pyknometer

A pycnometer is a laboratory instrument used to determine the density or specific gravity of a substance by measuring a known volume and the mass of the substance contained within it. There are two main types: liquid pycnometers and gas pycnometers. A liquid pycnometer is a sealed vessel with a precisely defined internal volume. To determine the density of a liquid, the empty pycnometer is weighed (M0). The liquid is then introduced to fill the chamber to its marked line, and the filled pycnometer is weighed again (M1). The density is calculated as ρ = (M1 − M0) / V, where V is the known internal volume. Temperature control is essential because liquid density depends on temperature, and corrections may be required to reference values.

A gas pycnometer measures the volume of solids by gas displacement, typically using helium or nitrogen. The sample is placed in a sealed chamber and the amount of gas required to reach a specified pressure is measured. Applying the ideal gas law, the sample volume is derived, and density is computed as mass divided by this volume. Gas pycnometry is particularly useful for porous or irregular solids where liquids may not fully penetrate pores or may cause adsorption.

Applications include determining the density and specific gravity of liquids (such as oils and solvents) and solids (ceramics, powders, catalysts), as well as in quality control and materials research. Pycnometers provide high-accuracy density measurements but require careful calibration and temperature control. Common limitations involve potential entrapped air, moisture sensitivity, and the need for clean, dry samples to avoid measurement bias.