Pyknometria

Pyknometria, also known as pycnometry, is a laboratory method for determining the density of liquids or solids by measuring a material's volume with a pycnometer. The technique is widely used in chemistry, materials science, and geology to assess solvent densities, purity, and material density. It is valued for its simplicity and the ability to perform measurements at controlled temperatures.

In principle, a pycnometer is a small, precisely shaped flask with a known internal volume. The density ρ of a liquid is its mass m divided by this volume V. Because V can be fixed, the method often uses a calibration step with a liquid of known density (usually water) at the measurement temperature to determine V, after which ρ_liq = m_liq / V.

Procedure: clean and dry the pycnometer, weigh it empty (m_e). Fill with the test liquid, ensure complete filling with no air bubbles, seal, and weigh (m_f). Repeat to obtain an average mass difference m_liq = m_f − m_e. For calibration, fill with water and determine V from V = m_w / ρ_w. Then compute ρ_liq = m_liq / V. Correct for air buoyancy if high precision is required.

Temperature control is essential, as density changes with temperature. Common sources of error include incomplete filling, trapped air, surface impurities, and leaks. Repeated determinations and proper cleaning minimize these effects. In solids, gas pycnometry (often using helium) can determine skeletal densities and is widely used for powders and porous materials.