Pycnometer

Pycnometer with thermometer and cap to reduce evaporation, filled with iso-propanol and copper particles

The pycnometer (from Greek : πυκνός, densely packed) is a measuring device for determining the density of solids or liquids by weighing. The pycnometer consists of a glass flask with a special ground- glass stopper that contains a thin vertical passage, the capillary (see illustration), the filling volume is precisely adjusted.

The first pycnometer was constructed by Abu Raihan Muhammad al-Biruni (973-1048).

Measuring principle

The pycnometer is used to provide a precisely reproducible volume of liquids. After weighing the empty and the filled pycnometer, the density of the filling can be calculated.

General handling

The pycnometer must be completely clean and dry before use.
If its empty mass is not known, it must be determined.
The vessel is filled so high that when the stopper is inserted, liquid penetrates outwards through the capillary of the stopper.
After filling, the outside is dried well. The vessel and capillary must be completely filled and free of bubbles.

In all measurements, the temperature of the liquid used is of significant importance, as the density can change greatly depending on the temperature. That is why the filling and temperature control deserve special attention:

Pre-fill with the liquid
Temperature compensation (e.g. in a water bath at the desired temperature)
Insertion of the stopper (firm, bubble-free, complete filling of the capillary)
Possibly. wait and see whether the level at the end of the capillary is stable
Take out of the bath and dry well
Weigh

In order to avoid a measurement error with regard to the difference between the external temperature control and the actual temperature of the medium contained in the pycnometer, there are variants of pycnometers that have an additional side arm. A correspondingly high-resolution liquid glass thermometer is then inserted either in this side arm (rarely) or through the regular opening (as a rule) in the pycnometer, thus determining the exact temperature of the medium filled. In the latter case, the side arm contains the vertically arranged capillary through which excess medium escapes when the thermometer is inserted into the filled pycnometer.

The volume of these pycnometers is often precisely determined by the manufacturer at a defined temperature and ground directly into the pycnometer. There are also often markings on pycnometers, thermometers, capillary stoppers and caps, which mark the exact position when the volume is determined by the manufacturer, in order to enable the most accurate reproducibility possible in the laboratory.

When examining or using organic liquids instead of water, the problem of evaporation of this medium when handling the pycnometer can lead to large fluctuations in the density determination. For this reason, with some pycnometers it is also possible to place a ground glass cap over the capillary in order to prevent evaporation or to significantly reduce the gas space available for evaporation compared to the room air, usually below 1 ml reduce.

The formulas given below can be transferred directly to other liquid media (e.g. isopropanol ) if the respective parameters of the water are exchanged accordingly.

Density of liquids

First the empty pycnometer is weighed, then the filled and heated to 20 degrees Celsius.

For the density determination of liquids with unknown pycnometer volume, the following applies:

Be

${\ displaystyle m_ {0}}$ the mass of the empty pycnometer,
${\ displaystyle m_ {1}}$ the mass of the pycnometer filled with water,
${\ displaystyle m_ {2}}$ the mass of the pycnometer filled with the liquid to be examined,
${\ displaystyle \ rho _ {W}}$ the density of the water at the given temperature,

then the density of the liquid results in:

{\ displaystyle \ rho _ {Fl} = {\ frac {m_ {2} -m_ {0}} {m_ {1} -m_ {0}}} \ cdot \ rho _ {W}}

Density of solids

The measuring principle is based on the displacement of the liquid in the vessel by the added solid. First the empty vessel or the one filled with liquid is weighed and then the body to be measured is placed in the vessel.

