Relative density#Pycnometer

The pycnometer ( from Greek: πυκνός, crowded ') is a measuring instrument for determining the density of solids or liquids by weighing. The pycnometer consists of a glass bulb with a special ground-glass stopper containing a thin vertical passage, the capillary ( see Figure ), the filling volume is accurately adjusted ..

The first pycnometer designed Abu Raihan Muhammad al -Biruni ( 973-1048 ).

Principle of measurement

The pycnometer is used to represent a precisely repeatable 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 unladen mass is not known, they must be determined.
• Filling is so high that when the plug is inserted liquid penetrates through the capillary of the stopper out.
• After filling, the outside is dried thoroughly. Vessel and capillary must be completely filled and free of bubbles.

For all measurements, the exact temperature of the liquid used is important. Therefore, the filling deserves special attention:

• Priming with the liquid
• Temperature compensation (e.g., in a water bath to the desired temperature)
• Insertion of the plug (solid, bubble-free, complete filling of the capillary)
• Possibly. wait and watch whether the mirror at the end of the capillary is stable
• Remove from the bath and dry thoroughly

Density of liquids

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

To determine the density of liquids applies:

Be

• The mass of the empty pycnometer,
• The mass of the pycnometer filled with water,
• The mass of the pycnometer filled with the liquid to be examined,
• The density of water,

Then yields the density of the liquid to

Density of solids

The measuring principle is based on the displacement of the liquid contained in the vessel. In this case, the empty and weighed, the liquid-filled vessel and the body to be measured is first added to the vessel.

From the weight differences, the density of the body is calculated:

Be

• The mass of the empty pycnometer,
• The mass of the pycnometer filled with water,
• The mass of the pycnometer with the solid state,
• The mass of the pycnometer with the solid, filled with water,
• The density of water,

Then yields the density of the solid to

Example description of a measurement

Implementation in accordance with Pharm:

• Now the bottle is rinsed with a volatile solvent (ethanol, methanol, acetone, etc.) and blown out with compressed air - in the oven the pycnometer should not be dried because the calibration could be lifted to the specified volume. Is the pycnometer completely dry again, it will be filled to overflowing with the test liquid

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