Gas pycnometer

A gas pycnometer is a device used to determine the volume of solids regardless of their geometric shape. Powdery or porous samples can also be measured. This measurement is used to determine the density of samples, which is also referred to as "true density". The physical basis is Boyle-Mariotte's law .

General

Gas pycnometers are based on the fact that the solid bodies to be measured displace their volume of a test gas in a sample chamber. This difference in the volume of the test gas compared to the empty sample chamber or a reference chamber is recorded by measurement. Helium is usually used as the test gas because it interacts little with solids (i.e. is adsorbed) and, secondly, is so small and light that it can penetrate into porous material. There are two basic types of gas pycnometer.

Constant pressure gas pycnometer

Scheme - constant pressure gas pycnometer

The structure consists of two chambers, a sample and a reference chamber, of known and equal volumes. These chambers are provided with pistons and connected to one another by means of a differential pressure meter so that the same pressure always prevails in both chambers. If the test gas present in the reference chamber is compressed by pushing the piston inwards, the piston of the sample chamber is pushed outwards and the pressure in the chambers adjusts to the existing external pressure and there is no pressure difference between the chambers. If there is no sample in the chamber, the length of the displacement path of both pistons is the same. Introducing a sample into the sample space reduces the volume of the sample chamber and thus the volume of gas present. If the compression is then carried out again in the manner described above, the length of the displacement path of the piston of the sample chamber is less than in the case of the unloaded sample chamber, due to the incompressibility of the solid volume of the sample.

The solid volume of the sample depends on the known parameters with the difference in the displacement path of the two pistons as follows:

${\displaystyle V_{P}={\frac {(a\times A_{0}\times V_{1})}{(V_{1}-V_{2})}}}$

in which

${\displaystyle V_{P}}$ = Sample volume

${\displaystyle V_{1}}$ = Volume of the chambers before compression

${\displaystyle V_{2}}$ = Volume of the chambers after compression

${\displaystyle a}$ = Difference in the displacement path of the pistons

${\displaystyle A_{0}}$ = Surface of the piston.

Constant volume gas pycnometer

Scheme - constant volume gas pycnometer

This type of measurement is the most widespread due to its simple structure and implementation and is also directly named as a method in various standards and pharmacopoeias .

The structure consists of a sample and a reference / expansion chamber, which are connected to one another via a valve. The volumes of the two chambers are known through calibration. After the sample has been introduced into the sample chamber, test gas is introduced, possibly also into the reference chamber. For the actual measurement, there must be a pressure difference between the chambers, the sample chamber having the higher pressure. After closing the intermediate valve, this can be done by increasing the pressure in the sample chamber or by evacuating the expansion chamber. After measuring the pressure in the sample chamber, the intermediate valve is opened and the pressure is measured after the pressure has been equalized with the expansion chamber.

The solid volume of the sample depends on the known parameters with the ratio of the initial and final pressure as follows:

${\displaystyle V_{P}=V_{K}+{\frac {V_{R}}{1-{\frac {p_{i}}{p_{e}}}}}}$

in which,

${\displaystyle V_{P}}$ = Sample volume

${\displaystyle V_{K}}$ = Volume of the sample chamber

${\displaystyle V_{R}}$ = Volume of the reference / expansion chamber

${\displaystyle p_{i}}$ = initial print

${\displaystyle p_{e}}$ = Final pressure

Density determination

If the mass of the sample is known, the density of the solid can be calculated. This is also known as true density.

See also

• Aerometer for density measurement of gases
• Hydrometers for density measurement of liquids
• Flexural transducer for density measurement of liquids and gases
• Capillary pycnometer for measuring the grain density of a soil
• Pycnometer

literature

• Alfred Fahr , Gerrit L. Scherphof (Translator): Voigt's Pharmaceutical Technology. 12th edition, ISBN 978-1-118-97262-5 , page 83ff
• S. Tamari: Optimum design of the constant-volume gas pycnometer for determining the volume of solid particles. Meas. Sci. Technol. 2004, 15, pp. 549-558
• PA Webb, C. Orr: Analytical Methods in Fine Particle Technology. Micromeritics Instrument Corp, Norcross, GA, 1997
• S. Lowell, JE Shields, MA Thomas, M. Thommes: Characterization of Porous Solids and Powders: Surface Area, Pore Size and Density. Kluwer Academic Publishers, Dortrecht 2004

Individual evidence

1. ↑ European Pharmacopoeia 9.0, Gas pycnometric density of solids , Paragraph 2.9.23., Page 339
2. ↑ ISO 12154: 2014 Determination of density by volumetric displacement - Skeleton density by gas pycnometry
3. ↑ DIN 51913: 2013-05 Testing of carbon materials - Determination of density with the gas pycnometer (volumetric) using helium as the measurement gas - Solids