The density of the body with an unknown pycnometer volume is calculated from the weight differences:

Be

${\ displaystyle m_ {0}}$ the mass of the empty pycnometer,
${\ displaystyle m_ {1}}$ the mass of the pycnometer filled with water,
${\ displaystyle m_ {2}}$ the mass of the pycnometer with the solid,
${\ displaystyle m_ {3}}$ the mass of the pycnometer with the solid, filled with water ,
${\ displaystyle \ rho _ {W}}$ the density of the water at the given temperature,

then the density of the solid is given by:

{\ displaystyle \ rho _ {F} = {\ frac {(m_ {2} -m_ {0})} {(m_ {1} -m_ {0}) - (m_ {3} -m_ {2}) }} \ cdot \ rho _ {W}}

Example description of a measurement

Implementation according to Ph.Eur . :

The empty and dry pycnometer is weighed with the stopper, the weight noted

{\ displaystyle m_ {0}}

Now the pycnometer is filled with water until it overflows

Now the pycnometer is tempered without a stopper (usually 20 ° C or 25 ° C) and refilled again and again until the liquid level remains at the same level

The stopper is placed on the pycnometer so that liquid emerges from the capillary , it is dried and weighed again |

{\ displaystyle m_ {1}}

The difference between this weighing and the previous one gives the weight of the water ( )

{\ displaystyle m_ {1} -m_ {0} = m_ {W}}

The exact volume of the pycnometer is now determined from the density , which we take from a table for our temperature ${\ displaystyle \ rho _ {W}}$ ${\ displaystyle V_ {P}}$

20 ° C: = 0.9982 g / cm³, 25 ° C: = 0.9971 g / cm³

{\ displaystyle \ rho _ {w}}

{\ displaystyle \ rho _ {w}}

{\ displaystyle V_ {P} = {\ frac {m_ {1} -m_ {0}} {\ rho _ {W}}}}

Now the pycnometer is rinsed out with a volatile solvent (ethanol, methanol, acetone or similar) and blown out with compressed air - the pycnometer should not be dried in the drying cabinet, as the calibration to the specified volume could be canceled. When the pycnometer is completely dry again, it is filled with the test liquid until it overflows

The pycnometer is kept at the same temperature as the water before, without a stopper, until the liquid level remains the same
Then the stopper is put back on in such a way that liquid emerges from the capillary and it is dried off
Now it is weighed again | ${\ displaystyle m_ {2}}$
We get the weight of the test liquid from the difference between this weighing and that of the empty pycnometer ( ) ${\ displaystyle m_ {2} -m_ {0} = m_ {Fl}}$
This is now used in the formula together with our previously calculated volume ${\ displaystyle V_ {P}}$

{\ displaystyle \ rho _ {Fl} = {\ frac {m_ {2} -m_ {0}} {V_ {P}}}}

Another possibility of determining density is to determine the volume of a sample by gas displacement (e.g. He pycnometry) and the weight with a balance.

Web links

Commons : Pycnometers - collection of images, videos and audio files

Individual evidence

^ Brockhaus ABC Chemie , VEB FA Brockhaus Verlag Leipzig 1965, p. 1144.
↑ ^a ^b Gerhard Meyendorf: Laboratory Equipment and Chemicals ; People and Knowledge Volkseigener Verlag Berlin, 1965; P. 45.
↑ Otto-Albrecht Neumüller (Ed.): Römpps Chemie-Lexikon. Volume 5: Pl-S. 8th revised and expanded edition. Franckh'sche Verlagshandlung, Stuttgart 1987, ISBN 3-440-04515-3 , pp. 3409-3410.

[ABC_Chemie-1] Brockhaus ABC Chemie , VEB FA Brockhaus Verlag Leipzig 1965, p. 1144.

[Meyendorf-2] Gerhard Meyendorf: Laboratory Equipment and Chemicals ; People and Knowledge Volkseigener Verlag Berlin, 1965; P. 45.

[RÖMPP-3] Otto-Albrecht Neumüller (Ed.): Römpps Chemie-Lexikon. Volume 5: Pl-S. 8th revised and expanded edition. Franckh'sche Verlagshandlung, Stuttgart 1987, ISBN 3-440-04515-3 , pp. 3409-3410.