limitations: The method relies on accurate knowledge of the reference volume and careful handling to prevent air pockets or sedimentation. Some liquids or samples may require alternative fluids or specialized pycnometers.